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The seismology of Greece
Tectonophysics,
98 (1983)
165
165-208
Elsevier Science Publishers
B.V., Amsterdam
- Printed
in The Netherlands
Review THE SEISMOLOGY
MARKUS
OF GREECE
BATH
Seismological
(Received
Section,
December
Box 12019,
S-750
28, 1982; accepted
I2 Uppsulu (Sweden)
April 21. 1983)
ABSTRACT
BLth, M.. 1983. The seismology
of Greece.
A review and a bibliography period
from 1950 up to the present
instrumental structural
installations, properties
has an interest geophysical
passing
expeditions
98:
165-208
are given of the seismological
literature
time. The aim is to incorporate on to earthquake
and seismotectonics.
in this tectonically
Tectonophysics.
complex
catalogues.
The purpose
for the area of Greece
all aspects of seismology, statistics
is to provide
area and particularly
m the
starting
and effects, and ending
with
up with
a service to every geophysicist
to the numerous
researchers
who
engaged
in
to Greece.
INTRODUCTION
Greece has the highest seismic activity in Europe with about 2% of the whole world’s seismic energy release. This is accomplished within an area amounting to only 0.09% of the total area of the world, truly a “ vast seismological laboratory”, as Galanopoulos
(1972a) formulates
it. See Fig. 1.
In 1962, Greece was visited by one of UNESCO’s seismological survey missions with the purpose to explore the state of seismological studies and research, including engineering
aspects
(Bath
et al., 1962). Beyond
this, the early exploration
of the
seismological conditions in Greece was essentially left to the Greek seismologists. However, in the last decade a notable change has taken place, and the Greek seismicity now stands in the center of interests of many of the world’s seismologists. Two recent earthquakes have especially directed the attention to the Greek seismicity: partly an earthquake in the Thessaloniki area on June 20, 1978 (surfacewave magnitude M = 6.5; 50 deaths), partly an earthquake in the Corinth-Athens area on February 24, 1981 (M = 6.7; about 25 deaths). Even though the number of deaths were comparatively modest, the material damage was considerable in both events. Beside this, there are other aspects in which these two earthquakes exhibit similarities, e.g., the obvious panic among the population, the very pronounced and efficient sense of responsibility among the authorities, and finally a very great O&ICI-1951/83/$13.20
0 1983 Elsevier Science Publishers
B.V
”
0 0
0
0
0 0
MEDITERRANEAN
EPSTEIN
Fig. I. The seismically
0 0
active trends
of Greece
0
in 1902- 1976. Open clrclea correspond
h i 90 km and filled circles to h > 90 km. After Galanopoulos
international interest among seismologists research teams from abroad for examination
to focal depths
(1977a).
and engineers, leading to numerous of the effects both in the field and on
buildings (Papazachos and Carydis. 1982). The present review has been prepared as a service to this extensive international interest in the Greek area. In particular, our review is defined as follows with regard to subject, time and space: (1) Subject. The listed publications are restricted to seismological research, including seismotectonics and earthquake engineering. On the other hand, the very rich geological literature on Greece, often with important bearings on the tectonics of the area, will not be included, unless seismology enters as a part.
167
30 -
N/year BIBLIOGRAPHICAL
u 0
’
I
1950 Fig. 2. Annual
60 number
year
I
0
I
60
70 of items (N/year)
in our bibliography
versus time.
and no papers before 1950 are included. The earlier seismological literature deals mostly with macroseismic effects of earthquakes (a list with 54 items of this early literature can be found in (2) Time. The period
covered
is from 1950 onwards,
Trikkalinos, 1955). (3) Space. The space is restricted to the Greek area, and papers dealing with the Mediterranean region, with no particular reference to Greece, are in general omitted. The bibliography (see References) includes 329 items, to which Greek authors have contributed in whole or in part by no less than 58%. About 25% of all listed items have appeared in Greece, the remainder in various international journals. mostly European. Perhaps number of listed references the overall activity
most striking, however, is a graph showing the annual (Fig. 2). This figure demonstrates a relatively low level of
in the period
1950-1965,
This time variation is in fact quite concerned with the Greek area. INSTRUMENTAL
typical
after which a very active period sets in. of all kinds
of seismological
activities
RECORDINGS
Permanent installations. As instrumental records are crucial for the development of modern, quantitative seismological research, I will give a brief historical review of the permanent seismograph installations in Greece, collected from scanty, scattered information in the literature (Z, N, E refer to vertical, north-south and east-west seismograph components, respectively): 1898: First seismograph
in Greece,
type Agamennone,
in Athens.
168
1911: Mainka
N, E, Seismological
recording major earthquakes Aegean region.
Laboratory
in Greece,
seismology in Greece. 1927: Wiechert Z, N, E, Seismological Mainka
N, E transferred
University.
this is the first reliable
The year 1911 is generally
1954-19.56:
of Athens
quoted Institute,
seismograph
as the beginning National
from Athens
Intended
in the
of instrumental
Observatory
University
for
of Athens.
to Athens Observa-
tory. Kritikos N, Athens Observatory, installed for the recording of major local shocks. Athens Observatory becomes the headquarters of instrumental seismology in Greece. 19.57: Benioff Z, Athens seismograph in Greece. 1962: WWSSN (World
Observatory,
being the first modern
Wide Seismograph
short-period), Athens Observatory. 1963: Wiechert Z, Patras, this becoming by Riolos. 1964: Hiller (Stuttgart) 1965: Wood-Anderson
Station
Network)
the first auxiliary
high-magnification Z, N. E (long- and
station,
later replaced
Z, N, E, Athens Observatory. N, E, Athens Observatory.
1965-1966: Sprengnether Z, N, E at four new stations on four islands. This marks the initiation of the modern Greek seismograph network. 1966: One strong motion seismograph at Argostoli (Cephalonia). Seventeen seismoscopes installed as a network in western Greece. The engineering aspects (strong motion, acceleration) begin to receive a greater attention than hitherto. 1968: Sprengnether Z, N, E at two new stations on the mainland. 1971: A network of strong motion seismographs begins to be installed in Greece. 1971-1981: The network of seismograph stations is further developed and modified and consists presently of 13 stations (Fig. 3 and Table I). In addition, there are now nearly 20 strong motion seismographs well distributed over the Greek area and about equally many seismoscopes, the latter concentrated to the Ionian Islands and adjacent
parts of Peloponnesus
A remarkable
fact, evident
and the mainland.
from this historical
being a highly seismic area, got a modern Moreover,
the station
density
network
is lower than
review, is that Greece, very late, starting
in some neighbouring
in spite of
not until
1965.
countries,
e.g.,
Italy with its 54 stations. Nevertheless, we note with satisfaction that the present network covers the Greek area quite well, and permits an adequate account of earthquakes within this area. This is evidenced especially by the Athens Monthly Seismological Bulletins, which contain detailed information both on record readings and on calculated source parameters. Cf. also Giannakopoulos (1972). Combination with a relatively dense network in western Turkey could enhance the outputs still further. Temporary installations. Although the present network can be considered adequate for a supervision of the whole area concerned, it may be insufficient for more special studies (Psycharis, 1978). Therefore, beside the more permanent installations,
169
I
3OOkm
I
Fig. 3. The present
seismograph
station
network
in Greece.
The numbers
refer to Table I
temporary or special-purpose seismograph installations have been used sporadically. mostly in the last decade and often by research workers from abroad: (1) With the purpose reports recordings near
to select sites for permanent stations, Steinwachs (1971) Kremasta dam in 1966 with a mobile three-component
station. With a similar purpose, Crampin (1975) performed recordings at several localities in Greece for a few days at each place. Measurements of the seismic noise were then of dominant
interest.
(2) As to the Kremasta Corporation, the installation
together
dam,
Therianos
with the Geodynamic
of four seismograph
stations
( 1974) reports institute
that
of Athens,
the Public is proceeding
at selected places around
Power with
the complex
of the Acheloos River at existing and future hydroelectric projects, in order to secure reliable records of any seismic activity in that area. (3) Earthquake sequences-swarms as well as regular aftershocks-have been studied by temporary installations. Drakopoulos and Delibasis (1973) operated a mobile station for recording an earthquake swarm on the Melos Island in August 1971. Aftershocks near the Trichonis Lake in central Greece in June-December 1975 were recorded by two portable seismographs and a seismoscope (Delibasis and Carydis, 1977). The Thessaloniki earthquake of June 20, 1978, is obviously the first one in Greece, for which major efforts were made at recording the aftershocks. The Institute of Earthquake Engineering in Skopje operated three strong motion accelerographs in the area (Papastamatiou, 1978; Petrovski et al., 1978; Maley et al., 1979). The U.S. Geological Survey installed a local array of ten stations, operating
170
TABLE
1
The present
seismograph
station
(After Nat]. Obs. Athens,
Location
Station
1.
Athens ATH
1911
Limestone
2.
3.
5.
6.
9.
10.
Instrument
Mass
r,
T,
(kg)
(set)
(see)
c
37”58’2O”N
Benioff
Z
107.5
1.0
0.74
12500
23”43’00”E
Benioff
107.5
1.0
0.77
12500
95 m above
Wood-Anderson
N, E N, E
m.s.1.
Sprengnether
2
II.2
14.9
99
1500
Sprengnether
N, E
10.7
14.8
99
1500
Wiechert
Z
I300
1.5
150
Wiechert
N
5.0
170
Wiechert
E
IO00 IO00
5.0
185
Mainka
N
135
2.8
50
Mainka
E
135
3.5
60
Kritikos
N
40
2.5
4
0.8
2800
37”04’08”N
Sprengnether
Z
1.14
0.5
0.5
70000
25’31’50”E
Sprengnether
N
1.14
0.5
0.5
40000
Limestone
620 m
Sprengnether
E
1.14
0.5
0.5
20000
Archangelos
36” 12’58”N
Sprengnether
Z
1.14
0.5
0.5
30000
ARG
28’07’34”E
Sprengnether
N
1.14
0.5
0.5
45000
170 m
Sprengnether
E
1.14
0.5
0.5
10000
1966
Ithomi
37’10’47”N
SVK-M3
Z
5.0
1.0
0.5
25000
ITM 1974
2 l”55’36”E
SGK-M3
N
5.0
1.1
0.5
25000
Flysch
400 m
SGK-M3
E
5.0
1.1
0.5
15000
Jannina
39’39’24”N
Sprengnether
Z
1.14
0.5
0.5
40000
JAN 1968
20’5 1’03”E
Sprengnether
N
1.14
0.5
0.5
20000
Limestone
540 m
Sprengnether
E
1.14
0.5
0.5
5000
Kozani
40” 18’24”N
Sprengnether
Z
1.14
0.5
0.5
65000
21’46’15”E
Sprengnether
N
1.14
0.5
0.5
25000
900m
Sprengnether
E
1.14
0.5
0.5
20000
1972
Limestone
8.
Bull. Nov. 1981)
Apeiranthos
KZN
7.
in Greece
APE 1975
Marl, sand 4.
network
Seismol. Inst., Monthly
Neapolis
35O15’45”N
SVK-M3
Z
5.0
1.0
0.4
50000
NPS 1973
25’36’45”E
SGK-M3
N
5.0
1.0
0.4
50000
Limestone
370 m
SGK-M3
E
5.0
1.0
0.4
25000
Paraskevi
39” 14’46”N
Sprengnether
Z
1.14
0.5
0.5
40000
PRK 1965
26O16’18”E
Sprengnether
N
1.14
0.5
0.5
20000
Rhyolite
100 m
Sprengnether
E
1.14
0.5
0.5
10000
Penteli
38”02’56”N
Benioff
Z
1.0
23”51’53”E
Press-Ewing
Z
6.9
10.0
(0.7) 85
12000
PTL 1971 Limestone
500m
Polygyros
40°22’26”N
Sprengnether
Z
I,14
0.5
0.5
70000
PLG 1968
23’26’44”E
Sprengnether
N
1.14
0.5
0.5
20000
Gneiss
580 m
Sprengnether
E
1.14
0.5
0.5
10000
100
2500
171
TABLE
I (continued) Location
Station
1 I.
12.
13.
Instrument
Riolos
38”03’28”N
RLS 1976
21”28’0O”E
Sandstone
100 m
Mass
7;)
(kg)
(set)
Willmore
2
5.0
I.0
TK (see)
V
25000
Valsamata
38O 10’3O”N
Sprengnether
Z
1.24
0.5
0.5
45000
VLS 1965
20”35’23”E
Sprengncther
N
1.14
0.5
0.5
25000
Limestone
375 m
Sprengnether
E
1.14
0.5
0.5
10000
Wood-Anderson
N, E
2050
0.8
Vamos
35’24’25”N
Sprengnether
Z
1.14
0.5
0.5
70000
VAM 1965
24’11’59”E
Sprengnether
N
1.14
0.5
0.5
I5000
Limestone
225 m
Sprengnether
E
1.14
0.5
0.5
5000
The station activity,
name is followed
and its geological
T,, = seismometer
period;
by its internationally
foundation.
SVK-M3
T, = galvanometer
used three-letter and SGK-M3
period;
abbreviation,
are seismographs
V = maximum
dynamic
the year of its start of of Soviet construction.
magnification.
for nearly three weeks in July, 1978 (Maley et al., 1980; Carver and Bollinger, 1981) and a team from the University of Cambridge, England, operated a variable number of portable stations during the period July 20 (4) Some efforts have also been made to use the regular, continuous activity, i.e., not just accuracy of hypocentral locations, Leydecker 1978) operated a tripartite station with 60 km EARTHQUAKE
to August 31 (Soufleris et al., 1982). temporary installations for recording aftershocks. In order to increase the (1975) and Leydecker et al. ( 1975, side on Peloponnesus in 1972- 1974.
CATALQGUES
Earthquake catalogues, homogeneous and complete, are a prerequisite for many investigations in seismology. They form the basis for the production of seismicity maps as well as for all kinds of statistical evaluations, including estimation of seismic risk for engineering purposes. Likewise, accurate hypocenter data are needed in the evaluation of the seismotectonics of any region. The complicated tectonics of the Greek area calls for exceptionally accurate determinations of earthquake foci. However, the production of earthquake catalogues fulfilling all such requirements has to face practically insurmountable difficulties. The instrumental recording period is limited to the present century or less, and for earlier centuries we have to rely on descriptions of macroseismic effects only. In both cases, it is impossible to achieve a perfect homogeneity, i.e., a consistent completeness and accuracy over the whole observation period. The historical record for Greece goes back a few thousand years, but the data become more and more scanty and unreliable the further back in
172
time we go. And a similar present
century
problem
due to the gradual
is faced during improvement
works. In both cases, the degree of completeness (cf. Drakopoulos, significance
1976a). These problems
and there is no remedy
the instrumental
period
of the seismograph and accuracy
are not unique
increases
of the
station
net-
with time
for Greece but of universal
for them.
It is frequently assumed that catalogues are representative to such a degree that they permit linear extrapolations, e.g., for the calculation of seismic risk. However. strictly speaking, we have no guarantee against the possibility that catalogues. even those encompassing a few thousand years, just represent a part of a far longer geological cycle with varying tectonic activity. Even though tectonics may provide additional valuable information, there is presently no reliable way of eliminating such deficiencies in homogeneity, completeness and representative value. But it is important to bear them in mind in any application of earthquake catalogues. The available information on catalogues for the Greek area is summarized in Table II, arranged in chronological order. The catalogues are essentially of three different kinds, with regard to their coverage of space and time: Catalogues for larger areas, of which Greece forms a part. The world-wide catalogues of Gutenberg and Richter (1954) and Rothe (1969) belong to this category. Though not listed in Table II, we may also include the earthquake lists published by several seismological agencies: International Seismological Summary (ISS) for 191% 1963, International Seismological Centre (ISC) since 1964, National Earthquake Information Service (NEIS) of the U.S. Geological Survey. Seismologique Europeen-Mediterranten (CSEM) in Strasbourg. Catalogues
Centre of the
European area (Karnik, 1968, 1971) and of the Balkan area (Shebalin et al., 1974) also belong to this group. The Greek contribution to Shebalin et al. (1974) derives from compilations by Drakopoulos Catalogues covering the Greek
and Delibasis ( 1972, 19722 1973). area and a fairly long period of time.
These
catalogues are the most important ones for detailed studies of the whole area. From the data statements, we note that in general the following correspondence holds between observation period and maximum intensity Z,: before 1800, I, 2 8; 1801- 1900, I,, 2 7; 1901- 1970, I,, 2 6. For the two first-mentioned periods, Galanopoulos (1960b, 1961~) proceeds one unit lower on the I,, scale. The recent catalogue by Makropoulos (1978), cf. Makropoulos and Burton (198 1), is important because all locations, times and magnitudes are recalculated in a consistent manner applying modern techniques. The completeness of this catalogue is investigated in detail as well as the epicentral shifts versus earlier determinations. Believed to be complete for M 2 5.5 for the last 60 years, this catalogue is probably the most reliable one for the instrumental era now available for Greece. Catalogues covering limited areas (within Greece) or limited periods of time. They are also listed in Table II to the extent that I have been able to locate them in the available literature. In most cases, such catalogues are not obvious from the titles of the papers, but may appear hidden behind almost any title concerned with Greek seismology.
173
TABLE
II
Earthquake
catalogues
for the Greek area
Reference Platakis
(1950)
Area covered
Period covered
Crete
antiquitypresent
Galanopoulos
(1952b)
Leukas
Island
Comment
time
1469- I948
macroseismic observations
Galanopoulos
(1953b)
1879- 1892
Greece
macroseismic observations
Galanopoulos
(1954b)
138991949
Chios Island
macroseismic observations
Gutenberg
and Richter
the World
190441952
M > 5.5
Greece
antiquity-
damaging
and
disastrous
earth-
(1954) Galanopoulos
(1955~)
present
time
quakes Galanopoulos
(1960b)
Greece
1801-1958
I,r6;
Galanopoulos
(1961~)
Greece
Prior to 1800
I, Z 7
17Ot- 1960
M > 4.75
Galanopoulos
(1963)
34’-42“N.
Gaianopoulos
(1965a)
Greece
19*-29”E
1843- 1962
M25
M 2 5.5; only dates and magnitudes
Galanopoulos
( 1966)
Greece
Galanopoulos
(1967~)
Greece
(1967)
Garagunis Papazachos
et al. (1967a)
1800-1957
Corinth
M z 7; N= 28
480 B.C.- 1954
area
1926-1964
Greece
40 aftershock sequences
Gaianopoulos Karnik
(1968a)
(1968, 1971)
31”-36ON,
24’-36’E
European
1908-1967
M > 4.5
I:
I0 > 6; M > 4.5
1901-1955
II: 1801-1900
I, z 7
Rothe ( 1969)
the World
195331965
incl. macroseismic
Tamrazyan
Greece
1900-1958
M > 6.5
1900-1970
Ma7;N=27
effects (1970)
Galanopoulos
( 197 1a)
34”-42”N,
Galanopoulos
( 1971b)
within
19”-31”E
100 km of
1805-1969
Athens Papazachos
(1971)
Greece
191 I-1969
2 16 aftershock sequences
Papazachos
and
Comninakis
(1971)
Drakopoulos Delibasis
1911-1969
intermediate
Greece
2100 B.C.-1799
I, r 8
1800- 1900
I, 2 7
(1972)
Drakopoulos Delibasis
and
Greece
and
Greece
events
1901-1970
(1972- 1973)
Galanopoulos
( 1972b)
Papazachos
and
Comninakis
(1972)
Galanopoulos
(1973a)
34O-42”N.
19’-29T
Greece 34’-42’N,
19”-29°E
1950-1972
M z 5.5; N = 325
1911-1971
M t 4.9
1950-1973
M > 5.5
174
TABLE
II (continued)
Reference
Period covered
Area covered
Comment
et al. (1974)
Shebalin
Balkan
I:
1901-1970
M>4
I,>6;
II: prior to 1901
I,, > 7 (1801-1900) I, 2 X (prior to 1801)
( 1975)
Galanopoulos Leydecker Papazachos
(1975) ( 1975b)
E. Mediterranean
1954-1975
h>90km:N=105
36’-39”N.
1972- 1974
N=321
1911-1965
M 2 5.6
1966- 1973
M 2 5.1; N = 274;
20°-24.25”E
Greece
fore- and aftershock lists for 218 events Comninakis
and
Papazachos
(1977,1978)
30”-50”N.
10°W-40”E
Galanopoulos
(1977a)
32”-43ON,
Makropoulos
(1978);
33O-42.5”N.
Makropoulos
and Burton
17”-30”E 19O-29”E
1901-1910
M 2 6.5
1911-1949
M > 5.5
1950- 1963
M 2 5.0
1964- 1975
M 2 4.5; N = 2324
1902- 1976
M 2 5.5; N = 164
1901~1978
M 2 5.5 after 1920; N=1806
(1981) Yerkes et al. (1979)
39.5”-41.75’N.
Papazachos
Greece
(1980a)
22”-25OE
1759-1978 1974-1980
M > 5: N = 109 M >_5.1: fore- and aftershocks
Papazachos
and
Comninakis
( 1981)
I, = maximum events;
Hellenic
intensity
trench
1919-1979
main shocks and largest aftershocks
(12-degree
scale);
M = surface-wave
magnitude:
N = total
number
of listed
h = focal depth.
In this connection,
I like also to mention
a catalogue
by Alsan et al. (1975). where
all source parameters are recalculated for the area 35.5”-42S”N, 25.5”-45.0”E. in its western part overlapping the Greek area. Grigorova and Palieva (1965) describe the seismicity along the Mesta River, which in part also concerns Greece. EARTHQUAKE
STATISTICS
Using the material collected in earthquake catalogues, the earthquake statistics aims at finding relations between certain dependent quantities (Y), such as frequency of events, seismic energy release, strain release, and one or several independent quantities (X), such as space, time and magnitude. That is, we seek relations Y =f( X). Such procedures are applicable to any earthquake sequence, and it is convenient to divide the efforts according to the time scale of the sequences. Short-term statistics, extending over days, months or at most a few years. Such
175
sequences
are offered
frequency-magnitude
by foreshocks-main relations
for
shock-aftershocks.
foreshock
sequences
Frequency-time as well
and
as magnitude
relations between foreshocks and the main shock have been investigated extensively in Greece. The results are suggested to be useful in forecasting the time of occurrence obvious,
and
the magnitude
as evidenced
these cases, relatively shocks. The short-term connection
of the main
both by Cephalonia large foreshocks
statistics
with volcanic
encompasses activity.
shock.
However,
the difficulties
are
in 1953 and Thessaloniki
in 1978. In both
were at first misinterpreted
to be the main
also earthquake
A problem
concerning
swarms. often occurring prediction
appears
in
here.
because at least in the initial stages, it may be difficult to distinguish a swarm from a true foreshock sequence. After this, we proceed to a more detailed discussion of the relevant literature: (1) Fore- and aftershocks. A paper by Papazachos et al. (1967a) is the first more extensive attempt to investigate the properties of seismic sequences in Greece and the surrounding area. In all, 40 aftershock sequences from 1926 to 1964 are used. with all data listed (Table II). Papazachos (1971) studies 216 aftershock sequences for the period 1911-1969. for which all pertinent information is collected in a table. Ranalli (1969) presents in considerable detail the statistical methods he used. Of his investigated 15 sequences, there are six from Greece. There is quite general agreement about some results. This concerns the recurrence relation: log N = a-hh4 (N = frequency, M = magnitude, a. b = constants). Thus, the b-coefficient is found to be lower for foreshocks than for aftershocks, to be constant during an aftershock sequence, and to decrease with depth. The h-coefficient is negatively findings. Further studies
correlated
with the acting
of the b-coefficient
stress, which explains
are reported
by Ambraseys
some of these (1965). Niklova
and Karnik (1969), Giannakopoulos (I 972). Papazachos ( 1974e), Galanopoulos and Ekonomides (1975), Karnik and Prochazkova (1976) and Bath (1983). Moreover, there is a rather general agreement about magnitude differences with M, - M, = 1. I - 1.2 in average (confirming Bath’s law) and M, - M_, = 1.9-2.0. while also some variation of these differences with M, and focal depth h is suggested and put into mathematical form in some papers (M,, M,, M_ , = surface-wave magnitudes of the main shock, the largest aftershock and the largest foreshock, respectively). The relations between frequency and time for fore- and aftershocks are in general agreement with results found in other seismic areas, especially in Japan. In addition to the papers already quoted, information on earthquake sequences, in particular fore- and aftershocks, is given in the following papers: Galanopoulos ( i 955a), Drakopoulos (1968, 197 I, 1978a), Drakopoufos and Srivastava ( 1970) Drakopoulos and Ekonomides (1972), Drakopoulos and Delibasis ( 1973) Prochazkova (1972, 1973), Papazachos (1974c, d, 1975a. b, c, 1980a). Purcaru ( 1974), Delibasis and Carydis (1977), Carver and Henrisey (1978). Comninakis (1978).
Comninakis
and Papazachos
(1979a), Papazachos
and Leventakis
Bollinger (1981), Karnik et al. (1982), Soufleris et al. (1982). (2) Swarms. The information on swarm activity in Greece than on fore- and aftershocks, rare in Greece.
Drakopoulos
on the Melos Island the h-coefficient than
between
and
(1973) investigate
in August
more marked aftershocks-a
is much more scanty
reason being that volcanic
and Delibasis
which occurred
exhibits fore-
a probable
(1980). Carver and
1971. Drakopoulos
differences result
between that
earthquakes
an earthquake
(1978a) finds that
foreshocks
could
are
swarm
suggest
and swarms a method
to
distinguish swarms from foreshocks already at an early stage. Long-term statistics or secular statistics, usually extending over the total period of available homogeneous data. Frequency--magnitude relations can then be used for the estimation of seismic risk and/or the maximum magnitude likely to occur, presented in tables or maps. Space relations include seismicity mapping, e.g., frequency and energy mapping, providing significant generalizations of point maps (epicenter maps). Time relations lead to investigations of periodicities-a field that attracted much interest in the early days of seismology. Nowadays, it is to a great extent dismissed, because many of the earlier suspected periodicities prove not to be real. Migrations and oscillations of the seismic activity, involving space-time relations, belong either to short-term or long-term statistics. depending upon their time scale. After this, we proceed to a brief review of the literature relevant to the Greek area. Here, we are exclusively concerned with the instrumental data statistics. The macroseismically based statistics, including intensities, etc.. is dealt with in the next section. (1) Seismic risk and maximum for the estimation of the maximum theory of extreme values (Schenkova
magnitude.
Several different
methods
are applied
magnitude and the seismic risk, such as (a) the and Karnik, 1973, 1977; Karnik and Schenkova,
1974; Makropoulos, 1978; Burton, 1979), (b) the vertical separation between the upper and lower boundary lines in diagrams of strain release versus time (Galanopoulos, 1972b; Karnik and Radu, 1977; Makropoulos, 1978), or (c) from a/b, the “once per year earthquake”, advocated especially by Galanopoulos (1968b, 1971 b, 1976, 1979b). Cf. also Comninakis (1975). Galanopoulos (1976. 1979b) suggests that the maximum possible magnitude is at most two units greater than the annual maximum (a/b) of the region, and that this method is better than relying on the theory of extreme values. It is of interest to note that this suggestion of Galanopou10s is practically identical with defining the maximum magnitude as corresponding to a seismic risk of R = 0.01. The various measures of seismic risk are usually displayed on maps. Discrepancies between various estimates of seismic risk and maximum magnitude are in part due to different observational material and different methods of calculation. Purely seismological estimations should be compared with results from tectonics, stress measurements, etc.
177
In addition (197(i),
to the papers
Hattori
(1979),
already
Cosentino
mentioned,
( 1981) deal with seismic risk and maximum (2) Space characteristics
also those of Algermissen
( 1980). and
of seismicity.
Galanopouios
magnitude
A number
and
et al.
Makropoulos
in the Greek area.
of different
seismicity
maps has
been
prepared for Greece, which all differ somewhat in detail, but nevertheless emphasize the main features, e.g., the two main centers of activity in the Ionian Islands and near the Dodecanese Islands. Different quantities are used for the mapping, Delibasis
such as (a) the released seismic wave energy E (Galanopoulos. 1963; and Galanopoulos, 1965, 1967; Niklova and Karnik, 1969; ~znichenko et
al., 1973; Maaz et al., 1974; Bath, 1983), (b) the released seismic strain, calculated as E 1/2 (Galanopoulos, 1955a, 1956a; Delibasis and Galanopoulos, 1965, 1967), (c) the frequency of events, expressed as a, b, a/b (Kaila and Narain, 197 1: Kaila and Rae, 1975: Comninakis, 1975) or (d) they are given as seismotectonic maps, mostly equivalent to epicenter maps but with tectonic units included (Beloussov et al., 1968: Bornovas et al., 1971; Bune et al., 1971; Gorshkov et al., 1974: Beuzart. 1975; and in numerous papers In examining terminology
referred to in the last section). the publications on seismicity
of a more special type, deviating
unambiguous, for example: “Seismic efficiency” (Galanopoulos,
mapping.
we come
from the standard
1956a) or “ tectonic
across
some
one and not fully
flux”
(Delibasis
and
Galanopoulos, 1965): strain release map in terms of E’j2. “Seismic efficiency” (Galanopoulos, 1963): earthquake energy per square degree and 100 years, shown by isoenergetical “‘Earthquake efficiency” (Delibasis
lines. and Galanopoulos,
1965): energy
map. with
energy expressed as the number of shocks with M = 6.8 per square degree and 100 years. “Seismic activity” (Riznichenko et al., 1973): energy map, showing the number of earthquakes corresponding to a certain energy or a certain magnitude. ““Shakeability” (Riznichenko et al., 1973): distribution of maximum expected intensity or mean recurrence time (years) of earthquakes of any given intensity (for example, for Athens the intensities 7, 8, 9. 10 recur at intervals of 9, 50, 1200. 50.000 years, respectively). (3) Space-time
characteristics
of seismicity.
The information
about
oscillations,
migrations and periodicities is usually rather scanty and often unreliable. This is true universally and Greece is no exception. It is well known that the stress is concentrated to those areas where seismic belts end. split or change direction. Osciilations may occur between the stress concentrations at the end points of a belt. Likewise, a passive, practically aseismic plate like the Aegean Sea is supposed to exhibit oscillations between opposite borders of the plate (Galanopoulos, 1955b), while Tamrazyan (1970) finds an oscillation pattern between north and south Greece. The two major concentrations of activity (near Ionian and Dodecanese Islands) are also suggested to show an oscillation pattern with a period of 52 years
17X
(Delibasis and Galanopoulos, 1967; Galanopoulos, Galanopoulos (1956a) points to the possibility scale, mentioning
that in 1950, the earthquake
197 1a). of oscillations
activity
also on a bigger
in Greece was at a minimum
while the earth as a whole then suffered more from earthquakes than in any year since 1906. Karnik (1972) finds an upward migration of the seismic activity in the Aegean area during the present century (cf. also Papazachos, 1977). while Galanopoulos (1965a) maintains that periods of greater seismic activity are initiated by intermediate
shocks, i.e., the Greek seismic activity
is induced
by processes
occurring
under the earth’s crust (however, denied by Purcaru and Berckhemer, 1982). Comninakis and Papazachos (1980) suggest a periodic pattern for intermediate earthquakes with quiet periods of about six decades followed by very active periods of about two decades. Purcaru and Berckhemer (1982) find a migration from west to east in the Hellenic arc at time intervals between 20 and 50 years. Trigger effects. In such a highly stressed and fractured area as Greece, it is understandable that various human and other impacts on the crust could release the strain, i.e., trigger earthquakes. There are several kinds of such impacts that could be of importance: (1) Mining operations. It is well known that mining operations lead to rockbursts. However, I have not found any account of Greek rockbursts in the literature, although these would be expected to be a safety risk factor of significance in the vicinity
of Greek mines.
(2) Dam constructions. There are several descriptions of such effects, mainly concerned with the Kremasta Lake but also some others. Based on a detailed investigation of the time, magnitude and spatial distribution of the shocks. Comninakis
et al. (1968) maintain
that the foreshocks
and the main shock (February
1966, M = 6.3) at the Kremasta dam were triggered natural events, the time distribution of dam-generated
5,
by the waterloading. As for foreshocks is suggested as a
means of predicting the time of occurrence of the main shock (Papazachos, 1973a). Contrary to natural sequences, there is an indication that the b-coefficient is higher for the dam-generated foreshocks than for the aftershocks (Gupta et al.. 1972; Papazachos, 1974a; Delibasis and Carydis, 1977). Additional information on damreleased activity is found in Galanopoulos (1967b). Galanopoulos and Ekonomides ( 1973), Drakopoulos (1974), and Therianos (1974). (3) Volcanic eruptions. In connection with trigger effects it is appropriate to point to a suspected relation between earthquakes and volcanic eruptions, All but one of the historically known eruptions of the Santorini volcano were either preceded or followed by tectonic earthquakes (Komlb et al., 1978; Hedervari, 1979). This phenomenon which has also been observed elsewhere, may be due to mutual triggering effects. And it is certainly different, both in origin and properties. from the typical earthquake swarms that accompany volcanic eruptions. We also note that the Atlantis myth, among various conflicting hypotheses, has been located to the Aegean Sea (Galanopoulos and Bacon, 1969; Milanovskiy, 1969).
179
(4) Cosmic moon
influences.
Especially
and the new moon,
the moon’s
have been suspected
Greece (Tamrazyan, 1970). Earthquake prediction. Earthquake Mediterranean
belt, compared
important
only
not
passage
over the meridian,
to act as triggers of earthquakes
prediction
has been
rather
neglected
to several other seismic areas. Prediction
for solving
just
the full
the prediction
problem
itself.
in
in the
research but
also
is for
revealing many of the otherwise unknown properties of earthquakes and earthquake sequences. The prediction problem has two main aspects. differing bo$h in methods and purpose: (1) Statistical estimation
prediction.
of maximum
This
magnitude,
concerns
the calculation
of seismic
etc. This type of prediction
risk,
is indispensable
the for
engineering purposes, e.g., the formulation of building codes. On the other hand, the time of occurrence of single events is not included in this prediction. (2) Deterministic prediction. This aims at predicting the time, location and magnitude of single earthquakes. The purpose is to make it possible to take precautions for a coming event, including the evacuation of people. Since 1960, much research is done with this goal in the U.S.A., Japan, China and the U.S.S.R. As far as Greece is concerned, the efforts to predict single forthcoming earthquakes have so far been rather modest, and they can be summarized apart from some isolated observations of animal behaviour (a) Until recently, the only method suggested is based foreshock sequences, enabling an estimate of the occurrence of a coming main shock. Cf. Papazachos et al. (1982). (b) Seismic gaps-in space and time-are indications of an increased likelihood of a forthcoming larger earthquake.
in the following points. (Papakis, 1962): on statistical studies of time and the magnitude accumulating strain and This technique, already
used in some other parts of the world, is recently applied also to Greece. Wyss and Baer (1981a, b) and Papazachos and Comninakis (1982) expect on this basis that .some larger earthquakes will occur during the 1980’s. according to Wyss and Baer (1981 b) both in the western re-evaluated prediction
magnitudes, and maintains
and the eastern
Ambraseys
Hellenic
(198 1) expresses
that the available
arc. However, criticism
data are insufficient
on the basis of
against
this mode of
to support
predict-
ions with an acceptable degree of certainty (cf. also Purcaru and Berckhemer, 1982). Karnik et al. (1982) study time gaps, among others for the Thessaloniki sequence in 1978. (c) Another more original approach is due to Papazachos (1980b, 198 1) and Papazachos and Comninakis (1982), who find a remarkable sinusoidal variation with time of M, - 44, (the magnitude difference between the main shock and its largest aftershock) with a period of 29 years. The exact physical reason for this sinusoidal time variation remains to be clarified, but larger earthquakes appear to occur around the minima of M, - M,. And it is noteworthy that this model leads to results agreeing in time with (b). A global test of the method is challenging. (d) A note
in some
news
media
in
1981 reports
that
Greek
physicists,
K.
Alexopoulos, earthquakes
P. Varotsos
and K. Nomikos
six to seven hours
crystal for field measurements earthquakes.
The method
detection and location 200 km is suggested.
in Athens,
in advance.
of electricity, appears
generated
to be both
and
stress before
promising
and
for the
areas
and
suggested
every have
to be used
for
EFFECTS
The macroseismic categories: (1) Effects
by the increasing
cheap
in predicting
they use a piezoelectrical
of oncoming events, a network of observation points Electrical conductivity variations and earth currents
already been observed elsewhere in seismic prediction (cf. Bath, 1979a, pp. 297-304). EARTHQUAKE
have succeeded
Apparently,
effects,
on nature:
directly
geological
observable relations,
in the field,
fault scarps,
soil deformation, landslides, avalanches. (2) Effects on human constructions: buildings,
bridges,
fall into
two main
topographical
changes.
power plants,
with partial
or total destruction,
and secondary effects, especially fires. It is natural that the earlier literature is largely dominated by descriptions of various macroseismic effects. This is true not only for the pre-instrumental period, when macroseismic data were the only available information, but also for the early instrumental period, when the information from seismograph recordings was scanty. But it would be incorrect to believe that the macroseismic observations have lost their significance in modern seismology. On the contrary, they may be considered as more important than ever, by providing means to establish relations between instrumental seismology on the one hand and geology, tectonics and engineering on the other hand. Such relations will then provide for more reliable evaluation of historical earthquakes, for which only macroseismic information is available. The present-day significance of macroseismic observations Macroseismic
is very clearly
observations.
testified
by the engineering
reports
of numerous
teams
examining
recent Greek earthquakes, e.g., the Thessaloniki event on June 20, 1978. The following publications contain macroseismic information: Galanopoulos ( 1950, 1952a, b, 1953b, 1954a, b, 1956b, 1959, 196 1a, b), Georgalas
and Galanopou-
10s (1953) Grandazzi (1954) Trikkalinos ( 1955), Anonymous ( 1956) Kiskyras (1956-58a, b), Papakis (1962) Kokinopoulos and Tasios (1966) Ambraseys (1967), Papageorgakis (1968) Rothe (1969) Papazachos (1975d), Bairaktaris and Roussopoulos (1976), R~us~opoulos (1976) Drakopoulos et al. (1978) Fintel (1978), Kalevras (1978) Kalevras and Constantopoulos (1978) Kalevras et al. ( 1978), Kulhanek and Meyer (1978), Papastamatiou (1978) Blume and Stauduhar (1979) Comninakis and Papazachos (1979b), Maley et al. ( 1979, 1980), Papazachos et al. (1979), Yerkes et al. (1979), Zavliaris et al. (1980). In addition, the Athens Monthly Seismological Bulletins provide extensive macroseismic data beside the instrumental evaluations. Cf. also Table II. Liquefaction, i.e., loss of shear strength in sands due
181
to earthquake
shaking,
is investigated
Roussopoulos (1980). Ii?te~~jt~-acceleration. degrees, nowadays number
a multitude
effects
are generally
in some 12-degree scale. intensity
of difficulties,
often influenced
Mactoseismic
with regard to Greece by Andrikopoulou
partly
because
by personal judgment,
of different
effects
the values
expressed
determinations
difficulties is that intensity lacks a unique, sity ratings prove to be immensely useful,
number.
in intensity
are subject
are not measured
partly because the estimates
by one single
and
to a
but estimated, aim at expressing
The main
reason
for the
physical definition. Nevertheless, intennot only for historical earthquakes, but
also nowadays in the instrumental era. Intensity estimates, based on damages to buildings, are subject to several factors which may lead to overestimates, rather than underestimates. Both the ground conditions and the structural properties of a building are important. For example. Galanopoulos (1952a) emphasizes that Larissa in 1941 showed exaggerated effects both due to soil conditions and to war effects (bombardments had weakened the structures). Also structures weakened by earlier earthquakes may lead to exaggerated intensities. In a study of two Peloponnesus earthquakes in 1965 and 1966. Ambraseys (1967) points out that exaggerated intensities are obtained by including secondary
effects, caused by landslides
The geographical
distribution
and slumping.
of intensity
for example.
is generally
presented
in isoseismal
maps, of which many have been produced for Greece (see, for example, Grandazzi, 1954; Delibasis and Drakopoulos, 1974; Shebalin et al., 1974; Papazachos, 1975d; Blume and Stauduhar, 1979). In general, the intensity distribution appears to be rather irregular, at least when a larger number of observations are available. In the case of Greece, some handicap is experienced by the vast extent of the seas with no observations, except on some islands. The intensity distribution depends on several factors, such as the focal mechanism, the focal depth, the structural trend of faults, the attenuation along different paths, the ground conditions at the receiving sites, etc. The properties of such distributions are studied by Trikkalinos (1955) Galanopoulos (1958, 1959), and Drakopoulos (1976b, 1978b). Beside intensity
maps for single events, more generalized
showing the distribution of the maximum estimated intensity area (Galanopoulos, 1965b; Galanopoulos and Delibasis, Gorshkov estimation
maps are of importance, over the whole Greek 1972, reproduced in
et al., 1974; Shebalin et al.. 1976). Such evaluations are also used for the of the seismic risk in terms of intensity, particle acceleration or particle
velocity, as a correspondence to the risk values in terms of magnitude. Expected seismic effects may be represented also on microzoning maps, based on all available information of importance (DrakopouIos, 1973, 1976b, 197%; Roussopoulos and Syrmakezis, 1973; Sherif, 1973; Galanopoulos, 1974b, 1977a; Drakopoulos et al., 1978). Due to the numerous factors involved, not always under complete control, it is not surprising that such maps do not always hold true. For example, Papastamatiou (1978) finds that the nlicrozonation study of Thessalonik~
182
(Drakopoulos,
1973) failed
to predict
all the observed
motion. In order
to improve
the intensity
ratings
estimations
to parallel
instrumental
data
(Galanopoulos, instrumental
1961a. b; Karnik, recordings
it is necessary, from
1965), partly
of strong motion
variations partly
the permanent to supplement
accelerographs.
of the strong to relate
station
the
network
the estimations
by
There is now a rather good
distribution of strong motion instruments around Greece. Therefore. strong motion records form the basis of several recent studies of Greek earthquakes. e.g., by Galanopoulos and Drakopoulos analysis of strong motion records
( 1974). Carydis (1976a). and others. The first from Greece dates from 1973. i.e., it is of a rather ROUSSO~OU~OS.1973). Carydis and Sbokos ( 1976)
recent data (Drakopoulos and analyze Greek strong motion records from 1972- 1975 (cf. Brady et al.. 1978). Temporary installations of strong motion instruments are nowadays a rule after damaging earthquakes, such as after the Thessaloniki event of June 20. 1978 (Papastamatiou, 1978; Petrovski et al., 1978; Blume and Stauduhar. 1979; Maley et al., 1979). Building code. A building code for aseismic construction is applied in Greece since 1959, at least to important structures in towns (Ambraseys, 1967). A brief description of the code is given by Blume and Stauduhar (1979) and Carydis (1976b) provides a discussion of it. In this code. Greece is divided into three zones, depending on the seismic risk. Seismic coefficients are specified according to seismic intensity ratings and soil classifications. The major cities, Athens and Thessaloniki, naturally attract most interest in this connection. Galanopoulos (1956b) points to a local earthquake source about 15-20 km northeast of Athens, responsible for many shocks in the last few years. Athens is far from being immune to earthquakes and the extension of the city on dilluvial and alluvial grounds indicates that at least its newer quarters cannot be considered earthquake-safe (Galanopoulos, 1963). Cf. also Galanopoulos (1977b). The Thessaloniki events of May-June 1978 are noteworthy, because this is the first time in the Greek history that a big city with tall buildings was affected by an earthquake
sequence
of a long duration
and relatively
large magnitudes
(Papazachos
et al., 1979). The collapse of an eight-story building of reinforced concrete is particularly remarkable. The probable reason is poor ground conditions and also the fact that the building had been designed before the adoption of a modern building code (Psycharis, 1978; Maley et al., 1979). The ground conditions are exceptional in Thessaloniki, with large variations from very soft to hard ground (Drakopoulos, 1973). Corinth has frequently been exposed to earthquake catastrophes. After the damaging earthquake of April 22, 1928, M = 6 l/4 (Drakopoulos et al., 1978) Corinth was rebuilt according to Japanese aseismic standards, obviously to a great advantage. The seismic risk in Athens, Thessaloniki, Patra, Corinth. Heraklion and Rodos, six industrial and highly populated centers, has been calculated by Makropoulos ( 1978).
1x3
Since 1612, the Leukas
Island has been affected
every 16 years, which places this area among Greece (Galanopoulos, Tsunumis. Seismic
by devastating
earthquakes
the most earthquake-exposed
once ones in
1952b). Cf. also Anonymous ( 1978) and Kalevras ( 1980). sea waves or tsunamis constitute another earthquake effect
which may cause great destruction
to harbours.
Submarine
slides, set off by dip-slip
components of earthquakes, are their most likely reason. Greek tsunamis are the most frequent ones in the Mediterranean area. but the large, damaging ones are rare. For example, among 41 tsunamis between 479 B.C. and 1956, only 16 were really damaging (Galanopoulos. 196Oa). Atnbraseys (1962) has accumulated
an impressive
list of eastern
Mediterranean
tsunamis,
including
141
events since 1410 B.C. up to the present time (also given by Antonopoulos, 1978), and Antonopoulos (1980) presents a detailed account of 136 tsunamis in this area from the birth of Christ up to 1980. Sousa Moreira (1974) gives a catalogue of tsunamigenic
European
earthquakes
with M > 5.3 in the period
1901-1971.
Tsunami
warnings would be useless in Greece because of the proximity of the sources, whereas double or triple rows of pine trees along affected coasts have been suggested as a kind of protection
(Galanopoulos,
1972a).
Further information on Greek tsunamis is contained in papers by Galanopoulos ( 1954b, 1957, 1967d), Ambraseys (1960, 1963, 1967) Papakis (1962), Galanopoulos et al. (1966) Karnik ( 1968, 1971) Marinos and Melidonis ( 1971) and Antonopoulos ( 1973). Deuth toils. The fact that the number of victims in Greek earthquakes is generally very low for the event magnitudes is an equally remarkable as fortunate circumstance, emphasized among others by Galanopoulos (1952b. 1953b) and Papastamatiou (1978). Compared to other countries in the same earthquake belt (Italy, Turkey, Iran) where death tolls are reckoned in thousands, the number of Greek victims
mostly
stays within
a few tens for earthquakes
of the same magnitude
(cf.
Bath, 1979b). In the period 1953-1969, about 600 people were killed by earthquakes in Greece (Galanopoulos, 1972a). to which the lonian Islands quakes in August 1953 contributed by 460 victims. In the iast century, there is only one earthquake (Chios, April 3, 1881) that had a greater number
of victims, stated as 2829 in Galanopoulos
( 1953b), but 4181 in Galanopoulos (195413). A few historical examples can also be given, e.g., Corinth in the year 77 with about 4000 deaths, while the same area had only 20 deaths in the damaging (Garagunis, 1967).
earthquake
The exact reason for the relatively speculate on the following factors:
of 1928, thanks
to a warning
low death figures is not quite clear, but we can
( 1) More favourable constructional style of houses (Papastamatiou, (2) Population distribution in relation to the earthquake damaging ering also the extent of waters in the Greek area. (3) Foreshocks, serving as a warning whether foreshocks are more common
foreshock
1978). zones, consid-
for people, who stay outdoors. It is not clear in Greece than in the other Mediterranean
184
countries.
But the fact that the Greek crust is very much intersected
faults is favourable
to minor
by numerous
releases before a main shock occurs.
CRUST AND UPPER MANTLE STRUCTURE
While the previous sections deal with the properties of Greek earthquakes, this and the next section are devoted to a study of the structure and tectonics of the Greek area. In other words, we now proceed to an investigation of the underlying causes-a necessary step to deepen our knowledge. Seismology provides most information in this respect, and as in mechanics, we divide our approach into statics (equivalent to structure: this section) and dynamics (equivalent to acting forces and movements: next section). The structural studies are based upon the interpretation of seismic waves, which yield information about wave velocities and layer thicknesses. As to the methods used, we distinguish between artificial sources (usually explosions-seismic waves). Artificial
profiling) and natural sources (earthquakes-body wave sources provide the highest accuracy, but
and surface the depth of
penetration of the waves is rather limited, mostly restricted to the crust (except for nuclear explosions). On the other hand, earthquakes imply a lower accuracy but generally deeper penetration of the investigated waves. Seismic profiling. The seismic profiling methods are conveniently divided into two branches: seismic reflection and seismic refraction, according to the measurement techniques used. (1) Reflection profiles. With regard to our purpose of trying to elucidate the Greek earthquake dynamics, the reflection experiments have only a limited information value, although applied to a great extent in recent years in the waters around Greece. This is obvious if we consider both the geographical location of investigated profiles and their depth of penetration. An exception to this rule is offered by a recent paper by Myrianthis (1982), who utilizes reflection profiles to clarify the dynamics of the February-March 1981 earthquakes in the Gulf of Corinth. Jongsma (1975), Jongsma et al. (1975, 1977), and Brooks and Ferentinos (1981) are concerned with areas inside Greece, namely the Aegean Sea. Other authors mostly deal with various parts of the eastern Mediterranean, outside or marginal to the proper Greek area, especially the Mediterranean ridge, the Levantine Sea, the Nile cone, the area south and east of Crete, the Ionian Sea or the southwest coastal region of Peloponnesus (Wong et al., 1971; Lort, 1972, 1973; Finetti and Morelli, 1973; Sancho et al., 1973; Hinz, 1974; Lort et al., 1974; Finetti, 1976; Woodside, 1977; Le Quellec et al., 1980; Mascle et al., 1982; Monopolis and Bruneton, 1982). The depth of penetration is rather shallow and at most, the basement rock has been reached. The measurements concentrate on the sedimentary layering on the sea bottom, and the results are discussed in very great detail with regard to geological structure, bathymetry and geological history. In these discussions, also other relevant
1x5
information
is included,
thermy. (2) Refraction more
interest,
tectonophysical related
deriving
profiles. partly
especially
In our present
because
their
belts
context,
deeper
aspects, partly because
to the earthquake
from gravity,
the refraction
penetration
the locations
in Greece.
geomagnetics experiments
has more
of the profiles
This
is particularly
and geoattract
bearing
on the
are more closely true
of the five
important refraction profiles that Makris and his associates investigated 197 1- 1974, covering PeloponnesusIonian Sea, the Cretan area and the Aegean
in Sea
(Makris,
1973. 1975, 1977, 1978b. c; Makris and Vees, 1977; Makris et al.. 1977). In
addition,
refraction
measurements
by Hinz ( 1974) and Weigel (1974a.
are reported
b). The seismic waves generally observed and evaluated are Pg, Pn and PmP. The velocity of Pg is about 6.0 km/set, representing the upper part of the crust, assumed to increase to 6.2-6.8 km/set towards the lower crust, while Pn exhibits a velocity of 7.667.8 km/set, i.e., rather low for the upper mantle. PmP is very clear, indicating a sharp Moho discontinuity, whereas the crust has a more gradual velocity increase with depth, i.e., it lacks a Conrad discontinuity. A most striking result is the marked variation of the crustal thickness, clearly indicating the complicated structure of the whole region. From Makris ( 1973, 1977, 1978b), we get the following representative crustal thicknesses: Ionian Sea 24428 km, Hellenides (central Peloponnesus) 46648 km, Greece-Bulgaria border to central Greece 44-48 km, northern Aegean Sea 30-32 km, southern Aegean Sea 20-28 km, Cretan
Sea 20 km, Crete-CarpathosRhossos
30-34
km, south of Crete 30-34
One problem concerns the true nature of the crust, whether continental (Comninakis and Papazachos, 1976). In most of the places investigated,
km.
or oceanic the crust is
considered to be continental (Lort et al., 1974) but in some of the oceanic areas, the thinner crust is rather termed sub-continental, whereas a typical oceanic crust is hardly
found
eastern
Mediterranean
sediments.
anywhere
in the whole crust
eastern
Mediterranean.
Alternatively, covered
the
is considered
to be oceanic,
but
Cf. Lort (1971, 1972). Woodside
( 1977) Morelli
( 1978). and Le Pichon
by thick
(1981). As general
comments
Greek and adjacent
on the seismic profiling
(reflection
areas, we can state the following.
and refraction)
Practically
in the
all the experiments
were made within the last 15 years, and all relevant papers date from the last decade. Therefore, this marks quite a recent development for this area. Moreover, we note that the expeditions are mainly performed on the initiative from abroad (Italy, Germany, England, U.S.A.), but mostly in cooperation with various Greek institutions. The profiling experiments have recently become intensified in connection with the oil prospection in the Aegean Sea. The geotectonic investigation of this area has thus become of great practical interest, in addition to its more theoretical aspects. Body waves from earthquakes. Considering the complex structure, evidenced by varying crustal thickness and by the probable existence of subduction zones, it is
186
expected
that seismic waves passing
these structures
exhibit
significant
time residuals
versus standard travel time tables. It is also expected that such residuals vary practically from path to path in a way that corresponds to the complicated structure. It is quite natural structure encounter it would
seem
that the interpretations of travel time residuals in terms of great difficulty and ambiguity. In order to reach reliable results,
necessary
to combine
a large
number
of crossing
paths.
These
expectations are fully corroborated by the research which has been devoted to travel time studies of the Greek area and which can conveniently be classified as follows: (1) Travel time residuals for Greek earthquakes recorded at Greek or other nearby stations. using Pn, Pg (and Sg). Positive residuals for Pn are ascribed to a low velocity of the upper mantle, while positive residuals for Pg are ascribed to an influence of the sedimentary layering (Ekonomides. 1972). Cf. also Taner ( 1962). Comninakis
(1967), Delibasis
and Galanopoulos
( 1967).
Average crustal structures derived from travel times of body waves (P. S) from Greek earthquakes recorded in southeast Europe suggest that crustal thicknesses vary regionally between 32 and 47 km, that the average Pn velocity is 7.87 km/set and the Pg velocity 6.1-6.8 km/set, and that no low-velocity layer exists in the crust (Papazachos et al., 1966). (2) Travel time residuals for Greek earthquakes recorded teleseismically. Negative residuals for P waves recorded at Greenland stations from earthquakes near Rhodes are explained as due to a lithospheric plate of a high P velocity sinking below the Aegean Sea (Gregersen, 1977). (3) Travel time residuals for teleseisms recorded at Greek stations. using P waves. Residuals indicate a high-velocity sinking slab and a low-velocity upper mantle in the Aegean Sea (Agarwal et al., 1976; Jacoby et al., 1978). See also Hovland and Husebye (1981). Sokerova (1974, 1977) finds that the station correction variations are mainly due to the crust and the sediments beneath the stations. The complicated structure influences but also the recorded wave amplitudes
not only the travel times. as just mentioned, (cf. Delibasis, 1981). The absorption and
scattering as well as the geometrical spreading will vary practically from path to path. Therefore, as for the travel times, a similar detailed study is needed in order to determine earthquake magnitudes in a reliable way. The magnitude problem may be illustrated
by the fact that Athens
magnitude calibrations since around and Maamoun and Allam (1981).
Observatory has applied at least four different 1950. See also Papazachos and Vasilikou ( 1966)
Surfuce waves from earthquakes. Phase and group velocity dispersion of Love and Rayleigh surface waves, both fundamental and higher modes, provides important information on the average crustal and upper mantle structure along the total path or in more detail if several determinations are combined in a “crossing-path technique”.
An efficient propagation of Li, Lg and Rg across the Aegean Sea proves its continental structure with the following velocities: Li 3.80 km/set, Lgl 3.57 km/set,
Lg2 3.32 km/set
and Kg 3.03 km/set
(Rizhikova,
on higher mode surface waves but without As to the true nature or oceanic,
of the eastern
the information
sial as the information
dispersion
Mediterranean
information. crust, i.e., whether
from surface wave data appears from seismic
1969) suggests the eastern sediments, structure.
1966a, b). This is the only report
refraction
Mediterranean
data,
reported
to be of a continental
in contrast to the western Mediterranean, Similarly, Cloetingh et al. (1980) consider
continental
to be equally above.
controverPayo (1967.
structure
with thick
which is of a more oceanic the crustal structure of the
eastern Mediterranean to be of a continental margin type with a thickness of 35-40 km. For comparison with crustal thickness. earlier derived from body wave data. Papazachos et al. (1967b) and Papazachos (1969) investigate the dispersion of fundamental mode surface waves. On this basis, Papazachos (1969) maintains that the eastern Mediterranean crust has an oceanic character with a mean thickness of 20 km. An upper mantle Pay0 ( 1969). Further
literature
low-velocity
layer is advocated
by Papazachos
on surface waves in the Greek area is provided
et al. (1967c), Payo (1976). and Farrugia
(1969) and
by Papazachos
and Panza (1981).
SHSMOTECTONICS
The literature on the tectonics of the Greek area is extremely comprehensive. even if we restrict ourselves to seismotectonics, i.e., tectonophysical results based on seismological measurements, leaving aside the still more comprehensive geologicaltectonical literature. There is hardly any literature that is so controversial as the seismotectonical production concerned with the Greek area, and the need for further research is frequently emphasized. Part of the divergences is due to the fact that quite different seismological observations are used as the starting point, including not only hypocentral locations and fault plane solutions, but also reflection and refraction results, body and surface wave measurements, discussions geological
frequently history.
as well as field observations.
involve other geophysical
No doubt,
it would
and geological
be a challenging
Moreover.
observations
but difficult
the
and the project
to
explore all details combined. In the following I will try to give some glimpses of the more significant results. There are essentially two important aspects in which seismology can contribute to the knowledge about the present tectonics: (I) By determining accurate hypocentral locations. (2) By providing fault plane solutions. Fault p/me s~I~t~~ns. Fault plane solutions or focal mechanism solutions yield information on the present-day tectonic motions. Based on seismograph records, such solutions define the orientation of the fault plane and the direction of slip along the fault plane. In other words, the type of fault motion will be defined (strike slip, dip slip, etc.; cf. Bath, 1979a. p. 173). In addition, these solutions determine the
188
directions
of the stress axes, i.e., the compressional
results will then have to be combined covering
the whole area of interest,
both between
and tensional
and with all other
relevant
cially that which can be derived from geological investigations. It is noteworthy that practically all the Greek fault plane exclusively
on the first motion
(compression
or dilatation)
Such
directions.
a large number
of earthquakes,
information, solutions
espe-
are based
of the P wave, exception-
ally also of the PKP wave. On the one hand, this information is most easily available and usually the most reliable one; on the other hand. it constitutes only a small fraction of the data contained in a seismic record. More readings could give significant contributions Thus, Hodgson and Cock PPP and PcP, obviously and/or polarization) are 10s and Delibasis (1974)
to the solutions and (1957) use first motion to a great advantage. used by Delibasis and in addition to P waves.
P, is also reported by Shirokova (1967). A striking feature is that sometimes divergent
solutions,
as investigated
the
by different
restrict ambiguities to a minimum. not only of P, but also of PP, PKP, S waves (direction of first motion Drakopoulos (1974) and DrakopouSome use of S waves. in addition to same
earthquakes
authors.
are
given
quite
This fact is emphasized
by
Gertig (1972a) and Drakopoulos and Delibasis (1974). One example is offered by Di Filippo and Marcelli (1954) compared to Hodgson and Cock (1957). Such divergences may depend on a number of factors, such as on the data used (long period vs short period recordings), on the analysis methods and to some extent also on personal judgment. It is obvious that the divergent results deserve to be critically reexamined, using more information whenever available. Additional earthquake mechanism solutions for the Greek area are presented in the following papers: Papazachos ( 196 1, 1974f. 1975d,e, 1976a, 1977). Constantinescu et al. (1966) Petrescu et al. (1967), Delibasis ( 1968). Karnik ( 1968, 197 1), Papazachos and Delibasis (1969), Radu (1969), Delibasis and Drakopoulos (1972) Gertig (1972b), McKenzie (1972, 1978) Ritsema (1974, 1975a), Leventakis (1975), Carver
and
Henrisey
(1978)
Le Pichon
and
Angelier
(1979),
Papazachos
et al.
(1979), D’Ingeo et al. (1980) Carver and Bollinger ( 1981) Maamoun and Allam (1981) Soufleris and Stewart (198 1), Soufleris et al. (1982). As a contrast to the numerous studies of P (compression, dilatation), modern spectral methods (cf. Bath, 1974) and waveform analysis, which permit an estimation of source dimension, seismic moment, average displacement and stress drop, have been applied in relatively few cases. Although some of the numerical results so far appear to be rather vague or unreliable, there is no doubt that the modern methods deserve to be examined and applied much more than hitherto. It is particularly to be noted that considerably more information can be extracted from the seismic records by the modern methods than by the traditional ones. Relevant papers are given by North (1977), Kulhanek and Meyer (1979), Barker and Langston (1981, 1982) Soufleris and Stewart (1981), Langston (1982), Soufleris et al. (1982). Large scale tectonics. The tectonic structure of the eastern Mediterranean ranges
189
among
the most complicated
to fully understand Ocean
before
the huge but generally
embarking
that the tectonics
ones in the world, and it would be a good advice to try
on Greece.
of the eastern
understood
by simple
considered,
such as subsidence
Morelli
simpler
around
the Pacific
et al. (1975) and Morelli
(1978) state
area is too complicated
to be fully
Mediterranean
seismotectonic
models.
of crustal
blocks,
Other
structures
contributing
flow of material
agents
must
in the mantle
be and
bending of certain tectonic elements. Bornovas et al. (1974) emphasize that the geophysical data alone are insufficient and that geological and neotectonic data must be included in an interpretation. Cf. also Berckhemer and Hsii (1982). The Balkan region is like a “crushed zone” pressed between two major plates, the African plate, moving northwards, and the Eurasian plate. Several authors talk about “buffer plates” between the two major plates. Cf. McKenzie (1972) Roman (1973) and Karnik (1975). From simple rock dynamics. one would expect that the rocks pressed between two blocks moving northhsouth would be fractured in NW-SE and NE-SW directions. This is also what has apparently happened. These fracture directions produce a mosaic pattern, dominating a large part of the Balkan peninsula. But the plate motion is more complicated than this, and we have not only a compressed and crushed zone, but obviously also subduction of the African plate. This subduction appears in places where the African plate experiences resistance and the earthquake belt is concave towards the north, i.e., north of Crete as well as in the Calabrian arc in southern Italy, possibly also in southern Spain. In other places, where the African plate moves more freely northwards and creates a belt concave towards the south, e.g., in the Adriatic Sea, there is no subduction. But all this is controversial
and problematic.
As to the large scale tectonics of the eastern general agreement about the dominant influence
Mediterranean of the African
area, there is fairly and Eurasian plates.
approaching each other. But even in this respect, there are some divergent opinions. Ritsema (1975a, b) emphasizes that throughout the Mediterranean area, at shallow as well as at deeper levels, the prevalent
direction
of tectonic
transport
is in the E-W
or W-E directions, i.e.. in clear contrast to the generally assumed N-S collision of the African and Eurasian plates. As a consequence, the northward movement of the African plate would only play a subordinate role for the Mediterranean tectonics. A similar idea is expressed by Makris (1973), referring the compression in the Hellenides to the opening of the Atlantic, while Makris (1977) refers to the relative movement between Europe and Africa as the most probable initiator of the tectonic system. Including quantitative estimates, Le Pichon and Angelier (1979) explain the Hellenic trench and arc as a result of rotation around a pole in the southern Adriatic Sea, while McKenzie (1970) mentions a rotational movement of the African plate around a center in the Atlantic. Several of these suggestions are interesting in view of a possible eastward migration of the seismic activity in the entire Mediterranean belt (Bath,
1979a, p. 306). Fig. 4 gives a recent example.
Though
this migration
pattern
190
12 -
se
_
1000
6 _
60
/
a7
km
EASTWARD MIGRATION
/
04 /
4-
34 km/day
7” l2 0
0
I
3 100 days
o-
l 1’
I 0
I
Fig. 4. Suggested
I
I
2
eastward
I
I
I
4
migration
6
of seismic activity
in the Mediterranean
belt in 1980- 1981 (number
I is the origin of the coordinate system with distances measured along the belt): I. AroresIslands,Jun1,1980,M=7.0 5. Pakistan.Mu~2, 1981.M=6.7 6. Irun.JuneII. 1981.M=6.9 z AIgeria, Oct. 10. 1980, M = 7.5 3.
Iia!y. Noo. 23, 1980. M = 7.1
7.
Zrun,Ju!r~28.
4.
Greece, Feb. 24. 1981. M = 6.7
8.
Pakrstun.
198l.M=7.3
Sep. 12. 1981. M= 5.8
has appeared in clear form only a few times and therefore must still be considered hypothetical, it is important enough to be seriously taken into acc:‘ymt in any effort to explain the large scale tectonics of the whole belt. Moreover, it suggests that earthquake prediction should be extended over the entire Mediterranean belt, including interrelations between all parts of the belt, and not be restricted to Greece (Bath, 1979b). The Mediterranean resembles superficially it is agreed
ridge with its sinusoidal shape in the eastern Mediterranean an oceanic ridge, like the Atlantic rift, but quite unanimously
that the Mediterranean
ridge is of a different
nature
and origin.
This
conclusion is based on a variety of observations. such as the absence of a central rift. horizontal compression and not tension as in a rift zone, an unsymmetrical epicenter location around the ridge, existence of earthquakes deeper than normal, magnetic field properties, etc. (Galanopoulos, 1968a; Wong et al., 1971). The Mediterranean ridge consists of thick sedimens which have been uplifted due to compressive forces between the African and Eurasian plates (Lort, 1972; Finetti and Morelli. 1973: Finetti, 1976; Woodside, 1977) or due to vertical faulting (Sancho et al., 1973; Hinz, 1974). In summary, east-west, north-south of dominant tectonic importance-a whole region.
and vertical directions are all claimed to be good illustration of the complexity of this
191
Greek
there
is
tectonics.
fairly
controversial. arrangement
Passing
good
now to the more specific
agreement
Seismicity
about
some
Greek
results.
maps of the Greek area usually
of earthquakes
The areas of compressional
around
the Aegean
and tensional
tectonics.
while
others
we note that remain
exhibit a practically
Sea with its markedly
stress, evidenced
quite circular
lower activity.
by fault plane solutions,
are quite well defined (Mercier et al., 1972. 1976: Ritsema. 1974). and corroborated by direct stress measurements in shallow boreholes (Paquin et al.. 1982). Compression prevails perpendicularly to the seismic belts in the western and southern Greece (Constantinescu et al., 1966: Petrescu et al.. 1967; Comninakis and Papazachos. 1980). however. contradicted by Hinz ( 1974), 1972; Gertig, 1972a; Le Quellec et al., while the Aegean Sea is an extensional area (Jongsma et al., 1975, 1977: McKenzie, 1978; Le Pichon, 1981). According to Makris (1975. 1976, 1977), the Aegean Sea is the site of ascending and expanding hot mantle material-a “hot plume”-while McKenzie (1970, 1972) talks in terms of an Aegean microplate moving towards SW, just as the minor Turkish plate moves west (Fig. 5). The dominating
tectonic configuration
on the Greek mainland
is oriented
NW -SE
(or NNW--SSE) vs NE-SW (or ENE- WSW). the two intersecting (conjugate) systems apparently both seismically active (Hodgson and Cock. 1957: Kiskyras, 1959; Galanopoulos, 1967a; Delibasis and Drakopoulos. 1974; Kgrnik, 1975; Drakopoulos, 1976b; Kronberg and Giinther. 1978). A controversial problem concerns the westward extension of the north Anatolian fault (C’omninakis and Papazachos,
1979a). While some seismologists
hold the idea that this fault continues
up to
Fig. 5. Plate boundaries and motions in the Aegean area. Double lines = extensional plate boundaries; single heavy lines = transform faults; solid line crossed by short lines at right angles = boundaries where shortening
is occurring;
Redrafted
after McKenzie
A = Aegean (1972).
plate;
T= Turkish
plate;
H = Hellenic
arc;
c =
Cyprean
arc.
192
the Ionian Kg&k,
Islands,
as evidenced
in the northeast
Greece,
POU~OS,1974; McKenzie, 198 1; Makropoulos There are great Hellenic
by epicenter
1968, 1971, 1972; Maley and Johnson, breaking
and
198 1). concerning
arc. While many researchers
a Benioff zone, sometimes said to Comninakis, 1969, 1971; Caputo 1972; Papazachos, 1973b. 1977; Strobach, 1978; Comninakis and there are others who do not find regular
(Galanopoulos,
Sengor, the nature
maintain
1967a. C;
1971) others mean that it terminates
up into several branches
1978; Dewey
and Burton, controversies
distributions
(Delibasis
1979; Carver
and Drakoand
of the Aegean,
that the hypocenters
Bollinger, Cretan
or
are arranged
in
be of an amphitheatrical shape (Papazachos and et al., 1970; Maley and Johnson, 1971; Karnik. Beuzart. 1975; Gregersen. 1977; Richter and Papazachos. 1980; Maamoun and Allam. 198 1) or who deny that the hypocenters define such a
structure
(Lort, 1971, 1972; Galanopoulos. 1953a, 1973b. 1975; Jongsma. 1977; Woodside, 1977; Makropoulos, 1978: McKenzie. 1978; 1975; Hinz et al., Makropoulos and Burton, 1981). In the northern Aegean Sea. there are also shocks somewhat deeper than normal, interpreted by some as defining a Benioff zone (Papazachos,
1976a, b, 1977) while others question
or deny such a zone (Maley and
Johnson, 1971; Makris, 1977; McKenzie, 1978). The conflicting opinions about Benioff subduction zones in the Aegean area are in part due to inaccuracies of hypocenter locations. As soon as more accurate determinations have become possible, a more well-defined Benioff zone seems to result, at least in the investigated areas (Leydecker. 1975; Leydecker et al.. 1975). On the other hand, it is quite clear that there are scattered, intermediate-depth earthquakes south of the Aegean arc, i.e., on its convex side. These have been used as the main argument against the Benioff zone hypothesis. However, advocates of the Benioff zone offer explanations even for these apparently irregular occurrences of shocks.
According
to Papazachos
and
Papadopoulos
(1977) and Papazachos
Comninakis (1978b), the northern Aegean arc is presently dying out, while southern is active, and the intermediate-depth shocks south of the Aegean represent
the initial
stages of an oncoming
Benioff zone structure
and the arc
(cf. Galanopoulos,
1975). This means a gradual shift of this structure from north geological epochs. So far, this explanation is naturally hypothetical.
to south
over
An interesting solution is presented by Papazachos and Papadopoulos (1977). See Fig. 6; cf. also Makris (1975, 1976) and Papazachos (1977). In its essence, this involves a circulation cell with descending motion along the subducting slab in the Aegean arc, coupled with ascending motion further north, at the volcanic arc. This idea appears to be very promising, especially as it seems to combine several phenomena into a unified picture. Therefore, this suggestion deserves to be further explored by all available means. An alternative explanation of the seemingly irregular intermediate-depth shocks south of the Aegean arc is offered by Richter and Strobach (1978). They refer to the fact that the sharp curvature of the arc leads to tension along its direction. This
193
-..,
I Fig. 6. A lithospheric Aegean
area.
compression:
tension
model (vertical
M = Mediterranean t = tension.
is strong
section)
Sea;
Redrafted
enough
J interpreting
S = southern
after Papazachos
geophysical
Aegean
area;
and magmatic N = northern
and Papadopoulos
properties Aegean
of the
area;
c=
(1977).
to break up the arc into several pieces, responsible
for the
observed scatter of the hypocenters. For a better understanding of the Greek tectonics, it would be helpful if some other areas on the earth could be found with a similar tectonic structure. Areas of a lower seismic activity, like the Aegean Sea, surrounded on all sides by very active seismic belts, are found for example in the Caribbean and the Philippine areas. A detailed examination of similarities and dissimilarities of the tectonics among these and possibly
some other areas (marginal
seas around
the Pacific) would no doubt be
very informative. In addition to the papers quoted above in this section, also the following contain extensive information on the seismotectonics of the Greek area: Galanopoulos (1965a, 1973a, 1974a, 1979a), Papazachos and Delibasis ( 1969) Gertig (1972b), Hedervari (1973), Papazachos (1973c, 1974b, f, 1975e), Drakopoulos and Delibasis (1974), Weigel (1974b, 1978) BonEev (1975) Payo (1975, 1976), Agarwal et al. (1976) Karnik and Radu (1976), Papazachos and Comninakis (1976, 1978a), Dtirbaum
Leydecker
et al. (1978)
Makris (1978a), Mercier et al. (1979), D’Ingeo et al. (1980), Hovland ( 198 l), Sore1 et al. (198 l), King et al. (1982), Le Pichon et al. (1982).
et al. (1977),
Makris
et al. (1977),
Petrov
(1977),
and Husebye
CONCLUSION
If the reader of this review has got the impression that many results are controversial or inconclusive, then this is a correct impression. While there is a fairly unanimous agreement about fundamental results, such as the trends of the seismic belts, the controversies increase steadily as we proceed to more complicated problems in order to culminate in the interpretation of the tectonics of the whole area. Greece
occupies
a crucial
point
in the Mediterranean
earthquake
belt and
its
194
wheel-shaped seismicity pattern acts as a “rolling hoop” structures to the west and east, combined with subduction Although practical
the present
reasons,
the research
review is limited
it is necessary
in all directions,
(1) All other branches
solution
the more linear
zones.
to the seismology
for a complete
especially
between
of Greece,
mainly
of the problems
for
to extend
as follows:
of geophysics.
such as gravity,
geomagnetics.
geothermy,
vulcanology. moreover geodesy and geology should be involved in addition seismology. The problems are too complex for seismology alone to solve them. (2) The whole
Mediterranean
belt should
be involved.
as forming
to
a unity,
of
which Greece is only a part, let it be of crucial significance. (3) International groups from advanced institutes all over the world are needed for the solution of the problems, in cooperation with each other and with Greek institutions. ACKNOWLEDGEMENTS
I am grateful to the Ministry of Coordination, Athens. for the inspiration to make this report, and to the Scientific Research and Technology Agency, Athens, for the interest and concern shown for this work. I also like to express my gratitude and appreciation to libraries, institutes and numerous individual authors who provided me with books, reports and reprints of papers, indispensable for the preparation of this review of the Greek seismology. Professor Seweryn J. Duda provided a useful review of the manuscript.
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165-208
Elsevier Science Publishers
B.V., Amsterdam
- Printed
in The Netherlands
Review THE SEISMOLOGY
MARKUS
OF GREECE
BATH
Seismological
(Received
Section,
December
Box 12019,
S-750
28, 1982; accepted
I2 Uppsulu (Sweden)
April 21. 1983)
ABSTRACT
BLth, M.. 1983. The seismology
of Greece.
A review and a bibliography period
from 1950 up to the present
instrumental structural
installations, properties
has an interest geophysical
passing
expeditions
98:
165-208
are given of the seismological
literature
time. The aim is to incorporate on to earthquake
and seismotectonics.
in this tectonically
Tectonophysics.
complex
catalogues.
The purpose
for the area of Greece
all aspects of seismology, statistics
is to provide
area and particularly
m the
starting
and effects, and ending
with
up with
a service to every geophysicist
to the numerous
researchers
who
engaged
in
to Greece.
INTRODUCTION
Greece has the highest seismic activity in Europe with about 2% of the whole world’s seismic energy release. This is accomplished within an area amounting to only 0.09% of the total area of the world, truly a “ vast seismological laboratory”, as Galanopoulos
(1972a) formulates
it. See Fig. 1.
In 1962, Greece was visited by one of UNESCO’s seismological survey missions with the purpose to explore the state of seismological studies and research, including engineering
aspects
(Bath
et al., 1962). Beyond
this, the early exploration
of the
seismological conditions in Greece was essentially left to the Greek seismologists. However, in the last decade a notable change has taken place, and the Greek seismicity now stands in the center of interests of many of the world’s seismologists. Two recent earthquakes have especially directed the attention to the Greek seismicity: partly an earthquake in the Thessaloniki area on June 20, 1978 (surfacewave magnitude M = 6.5; 50 deaths), partly an earthquake in the Corinth-Athens area on February 24, 1981 (M = 6.7; about 25 deaths). Even though the number of deaths were comparatively modest, the material damage was considerable in both events. Beside this, there are other aspects in which these two earthquakes exhibit similarities, e.g., the obvious panic among the population, the very pronounced and efficient sense of responsibility among the authorities, and finally a very great O&ICI-1951/83/$13.20
0 1983 Elsevier Science Publishers
B.V
”
0 0
0
0
0 0
MEDITERRANEAN
EPSTEIN
Fig. I. The seismically
0 0
active trends
of Greece
0
in 1902- 1976. Open clrclea correspond
h i 90 km and filled circles to h > 90 km. After Galanopoulos
international interest among seismologists research teams from abroad for examination
to focal depths
(1977a).
and engineers, leading to numerous of the effects both in the field and on
buildings (Papazachos and Carydis. 1982). The present review has been prepared as a service to this extensive international interest in the Greek area. In particular, our review is defined as follows with regard to subject, time and space: (1) Subject. The listed publications are restricted to seismological research, including seismotectonics and earthquake engineering. On the other hand, the very rich geological literature on Greece, often with important bearings on the tectonics of the area, will not be included, unless seismology enters as a part.
167
30 -
N/year BIBLIOGRAPHICAL
u 0
’
I
1950 Fig. 2. Annual
60 number
year
I
0
I
60
70 of items (N/year)
in our bibliography
versus time.
and no papers before 1950 are included. The earlier seismological literature deals mostly with macroseismic effects of earthquakes (a list with 54 items of this early literature can be found in (2) Time. The period
covered
is from 1950 onwards,
Trikkalinos, 1955). (3) Space. The space is restricted to the Greek area, and papers dealing with the Mediterranean region, with no particular reference to Greece, are in general omitted. The bibliography (see References) includes 329 items, to which Greek authors have contributed in whole or in part by no less than 58%. About 25% of all listed items have appeared in Greece, the remainder in various international journals. mostly European. Perhaps number of listed references the overall activity
most striking, however, is a graph showing the annual (Fig. 2). This figure demonstrates a relatively low level of
in the period
1950-1965,
This time variation is in fact quite concerned with the Greek area. INSTRUMENTAL
typical
after which a very active period sets in. of all kinds
of seismological
activities
RECORDINGS
Permanent installations. As instrumental records are crucial for the development of modern, quantitative seismological research, I will give a brief historical review of the permanent seismograph installations in Greece, collected from scanty, scattered information in the literature (Z, N, E refer to vertical, north-south and east-west seismograph components, respectively): 1898: First seismograph
in Greece,
type Agamennone,
in Athens.
168
1911: Mainka
N, E, Seismological
recording major earthquakes Aegean region.
Laboratory
in Greece,
seismology in Greece. 1927: Wiechert Z, N, E, Seismological Mainka
N, E transferred
University.
this is the first reliable
The year 1911 is generally
1954-19.56:
of Athens
quoted Institute,
seismograph
as the beginning National
from Athens
Intended
in the
of instrumental
Observatory
University
for
of Athens.
to Athens Observa-
tory. Kritikos N, Athens Observatory, installed for the recording of major local shocks. Athens Observatory becomes the headquarters of instrumental seismology in Greece. 19.57: Benioff Z, Athens seismograph in Greece. 1962: WWSSN (World
Observatory,
being the first modern
Wide Seismograph
short-period), Athens Observatory. 1963: Wiechert Z, Patras, this becoming by Riolos. 1964: Hiller (Stuttgart) 1965: Wood-Anderson
Station
Network)
the first auxiliary
high-magnification Z, N. E (long- and
station,
later replaced
Z, N, E, Athens Observatory. N, E, Athens Observatory.
1965-1966: Sprengnether Z, N, E at four new stations on four islands. This marks the initiation of the modern Greek seismograph network. 1966: One strong motion seismograph at Argostoli (Cephalonia). Seventeen seismoscopes installed as a network in western Greece. The engineering aspects (strong motion, acceleration) begin to receive a greater attention than hitherto. 1968: Sprengnether Z, N, E at two new stations on the mainland. 1971: A network of strong motion seismographs begins to be installed in Greece. 1971-1981: The network of seismograph stations is further developed and modified and consists presently of 13 stations (Fig. 3 and Table I). In addition, there are now nearly 20 strong motion seismographs well distributed over the Greek area and about equally many seismoscopes, the latter concentrated to the Ionian Islands and adjacent
parts of Peloponnesus
A remarkable
fact, evident
and the mainland.
from this historical
being a highly seismic area, got a modern Moreover,
the station
density
network
is lower than
review, is that Greece, very late, starting
in some neighbouring
in spite of
not until
1965.
countries,
e.g.,
Italy with its 54 stations. Nevertheless, we note with satisfaction that the present network covers the Greek area quite well, and permits an adequate account of earthquakes within this area. This is evidenced especially by the Athens Monthly Seismological Bulletins, which contain detailed information both on record readings and on calculated source parameters. Cf. also Giannakopoulos (1972). Combination with a relatively dense network in western Turkey could enhance the outputs still further. Temporary installations. Although the present network can be considered adequate for a supervision of the whole area concerned, it may be insufficient for more special studies (Psycharis, 1978). Therefore, beside the more permanent installations,
169
I
3OOkm
I
Fig. 3. The present
seismograph
station
network
in Greece.
The numbers
refer to Table I
temporary or special-purpose seismograph installations have been used sporadically. mostly in the last decade and often by research workers from abroad: (1) With the purpose reports recordings near
to select sites for permanent stations, Steinwachs (1971) Kremasta dam in 1966 with a mobile three-component
station. With a similar purpose, Crampin (1975) performed recordings at several localities in Greece for a few days at each place. Measurements of the seismic noise were then of dominant
interest.
(2) As to the Kremasta Corporation, the installation
together
dam,
Therianos
with the Geodynamic
of four seismograph
stations
( 1974) reports institute
that
of Athens,
the Public is proceeding
at selected places around
Power with
the complex
of the Acheloos River at existing and future hydroelectric projects, in order to secure reliable records of any seismic activity in that area. (3) Earthquake sequences-swarms as well as regular aftershocks-have been studied by temporary installations. Drakopoulos and Delibasis (1973) operated a mobile station for recording an earthquake swarm on the Melos Island in August 1971. Aftershocks near the Trichonis Lake in central Greece in June-December 1975 were recorded by two portable seismographs and a seismoscope (Delibasis and Carydis, 1977). The Thessaloniki earthquake of June 20, 1978, is obviously the first one in Greece, for which major efforts were made at recording the aftershocks. The Institute of Earthquake Engineering in Skopje operated three strong motion accelerographs in the area (Papastamatiou, 1978; Petrovski et al., 1978; Maley et al., 1979). The U.S. Geological Survey installed a local array of ten stations, operating
170
TABLE
1
The present
seismograph
station
(After Nat]. Obs. Athens,
Location
Station
1.
Athens ATH
1911
Limestone
2.
3.
5.
6.
9.
10.
Instrument
Mass
r,
T,
(kg)
(set)
(see)
c
37”58’2O”N
Benioff
Z
107.5
1.0
0.74
12500
23”43’00”E
Benioff
107.5
1.0
0.77
12500
95 m above
Wood-Anderson
N, E N, E
m.s.1.
Sprengnether
2
II.2
14.9
99
1500
Sprengnether
N, E
10.7
14.8
99
1500
Wiechert
Z
I300
1.5
150
Wiechert
N
5.0
170
Wiechert
E
IO00 IO00
5.0
185
Mainka
N
135
2.8
50
Mainka
E
135
3.5
60
Kritikos
N
40
2.5
4
0.8
2800
37”04’08”N
Sprengnether
Z
1.14
0.5
0.5
70000
25’31’50”E
Sprengnether
N
1.14
0.5
0.5
40000
Limestone
620 m
Sprengnether
E
1.14
0.5
0.5
20000
Archangelos
36” 12’58”N
Sprengnether
Z
1.14
0.5
0.5
30000
ARG
28’07’34”E
Sprengnether
N
1.14
0.5
0.5
45000
170 m
Sprengnether
E
1.14
0.5
0.5
10000
1966
Ithomi
37’10’47”N
SVK-M3
Z
5.0
1.0
0.5
25000
ITM 1974
2 l”55’36”E
SGK-M3
N
5.0
1.1
0.5
25000
Flysch
400 m
SGK-M3
E
5.0
1.1
0.5
15000
Jannina
39’39’24”N
Sprengnether
Z
1.14
0.5
0.5
40000
JAN 1968
20’5 1’03”E
Sprengnether
N
1.14
0.5
0.5
20000
Limestone
540 m
Sprengnether
E
1.14
0.5
0.5
5000
Kozani
40” 18’24”N
Sprengnether
Z
1.14
0.5
0.5
65000
21’46’15”E
Sprengnether
N
1.14
0.5
0.5
25000
900m
Sprengnether
E
1.14
0.5
0.5
20000
1972
Limestone
8.
Bull. Nov. 1981)
Apeiranthos
KZN
7.
in Greece
APE 1975
Marl, sand 4.
network
Seismol. Inst., Monthly
Neapolis
35O15’45”N
SVK-M3
Z
5.0
1.0
0.4
50000
NPS 1973
25’36’45”E
SGK-M3
N
5.0
1.0
0.4
50000
Limestone
370 m
SGK-M3
E
5.0
1.0
0.4
25000
Paraskevi
39” 14’46”N
Sprengnether
Z
1.14
0.5
0.5
40000
PRK 1965
26O16’18”E
Sprengnether
N
1.14
0.5
0.5
20000
Rhyolite
100 m
Sprengnether
E
1.14
0.5
0.5
10000
Penteli
38”02’56”N
Benioff
Z
1.0
23”51’53”E
Press-Ewing
Z
6.9
10.0
(0.7) 85
12000
PTL 1971 Limestone
500m
Polygyros
40°22’26”N
Sprengnether
Z
I,14
0.5
0.5
70000
PLG 1968
23’26’44”E
Sprengnether
N
1.14
0.5
0.5
20000
Gneiss
580 m
Sprengnether
E
1.14
0.5
0.5
10000
100
2500
171
TABLE
I (continued) Location
Station
1 I.
12.
13.
Instrument
Riolos
38”03’28”N
RLS 1976
21”28’0O”E
Sandstone
100 m
Mass
7;)
(kg)
(set)
Willmore
2
5.0
I.0
TK (see)
V
25000
Valsamata
38O 10’3O”N
Sprengnether
Z
1.24
0.5
0.5
45000
VLS 1965
20”35’23”E
Sprengncther
N
1.14
0.5
0.5
25000
Limestone
375 m
Sprengnether
E
1.14
0.5
0.5
10000
Wood-Anderson
N, E
2050
0.8
Vamos
35’24’25”N
Sprengnether
Z
1.14
0.5
0.5
70000
VAM 1965
24’11’59”E
Sprengnether
N
1.14
0.5
0.5
I5000
Limestone
225 m
Sprengnether
E
1.14
0.5
0.5
5000
The station activity,
name is followed
and its geological
T,, = seismometer
period;
by its internationally
foundation.
SVK-M3
T, = galvanometer
used three-letter and SGK-M3
period;
abbreviation,
are seismographs
V = maximum
dynamic
the year of its start of of Soviet construction.
magnification.
for nearly three weeks in July, 1978 (Maley et al., 1980; Carver and Bollinger, 1981) and a team from the University of Cambridge, England, operated a variable number of portable stations during the period July 20 (4) Some efforts have also been made to use the regular, continuous activity, i.e., not just accuracy of hypocentral locations, Leydecker 1978) operated a tripartite station with 60 km EARTHQUAKE
to August 31 (Soufleris et al., 1982). temporary installations for recording aftershocks. In order to increase the (1975) and Leydecker et al. ( 1975, side on Peloponnesus in 1972- 1974.
CATALQGUES
Earthquake catalogues, homogeneous and complete, are a prerequisite for many investigations in seismology. They form the basis for the production of seismicity maps as well as for all kinds of statistical evaluations, including estimation of seismic risk for engineering purposes. Likewise, accurate hypocenter data are needed in the evaluation of the seismotectonics of any region. The complicated tectonics of the Greek area calls for exceptionally accurate determinations of earthquake foci. However, the production of earthquake catalogues fulfilling all such requirements has to face practically insurmountable difficulties. The instrumental recording period is limited to the present century or less, and for earlier centuries we have to rely on descriptions of macroseismic effects only. In both cases, it is impossible to achieve a perfect homogeneity, i.e., a consistent completeness and accuracy over the whole observation period. The historical record for Greece goes back a few thousand years, but the data become more and more scanty and unreliable the further back in
172
time we go. And a similar present
century
problem
due to the gradual
is faced during improvement
works. In both cases, the degree of completeness (cf. Drakopoulos, significance
1976a). These problems
and there is no remedy
the instrumental
period
of the seismograph and accuracy
are not unique
increases
of the
station
net-
with time
for Greece but of universal
for them.
It is frequently assumed that catalogues are representative to such a degree that they permit linear extrapolations, e.g., for the calculation of seismic risk. However. strictly speaking, we have no guarantee against the possibility that catalogues. even those encompassing a few thousand years, just represent a part of a far longer geological cycle with varying tectonic activity. Even though tectonics may provide additional valuable information, there is presently no reliable way of eliminating such deficiencies in homogeneity, completeness and representative value. But it is important to bear them in mind in any application of earthquake catalogues. The available information on catalogues for the Greek area is summarized in Table II, arranged in chronological order. The catalogues are essentially of three different kinds, with regard to their coverage of space and time: Catalogues for larger areas, of which Greece forms a part. The world-wide catalogues of Gutenberg and Richter (1954) and Rothe (1969) belong to this category. Though not listed in Table II, we may also include the earthquake lists published by several seismological agencies: International Seismological Summary (ISS) for 191% 1963, International Seismological Centre (ISC) since 1964, National Earthquake Information Service (NEIS) of the U.S. Geological Survey. Seismologique Europeen-Mediterranten (CSEM) in Strasbourg. Catalogues
Centre of the
European area (Karnik, 1968, 1971) and of the Balkan area (Shebalin et al., 1974) also belong to this group. The Greek contribution to Shebalin et al. (1974) derives from compilations by Drakopoulos Catalogues covering the Greek
and Delibasis ( 1972, 19722 1973). area and a fairly long period of time.
These
catalogues are the most important ones for detailed studies of the whole area. From the data statements, we note that in general the following correspondence holds between observation period and maximum intensity Z,: before 1800, I, 2 8; 1801- 1900, I,, 2 7; 1901- 1970, I,, 2 6. For the two first-mentioned periods, Galanopoulos (1960b, 1961~) proceeds one unit lower on the I,, scale. The recent catalogue by Makropoulos (1978), cf. Makropoulos and Burton (198 1), is important because all locations, times and magnitudes are recalculated in a consistent manner applying modern techniques. The completeness of this catalogue is investigated in detail as well as the epicentral shifts versus earlier determinations. Believed to be complete for M 2 5.5 for the last 60 years, this catalogue is probably the most reliable one for the instrumental era now available for Greece. Catalogues covering limited areas (within Greece) or limited periods of time. They are also listed in Table II to the extent that I have been able to locate them in the available literature. In most cases, such catalogues are not obvious from the titles of the papers, but may appear hidden behind almost any title concerned with Greek seismology.
173
TABLE
II
Earthquake
catalogues
for the Greek area
Reference Platakis
(1950)
Area covered
Period covered
Crete
antiquitypresent
Galanopoulos
(1952b)
Leukas
Island
Comment
time
1469- I948
macroseismic observations
Galanopoulos
(1953b)
1879- 1892
Greece
macroseismic observations
Galanopoulos
(1954b)
138991949
Chios Island
macroseismic observations
Gutenberg
and Richter
the World
190441952
M > 5.5
Greece
antiquity-
damaging
and
disastrous
earth-
(1954) Galanopoulos
(1955~)
present
time
quakes Galanopoulos
(1960b)
Greece
1801-1958
I,r6;
Galanopoulos
(1961~)
Greece
Prior to 1800
I, Z 7
17Ot- 1960
M > 4.75
Galanopoulos
(1963)
34’-42“N.
Gaianopoulos
(1965a)
Greece
19*-29”E
1843- 1962
M25
M 2 5.5; only dates and magnitudes
Galanopoulos
( 1966)
Greece
Galanopoulos
(1967~)
Greece
(1967)
Garagunis Papazachos
et al. (1967a)
1800-1957
Corinth
M z 7; N= 28
480 B.C.- 1954
area
1926-1964
Greece
40 aftershock sequences
Gaianopoulos Karnik
(1968a)
(1968, 1971)
31”-36ON,
24’-36’E
European
1908-1967
M > 4.5
I:
I0 > 6; M > 4.5
1901-1955
II: 1801-1900
I, z 7
Rothe ( 1969)
the World
195331965
incl. macroseismic
Tamrazyan
Greece
1900-1958
M > 6.5
1900-1970
Ma7;N=27
effects (1970)
Galanopoulos
( 197 1a)
34”-42”N,
Galanopoulos
( 1971b)
within
19”-31”E
100 km of
1805-1969
Athens Papazachos
(1971)
Greece
191 I-1969
2 16 aftershock sequences
Papazachos
and
Comninakis
(1971)
Drakopoulos Delibasis
1911-1969
intermediate
Greece
2100 B.C.-1799
I, r 8
1800- 1900
I, 2 7
(1972)
Drakopoulos Delibasis
and
Greece
and
Greece
events
1901-1970
(1972- 1973)
Galanopoulos
( 1972b)
Papazachos
and
Comninakis
(1972)
Galanopoulos
(1973a)
34O-42”N.
19’-29T
Greece 34’-42’N,
19”-29°E
1950-1972
M z 5.5; N = 325
1911-1971
M t 4.9
1950-1973
M > 5.5
174
TABLE
II (continued)
Reference
Period covered
Area covered
Comment
et al. (1974)
Shebalin
Balkan
I:
1901-1970
M>4
I,>6;
II: prior to 1901
I,, > 7 (1801-1900) I, 2 X (prior to 1801)
( 1975)
Galanopoulos Leydecker Papazachos
(1975) ( 1975b)
E. Mediterranean
1954-1975
h>90km:N=105
36’-39”N.
1972- 1974
N=321
1911-1965
M 2 5.6
1966- 1973
M 2 5.1; N = 274;
20°-24.25”E
Greece
fore- and aftershock lists for 218 events Comninakis
and
Papazachos
(1977,1978)
30”-50”N.
10°W-40”E
Galanopoulos
(1977a)
32”-43ON,
Makropoulos
(1978);
33O-42.5”N.
Makropoulos
and Burton
17”-30”E 19O-29”E
1901-1910
M 2 6.5
1911-1949
M > 5.5
1950- 1963
M 2 5.0
1964- 1975
M 2 4.5; N = 2324
1902- 1976
M 2 5.5; N = 164
1901~1978
M 2 5.5 after 1920; N=1806
(1981) Yerkes et al. (1979)
39.5”-41.75’N.
Papazachos
Greece
(1980a)
22”-25OE
1759-1978 1974-1980
M > 5: N = 109 M >_5.1: fore- and aftershocks
Papazachos
and
Comninakis
( 1981)
I, = maximum events;
Hellenic
intensity
trench
1919-1979
main shocks and largest aftershocks
(12-degree
scale);
M = surface-wave
magnitude:
N = total
number
of listed
h = focal depth.
In this connection,
I like also to mention
a catalogue
by Alsan et al. (1975). where
all source parameters are recalculated for the area 35.5”-42S”N, 25.5”-45.0”E. in its western part overlapping the Greek area. Grigorova and Palieva (1965) describe the seismicity along the Mesta River, which in part also concerns Greece. EARTHQUAKE
STATISTICS
Using the material collected in earthquake catalogues, the earthquake statistics aims at finding relations between certain dependent quantities (Y), such as frequency of events, seismic energy release, strain release, and one or several independent quantities (X), such as space, time and magnitude. That is, we seek relations Y =f( X). Such procedures are applicable to any earthquake sequence, and it is convenient to divide the efforts according to the time scale of the sequences. Short-term statistics, extending over days, months or at most a few years. Such
175
sequences
are offered
frequency-magnitude
by foreshocks-main relations
for
shock-aftershocks.
foreshock
sequences
Frequency-time as well
and
as magnitude
relations between foreshocks and the main shock have been investigated extensively in Greece. The results are suggested to be useful in forecasting the time of occurrence obvious,
and
the magnitude
as evidenced
these cases, relatively shocks. The short-term connection
of the main
both by Cephalonia large foreshocks
statistics
with volcanic
encompasses activity.
shock.
However,
the difficulties
are
in 1953 and Thessaloniki
in 1978. In both
were at first misinterpreted
to be the main
also earthquake
A problem
concerning
swarms. often occurring prediction
appears
in
here.
because at least in the initial stages, it may be difficult to distinguish a swarm from a true foreshock sequence. After this, we proceed to a more detailed discussion of the relevant literature: (1) Fore- and aftershocks. A paper by Papazachos et al. (1967a) is the first more extensive attempt to investigate the properties of seismic sequences in Greece and the surrounding area. In all, 40 aftershock sequences from 1926 to 1964 are used. with all data listed (Table II). Papazachos (1971) studies 216 aftershock sequences for the period 1911-1969. for which all pertinent information is collected in a table. Ranalli (1969) presents in considerable detail the statistical methods he used. Of his investigated 15 sequences, there are six from Greece. There is quite general agreement about some results. This concerns the recurrence relation: log N = a-hh4 (N = frequency, M = magnitude, a. b = constants). Thus, the b-coefficient is found to be lower for foreshocks than for aftershocks, to be constant during an aftershock sequence, and to decrease with depth. The h-coefficient is negatively findings. Further studies
correlated
with the acting
of the b-coefficient
stress, which explains
are reported
by Ambraseys
some of these (1965). Niklova
and Karnik (1969), Giannakopoulos (I 972). Papazachos ( 1974e), Galanopoulos and Ekonomides (1975), Karnik and Prochazkova (1976) and Bath (1983). Moreover, there is a rather general agreement about magnitude differences with M, - M, = 1. I - 1.2 in average (confirming Bath’s law) and M, - M_, = 1.9-2.0. while also some variation of these differences with M, and focal depth h is suggested and put into mathematical form in some papers (M,, M,, M_ , = surface-wave magnitudes of the main shock, the largest aftershock and the largest foreshock, respectively). The relations between frequency and time for fore- and aftershocks are in general agreement with results found in other seismic areas, especially in Japan. In addition to the papers already quoted, information on earthquake sequences, in particular fore- and aftershocks, is given in the following papers: Galanopoulos ( i 955a), Drakopoulos (1968, 197 I, 1978a), Drakopoufos and Srivastava ( 1970) Drakopoulos and Ekonomides (1972), Drakopoulos and Delibasis ( 1973) Prochazkova (1972, 1973), Papazachos (1974c, d, 1975a. b, c, 1980a). Purcaru ( 1974), Delibasis and Carydis (1977), Carver and Henrisey (1978). Comninakis (1978).
Comninakis
and Papazachos
(1979a), Papazachos
and Leventakis
Bollinger (1981), Karnik et al. (1982), Soufleris et al. (1982). (2) Swarms. The information on swarm activity in Greece than on fore- and aftershocks, rare in Greece.
Drakopoulos
on the Melos Island the h-coefficient than
between
and
(1973) investigate
in August
more marked aftershocks-a
is much more scanty
reason being that volcanic
and Delibasis
which occurred
exhibits fore-
a probable
(1980). Carver and
1971. Drakopoulos
differences result
between that
earthquakes
an earthquake
(1978a) finds that
foreshocks
could
are
swarm
suggest
and swarms a method
to
distinguish swarms from foreshocks already at an early stage. Long-term statistics or secular statistics, usually extending over the total period of available homogeneous data. Frequency--magnitude relations can then be used for the estimation of seismic risk and/or the maximum magnitude likely to occur, presented in tables or maps. Space relations include seismicity mapping, e.g., frequency and energy mapping, providing significant generalizations of point maps (epicenter maps). Time relations lead to investigations of periodicities-a field that attracted much interest in the early days of seismology. Nowadays, it is to a great extent dismissed, because many of the earlier suspected periodicities prove not to be real. Migrations and oscillations of the seismic activity, involving space-time relations, belong either to short-term or long-term statistics. depending upon their time scale. After this, we proceed to a brief review of the literature relevant to the Greek area. Here, we are exclusively concerned with the instrumental data statistics. The macroseismically based statistics, including intensities, etc.. is dealt with in the next section. (1) Seismic risk and maximum for the estimation of the maximum theory of extreme values (Schenkova
magnitude.
Several different
methods
are applied
magnitude and the seismic risk, such as (a) the and Karnik, 1973, 1977; Karnik and Schenkova,
1974; Makropoulos, 1978; Burton, 1979), (b) the vertical separation between the upper and lower boundary lines in diagrams of strain release versus time (Galanopoulos, 1972b; Karnik and Radu, 1977; Makropoulos, 1978), or (c) from a/b, the “once per year earthquake”, advocated especially by Galanopoulos (1968b, 1971 b, 1976, 1979b). Cf. also Comninakis (1975). Galanopoulos (1976. 1979b) suggests that the maximum possible magnitude is at most two units greater than the annual maximum (a/b) of the region, and that this method is better than relying on the theory of extreme values. It is of interest to note that this suggestion of Galanopou10s is practically identical with defining the maximum magnitude as corresponding to a seismic risk of R = 0.01. The various measures of seismic risk are usually displayed on maps. Discrepancies between various estimates of seismic risk and maximum magnitude are in part due to different observational material and different methods of calculation. Purely seismological estimations should be compared with results from tectonics, stress measurements, etc.
177
In addition (197(i),
to the papers
Hattori
(1979),
already
Cosentino
mentioned,
( 1981) deal with seismic risk and maximum (2) Space characteristics
also those of Algermissen
( 1980). and
of seismicity.
Galanopouios
magnitude
A number
and
et al.
Makropoulos
in the Greek area.
of different
seismicity
maps has
been
prepared for Greece, which all differ somewhat in detail, but nevertheless emphasize the main features, e.g., the two main centers of activity in the Ionian Islands and near the Dodecanese Islands. Different quantities are used for the mapping, Delibasis
such as (a) the released seismic wave energy E (Galanopoulos. 1963; and Galanopoulos, 1965, 1967; Niklova and Karnik, 1969; ~znichenko et
al., 1973; Maaz et al., 1974; Bath, 1983), (b) the released seismic strain, calculated as E 1/2 (Galanopoulos, 1955a, 1956a; Delibasis and Galanopoulos, 1965, 1967), (c) the frequency of events, expressed as a, b, a/b (Kaila and Narain, 197 1: Kaila and Rae, 1975: Comninakis, 1975) or (d) they are given as seismotectonic maps, mostly equivalent to epicenter maps but with tectonic units included (Beloussov et al., 1968: Bornovas et al., 1971; Bune et al., 1971; Gorshkov et al., 1974: Beuzart. 1975; and in numerous papers In examining terminology
referred to in the last section). the publications on seismicity
of a more special type, deviating
unambiguous, for example: “Seismic efficiency” (Galanopoulos,
mapping.
we come
from the standard
1956a) or “ tectonic
across
some
one and not fully
flux”
(Delibasis
and
Galanopoulos, 1965): strain release map in terms of E’j2. “Seismic efficiency” (Galanopoulos, 1963): earthquake energy per square degree and 100 years, shown by isoenergetical “‘Earthquake efficiency” (Delibasis
lines. and Galanopoulos,
1965): energy
map. with
energy expressed as the number of shocks with M = 6.8 per square degree and 100 years. “Seismic activity” (Riznichenko et al., 1973): energy map, showing the number of earthquakes corresponding to a certain energy or a certain magnitude. ““Shakeability” (Riznichenko et al., 1973): distribution of maximum expected intensity or mean recurrence time (years) of earthquakes of any given intensity (for example, for Athens the intensities 7, 8, 9. 10 recur at intervals of 9, 50, 1200. 50.000 years, respectively). (3) Space-time
characteristics
of seismicity.
The information
about
oscillations,
migrations and periodicities is usually rather scanty and often unreliable. This is true universally and Greece is no exception. It is well known that the stress is concentrated to those areas where seismic belts end. split or change direction. Osciilations may occur between the stress concentrations at the end points of a belt. Likewise, a passive, practically aseismic plate like the Aegean Sea is supposed to exhibit oscillations between opposite borders of the plate (Galanopoulos, 1955b), while Tamrazyan (1970) finds an oscillation pattern between north and south Greece. The two major concentrations of activity (near Ionian and Dodecanese Islands) are also suggested to show an oscillation pattern with a period of 52 years
17X
(Delibasis and Galanopoulos, 1967; Galanopoulos, Galanopoulos (1956a) points to the possibility scale, mentioning
that in 1950, the earthquake
197 1a). of oscillations
activity
also on a bigger
in Greece was at a minimum
while the earth as a whole then suffered more from earthquakes than in any year since 1906. Karnik (1972) finds an upward migration of the seismic activity in the Aegean area during the present century (cf. also Papazachos, 1977). while Galanopoulos (1965a) maintains that periods of greater seismic activity are initiated by intermediate
shocks, i.e., the Greek seismic activity
is induced
by processes
occurring
under the earth’s crust (however, denied by Purcaru and Berckhemer, 1982). Comninakis and Papazachos (1980) suggest a periodic pattern for intermediate earthquakes with quiet periods of about six decades followed by very active periods of about two decades. Purcaru and Berckhemer (1982) find a migration from west to east in the Hellenic arc at time intervals between 20 and 50 years. Trigger effects. In such a highly stressed and fractured area as Greece, it is understandable that various human and other impacts on the crust could release the strain, i.e., trigger earthquakes. There are several kinds of such impacts that could be of importance: (1) Mining operations. It is well known that mining operations lead to rockbursts. However, I have not found any account of Greek rockbursts in the literature, although these would be expected to be a safety risk factor of significance in the vicinity
of Greek mines.
(2) Dam constructions. There are several descriptions of such effects, mainly concerned with the Kremasta Lake but also some others. Based on a detailed investigation of the time, magnitude and spatial distribution of the shocks. Comninakis
et al. (1968) maintain
that the foreshocks
and the main shock (February
1966, M = 6.3) at the Kremasta dam were triggered natural events, the time distribution of dam-generated
5,
by the waterloading. As for foreshocks is suggested as a
means of predicting the time of occurrence of the main shock (Papazachos, 1973a). Contrary to natural sequences, there is an indication that the b-coefficient is higher for the dam-generated foreshocks than for the aftershocks (Gupta et al.. 1972; Papazachos, 1974a; Delibasis and Carydis, 1977). Additional information on damreleased activity is found in Galanopoulos (1967b). Galanopoulos and Ekonomides ( 1973), Drakopoulos (1974), and Therianos (1974). (3) Volcanic eruptions. In connection with trigger effects it is appropriate to point to a suspected relation between earthquakes and volcanic eruptions, All but one of the historically known eruptions of the Santorini volcano were either preceded or followed by tectonic earthquakes (Komlb et al., 1978; Hedervari, 1979). This phenomenon which has also been observed elsewhere, may be due to mutual triggering effects. And it is certainly different, both in origin and properties. from the typical earthquake swarms that accompany volcanic eruptions. We also note that the Atlantis myth, among various conflicting hypotheses, has been located to the Aegean Sea (Galanopoulos and Bacon, 1969; Milanovskiy, 1969).
179
(4) Cosmic moon
influences.
Especially
and the new moon,
the moon’s
have been suspected
Greece (Tamrazyan, 1970). Earthquake prediction. Earthquake Mediterranean
belt, compared
important
only
not
passage
over the meridian,
to act as triggers of earthquakes
prediction
has been
rather
neglected
to several other seismic areas. Prediction
for solving
just
the full
the prediction
problem
itself.
in
in the
research but
also
is for
revealing many of the otherwise unknown properties of earthquakes and earthquake sequences. The prediction problem has two main aspects. differing bo$h in methods and purpose: (1) Statistical estimation
prediction.
of maximum
This
magnitude,
concerns
the calculation
of seismic
etc. This type of prediction
risk,
is indispensable
the for
engineering purposes, e.g., the formulation of building codes. On the other hand, the time of occurrence of single events is not included in this prediction. (2) Deterministic prediction. This aims at predicting the time, location and magnitude of single earthquakes. The purpose is to make it possible to take precautions for a coming event, including the evacuation of people. Since 1960, much research is done with this goal in the U.S.A., Japan, China and the U.S.S.R. As far as Greece is concerned, the efforts to predict single forthcoming earthquakes have so far been rather modest, and they can be summarized apart from some isolated observations of animal behaviour (a) Until recently, the only method suggested is based foreshock sequences, enabling an estimate of the occurrence of a coming main shock. Cf. Papazachos et al. (1982). (b) Seismic gaps-in space and time-are indications of an increased likelihood of a forthcoming larger earthquake.
in the following points. (Papakis, 1962): on statistical studies of time and the magnitude accumulating strain and This technique, already
used in some other parts of the world, is recently applied also to Greece. Wyss and Baer (1981a, b) and Papazachos and Comninakis (1982) expect on this basis that .some larger earthquakes will occur during the 1980’s. according to Wyss and Baer (1981 b) both in the western re-evaluated prediction
magnitudes, and maintains
and the eastern
Ambraseys
Hellenic
(198 1) expresses
that the available
arc. However, criticism
data are insufficient
on the basis of
against
this mode of
to support
predict-
ions with an acceptable degree of certainty (cf. also Purcaru and Berckhemer, 1982). Karnik et al. (1982) study time gaps, among others for the Thessaloniki sequence in 1978. (c) Another more original approach is due to Papazachos (1980b, 198 1) and Papazachos and Comninakis (1982), who find a remarkable sinusoidal variation with time of M, - 44, (the magnitude difference between the main shock and its largest aftershock) with a period of 29 years. The exact physical reason for this sinusoidal time variation remains to be clarified, but larger earthquakes appear to occur around the minima of M, - M,. And it is noteworthy that this model leads to results agreeing in time with (b). A global test of the method is challenging. (d) A note
in some
news
media
in
1981 reports
that
Greek
physicists,
K.
Alexopoulos, earthquakes
P. Varotsos
and K. Nomikos
six to seven hours
crystal for field measurements earthquakes.
The method
detection and location 200 km is suggested.
in Athens,
in advance.
of electricity, appears
generated
to be both
and
stress before
promising
and
for the
areas
and
suggested
every have
to be used
for
EFFECTS
The macroseismic categories: (1) Effects
by the increasing
cheap
in predicting
they use a piezoelectrical
of oncoming events, a network of observation points Electrical conductivity variations and earth currents
already been observed elsewhere in seismic prediction (cf. Bath, 1979a, pp. 297-304). EARTHQUAKE
have succeeded
Apparently,
effects,
on nature:
directly
geological
observable relations,
in the field,
fault scarps,
soil deformation, landslides, avalanches. (2) Effects on human constructions: buildings,
bridges,
fall into
two main
topographical
changes.
power plants,
with partial
or total destruction,
and secondary effects, especially fires. It is natural that the earlier literature is largely dominated by descriptions of various macroseismic effects. This is true not only for the pre-instrumental period, when macroseismic data were the only available information, but also for the early instrumental period, when the information from seismograph recordings was scanty. But it would be incorrect to believe that the macroseismic observations have lost their significance in modern seismology. On the contrary, they may be considered as more important than ever, by providing means to establish relations between instrumental seismology on the one hand and geology, tectonics and engineering on the other hand. Such relations will then provide for more reliable evaluation of historical earthquakes, for which only macroseismic information is available. The present-day significance of macroseismic observations Macroseismic
is very clearly
observations.
testified
by the engineering
reports
of numerous
teams
examining
recent Greek earthquakes, e.g., the Thessaloniki event on June 20, 1978. The following publications contain macroseismic information: Galanopoulos ( 1950, 1952a, b, 1953b, 1954a, b, 1956b, 1959, 196 1a, b), Georgalas
and Galanopou-
10s (1953) Grandazzi (1954) Trikkalinos ( 1955), Anonymous ( 1956) Kiskyras (1956-58a, b), Papakis (1962) Kokinopoulos and Tasios (1966) Ambraseys (1967), Papageorgakis (1968) Rothe (1969) Papazachos (1975d), Bairaktaris and Roussopoulos (1976), R~us~opoulos (1976) Drakopoulos et al. (1978) Fintel (1978), Kalevras (1978) Kalevras and Constantopoulos (1978) Kalevras et al. ( 1978), Kulhanek and Meyer (1978), Papastamatiou (1978) Blume and Stauduhar (1979) Comninakis and Papazachos (1979b), Maley et al. ( 1979, 1980), Papazachos et al. (1979), Yerkes et al. (1979), Zavliaris et al. (1980). In addition, the Athens Monthly Seismological Bulletins provide extensive macroseismic data beside the instrumental evaluations. Cf. also Table II. Liquefaction, i.e., loss of shear strength in sands due
181
to earthquake
shaking,
is investigated
Roussopoulos (1980). Ii?te~~jt~-acceleration. degrees, nowadays number
a multitude
effects
are generally
in some 12-degree scale. intensity
of difficulties,
often influenced
Mactoseismic
with regard to Greece by Andrikopoulou
partly
because
by personal judgment,
of different
effects
the values
expressed
determinations
difficulties is that intensity lacks a unique, sity ratings prove to be immensely useful,
number.
in intensity
are subject
are not measured
partly because the estimates
by one single
and
to a
but estimated, aim at expressing
The main
reason
for the
physical definition. Nevertheless, intennot only for historical earthquakes, but
also nowadays in the instrumental era. Intensity estimates, based on damages to buildings, are subject to several factors which may lead to overestimates, rather than underestimates. Both the ground conditions and the structural properties of a building are important. For example. Galanopoulos (1952a) emphasizes that Larissa in 1941 showed exaggerated effects both due to soil conditions and to war effects (bombardments had weakened the structures). Also structures weakened by earlier earthquakes may lead to exaggerated intensities. In a study of two Peloponnesus earthquakes in 1965 and 1966. Ambraseys (1967) points out that exaggerated intensities are obtained by including secondary
effects, caused by landslides
The geographical
distribution
and slumping.
of intensity
for example.
is generally
presented
in isoseismal
maps, of which many have been produced for Greece (see, for example, Grandazzi, 1954; Delibasis and Drakopoulos, 1974; Shebalin et al., 1974; Papazachos, 1975d; Blume and Stauduhar, 1979). In general, the intensity distribution appears to be rather irregular, at least when a larger number of observations are available. In the case of Greece, some handicap is experienced by the vast extent of the seas with no observations, except on some islands. The intensity distribution depends on several factors, such as the focal mechanism, the focal depth, the structural trend of faults, the attenuation along different paths, the ground conditions at the receiving sites, etc. The properties of such distributions are studied by Trikkalinos (1955) Galanopoulos (1958, 1959), and Drakopoulos (1976b, 1978b). Beside intensity
maps for single events, more generalized
showing the distribution of the maximum estimated intensity area (Galanopoulos, 1965b; Galanopoulos and Delibasis, Gorshkov estimation
maps are of importance, over the whole Greek 1972, reproduced in
et al., 1974; Shebalin et al.. 1976). Such evaluations are also used for the of the seismic risk in terms of intensity, particle acceleration or particle
velocity, as a correspondence to the risk values in terms of magnitude. Expected seismic effects may be represented also on microzoning maps, based on all available information of importance (DrakopouIos, 1973, 1976b, 197%; Roussopoulos and Syrmakezis, 1973; Sherif, 1973; Galanopoulos, 1974b, 1977a; Drakopoulos et al., 1978). Due to the numerous factors involved, not always under complete control, it is not surprising that such maps do not always hold true. For example, Papastamatiou (1978) finds that the nlicrozonation study of Thessalonik~
182
(Drakopoulos,
1973) failed
to predict
all the observed
motion. In order
to improve
the intensity
ratings
estimations
to parallel
instrumental
data
(Galanopoulos, instrumental
1961a. b; Karnik, recordings
it is necessary, from
1965), partly
of strong motion
variations partly
the permanent to supplement
accelerographs.
of the strong to relate
station
the
network
the estimations
by
There is now a rather good
distribution of strong motion instruments around Greece. Therefore. strong motion records form the basis of several recent studies of Greek earthquakes. e.g., by Galanopoulos and Drakopoulos analysis of strong motion records
( 1974). Carydis (1976a). and others. The first from Greece dates from 1973. i.e., it is of a rather ROUSSO~OU~OS.1973). Carydis and Sbokos ( 1976)
recent data (Drakopoulos and analyze Greek strong motion records from 1972- 1975 (cf. Brady et al.. 1978). Temporary installations of strong motion instruments are nowadays a rule after damaging earthquakes, such as after the Thessaloniki event of June 20. 1978 (Papastamatiou, 1978; Petrovski et al., 1978; Blume and Stauduhar. 1979; Maley et al., 1979). Building code. A building code for aseismic construction is applied in Greece since 1959, at least to important structures in towns (Ambraseys, 1967). A brief description of the code is given by Blume and Stauduhar (1979) and Carydis (1976b) provides a discussion of it. In this code. Greece is divided into three zones, depending on the seismic risk. Seismic coefficients are specified according to seismic intensity ratings and soil classifications. The major cities, Athens and Thessaloniki, naturally attract most interest in this connection. Galanopoulos (1956b) points to a local earthquake source about 15-20 km northeast of Athens, responsible for many shocks in the last few years. Athens is far from being immune to earthquakes and the extension of the city on dilluvial and alluvial grounds indicates that at least its newer quarters cannot be considered earthquake-safe (Galanopoulos, 1963). Cf. also Galanopoulos (1977b). The Thessaloniki events of May-June 1978 are noteworthy, because this is the first time in the Greek history that a big city with tall buildings was affected by an earthquake
sequence
of a long duration
and relatively
large magnitudes
(Papazachos
et al., 1979). The collapse of an eight-story building of reinforced concrete is particularly remarkable. The probable reason is poor ground conditions and also the fact that the building had been designed before the adoption of a modern building code (Psycharis, 1978; Maley et al., 1979). The ground conditions are exceptional in Thessaloniki, with large variations from very soft to hard ground (Drakopoulos, 1973). Corinth has frequently been exposed to earthquake catastrophes. After the damaging earthquake of April 22, 1928, M = 6 l/4 (Drakopoulos et al., 1978) Corinth was rebuilt according to Japanese aseismic standards, obviously to a great advantage. The seismic risk in Athens, Thessaloniki, Patra, Corinth. Heraklion and Rodos, six industrial and highly populated centers, has been calculated by Makropoulos ( 1978).
1x3
Since 1612, the Leukas
Island has been affected
every 16 years, which places this area among Greece (Galanopoulos, Tsunumis. Seismic
by devastating
earthquakes
the most earthquake-exposed
once ones in
1952b). Cf. also Anonymous ( 1978) and Kalevras ( 1980). sea waves or tsunamis constitute another earthquake effect
which may cause great destruction
to harbours.
Submarine
slides, set off by dip-slip
components of earthquakes, are their most likely reason. Greek tsunamis are the most frequent ones in the Mediterranean area. but the large, damaging ones are rare. For example, among 41 tsunamis between 479 B.C. and 1956, only 16 were really damaging (Galanopoulos. 196Oa). Atnbraseys (1962) has accumulated
an impressive
list of eastern
Mediterranean
tsunamis,
including
141
events since 1410 B.C. up to the present time (also given by Antonopoulos, 1978), and Antonopoulos (1980) presents a detailed account of 136 tsunamis in this area from the birth of Christ up to 1980. Sousa Moreira (1974) gives a catalogue of tsunamigenic
European
earthquakes
with M > 5.3 in the period
1901-1971.
Tsunami
warnings would be useless in Greece because of the proximity of the sources, whereas double or triple rows of pine trees along affected coasts have been suggested as a kind of protection
(Galanopoulos,
1972a).
Further information on Greek tsunamis is contained in papers by Galanopoulos ( 1954b, 1957, 1967d), Ambraseys (1960, 1963, 1967) Papakis (1962), Galanopoulos et al. (1966) Karnik ( 1968, 1971) Marinos and Melidonis ( 1971) and Antonopoulos ( 1973). Deuth toils. The fact that the number of victims in Greek earthquakes is generally very low for the event magnitudes is an equally remarkable as fortunate circumstance, emphasized among others by Galanopoulos (1952b. 1953b) and Papastamatiou (1978). Compared to other countries in the same earthquake belt (Italy, Turkey, Iran) where death tolls are reckoned in thousands, the number of Greek victims
mostly
stays within
a few tens for earthquakes
of the same magnitude
(cf.
Bath, 1979b). In the period 1953-1969, about 600 people were killed by earthquakes in Greece (Galanopoulos, 1972a). to which the lonian Islands quakes in August 1953 contributed by 460 victims. In the iast century, there is only one earthquake (Chios, April 3, 1881) that had a greater number
of victims, stated as 2829 in Galanopoulos
( 1953b), but 4181 in Galanopoulos (195413). A few historical examples can also be given, e.g., Corinth in the year 77 with about 4000 deaths, while the same area had only 20 deaths in the damaging (Garagunis, 1967).
earthquake
The exact reason for the relatively speculate on the following factors:
of 1928, thanks
to a warning
low death figures is not quite clear, but we can
( 1) More favourable constructional style of houses (Papastamatiou, (2) Population distribution in relation to the earthquake damaging ering also the extent of waters in the Greek area. (3) Foreshocks, serving as a warning whether foreshocks are more common
foreshock
1978). zones, consid-
for people, who stay outdoors. It is not clear in Greece than in the other Mediterranean
184
countries.
But the fact that the Greek crust is very much intersected
faults is favourable
to minor
by numerous
releases before a main shock occurs.
CRUST AND UPPER MANTLE STRUCTURE
While the previous sections deal with the properties of Greek earthquakes, this and the next section are devoted to a study of the structure and tectonics of the Greek area. In other words, we now proceed to an investigation of the underlying causes-a necessary step to deepen our knowledge. Seismology provides most information in this respect, and as in mechanics, we divide our approach into statics (equivalent to structure: this section) and dynamics (equivalent to acting forces and movements: next section). The structural studies are based upon the interpretation of seismic waves, which yield information about wave velocities and layer thicknesses. As to the methods used, we distinguish between artificial sources (usually explosions-seismic waves). Artificial
profiling) and natural sources (earthquakes-body wave sources provide the highest accuracy, but
and surface the depth of
penetration of the waves is rather limited, mostly restricted to the crust (except for nuclear explosions). On the other hand, earthquakes imply a lower accuracy but generally deeper penetration of the investigated waves. Seismic profiling. The seismic profiling methods are conveniently divided into two branches: seismic reflection and seismic refraction, according to the measurement techniques used. (1) Reflection profiles. With regard to our purpose of trying to elucidate the Greek earthquake dynamics, the reflection experiments have only a limited information value, although applied to a great extent in recent years in the waters around Greece. This is obvious if we consider both the geographical location of investigated profiles and their depth of penetration. An exception to this rule is offered by a recent paper by Myrianthis (1982), who utilizes reflection profiles to clarify the dynamics of the February-March 1981 earthquakes in the Gulf of Corinth. Jongsma (1975), Jongsma et al. (1975, 1977), and Brooks and Ferentinos (1981) are concerned with areas inside Greece, namely the Aegean Sea. Other authors mostly deal with various parts of the eastern Mediterranean, outside or marginal to the proper Greek area, especially the Mediterranean ridge, the Levantine Sea, the Nile cone, the area south and east of Crete, the Ionian Sea or the southwest coastal region of Peloponnesus (Wong et al., 1971; Lort, 1972, 1973; Finetti and Morelli, 1973; Sancho et al., 1973; Hinz, 1974; Lort et al., 1974; Finetti, 1976; Woodside, 1977; Le Quellec et al., 1980; Mascle et al., 1982; Monopolis and Bruneton, 1982). The depth of penetration is rather shallow and at most, the basement rock has been reached. The measurements concentrate on the sedimentary layering on the sea bottom, and the results are discussed in very great detail with regard to geological structure, bathymetry and geological history. In these discussions, also other relevant
1x5
information
is included,
thermy. (2) Refraction more
interest,
tectonophysical related
deriving
profiles. partly
especially
In our present
because
their
belts
context,
deeper
aspects, partly because
to the earthquake
from gravity,
the refraction
penetration
the locations
in Greece.
geomagnetics experiments
has more
of the profiles
This
is particularly
and geoattract
bearing
on the
are more closely true
of the five
important refraction profiles that Makris and his associates investigated 197 1- 1974, covering PeloponnesusIonian Sea, the Cretan area and the Aegean
in Sea
(Makris,
1973. 1975, 1977, 1978b. c; Makris and Vees, 1977; Makris et al.. 1977). In
addition,
refraction
measurements
by Hinz ( 1974) and Weigel (1974a.
are reported
b). The seismic waves generally observed and evaluated are Pg, Pn and PmP. The velocity of Pg is about 6.0 km/set, representing the upper part of the crust, assumed to increase to 6.2-6.8 km/set towards the lower crust, while Pn exhibits a velocity of 7.667.8 km/set, i.e., rather low for the upper mantle. PmP is very clear, indicating a sharp Moho discontinuity, whereas the crust has a more gradual velocity increase with depth, i.e., it lacks a Conrad discontinuity. A most striking result is the marked variation of the crustal thickness, clearly indicating the complicated structure of the whole region. From Makris ( 1973, 1977, 1978b), we get the following representative crustal thicknesses: Ionian Sea 24428 km, Hellenides (central Peloponnesus) 46648 km, Greece-Bulgaria border to central Greece 44-48 km, northern Aegean Sea 30-32 km, southern Aegean Sea 20-28 km, Cretan
Sea 20 km, Crete-CarpathosRhossos
30-34
km, south of Crete 30-34
One problem concerns the true nature of the crust, whether continental (Comninakis and Papazachos, 1976). In most of the places investigated,
km.
or oceanic the crust is
considered to be continental (Lort et al., 1974) but in some of the oceanic areas, the thinner crust is rather termed sub-continental, whereas a typical oceanic crust is hardly
found
eastern
Mediterranean
sediments.
anywhere
in the whole crust
eastern
Mediterranean.
Alternatively, covered
the
is considered
to be oceanic,
but
Cf. Lort (1971, 1972). Woodside
( 1977) Morelli
( 1978). and Le Pichon
by thick
(1981). As general
comments
Greek and adjacent
on the seismic profiling
(reflection
areas, we can state the following.
and refraction)
Practically
in the
all the experiments
were made within the last 15 years, and all relevant papers date from the last decade. Therefore, this marks quite a recent development for this area. Moreover, we note that the expeditions are mainly performed on the initiative from abroad (Italy, Germany, England, U.S.A.), but mostly in cooperation with various Greek institutions. The profiling experiments have recently become intensified in connection with the oil prospection in the Aegean Sea. The geotectonic investigation of this area has thus become of great practical interest, in addition to its more theoretical aspects. Body waves from earthquakes. Considering the complex structure, evidenced by varying crustal thickness and by the probable existence of subduction zones, it is
186
expected
that seismic waves passing
these structures
exhibit
significant
time residuals
versus standard travel time tables. It is also expected that such residuals vary practically from path to path in a way that corresponds to the complicated structure. It is quite natural structure encounter it would
seem
that the interpretations of travel time residuals in terms of great difficulty and ambiguity. In order to reach reliable results,
necessary
to combine
a large
number
of crossing
paths.
These
expectations are fully corroborated by the research which has been devoted to travel time studies of the Greek area and which can conveniently be classified as follows: (1) Travel time residuals for Greek earthquakes recorded at Greek or other nearby stations. using Pn, Pg (and Sg). Positive residuals for Pn are ascribed to a low velocity of the upper mantle, while positive residuals for Pg are ascribed to an influence of the sedimentary layering (Ekonomides. 1972). Cf. also Taner ( 1962). Comninakis
(1967), Delibasis
and Galanopoulos
( 1967).
Average crustal structures derived from travel times of body waves (P. S) from Greek earthquakes recorded in southeast Europe suggest that crustal thicknesses vary regionally between 32 and 47 km, that the average Pn velocity is 7.87 km/set and the Pg velocity 6.1-6.8 km/set, and that no low-velocity layer exists in the crust (Papazachos et al., 1966). (2) Travel time residuals for Greek earthquakes recorded teleseismically. Negative residuals for P waves recorded at Greenland stations from earthquakes near Rhodes are explained as due to a lithospheric plate of a high P velocity sinking below the Aegean Sea (Gregersen, 1977). (3) Travel time residuals for teleseisms recorded at Greek stations. using P waves. Residuals indicate a high-velocity sinking slab and a low-velocity upper mantle in the Aegean Sea (Agarwal et al., 1976; Jacoby et al., 1978). See also Hovland and Husebye (1981). Sokerova (1974, 1977) finds that the station correction variations are mainly due to the crust and the sediments beneath the stations. The complicated structure influences but also the recorded wave amplitudes
not only the travel times. as just mentioned, (cf. Delibasis, 1981). The absorption and
scattering as well as the geometrical spreading will vary practically from path to path. Therefore, as for the travel times, a similar detailed study is needed in order to determine earthquake magnitudes in a reliable way. The magnitude problem may be illustrated
by the fact that Athens
magnitude calibrations since around and Maamoun and Allam (1981).
Observatory has applied at least four different 1950. See also Papazachos and Vasilikou ( 1966)
Surfuce waves from earthquakes. Phase and group velocity dispersion of Love and Rayleigh surface waves, both fundamental and higher modes, provides important information on the average crustal and upper mantle structure along the total path or in more detail if several determinations are combined in a “crossing-path technique”.
An efficient propagation of Li, Lg and Rg across the Aegean Sea proves its continental structure with the following velocities: Li 3.80 km/set, Lgl 3.57 km/set,
Lg2 3.32 km/set
and Kg 3.03 km/set
(Rizhikova,
on higher mode surface waves but without As to the true nature or oceanic,
of the eastern
the information
sial as the information
dispersion
Mediterranean
information. crust, i.e., whether
from surface wave data appears from seismic
1969) suggests the eastern sediments, structure.
1966a, b). This is the only report
refraction
Mediterranean
data,
reported
to be of a continental
in contrast to the western Mediterranean, Similarly, Cloetingh et al. (1980) consider
continental
to be equally above.
controverPayo (1967.
structure
with thick
which is of a more oceanic the crustal structure of the
eastern Mediterranean to be of a continental margin type with a thickness of 35-40 km. For comparison with crustal thickness. earlier derived from body wave data. Papazachos et al. (1967b) and Papazachos (1969) investigate the dispersion of fundamental mode surface waves. On this basis, Papazachos (1969) maintains that the eastern Mediterranean crust has an oceanic character with a mean thickness of 20 km. An upper mantle Pay0 ( 1969). Further
literature
low-velocity
layer is advocated
by Papazachos
on surface waves in the Greek area is provided
et al. (1967c), Payo (1976). and Farrugia
(1969) and
by Papazachos
and Panza (1981).
SHSMOTECTONICS
The literature on the tectonics of the Greek area is extremely comprehensive. even if we restrict ourselves to seismotectonics, i.e., tectonophysical results based on seismological measurements, leaving aside the still more comprehensive geologicaltectonical literature. There is hardly any literature that is so controversial as the seismotectonical production concerned with the Greek area, and the need for further research is frequently emphasized. Part of the divergences is due to the fact that quite different seismological observations are used as the starting point, including not only hypocentral locations and fault plane solutions, but also reflection and refraction results, body and surface wave measurements, discussions geological
frequently history.
as well as field observations.
involve other geophysical
No doubt,
it would
and geological
be a challenging
Moreover.
observations
but difficult
the
and the project
to
explore all details combined. In the following I will try to give some glimpses of the more significant results. There are essentially two important aspects in which seismology can contribute to the knowledge about the present tectonics: (I) By determining accurate hypocentral locations. (2) By providing fault plane solutions. Fault p/me s~I~t~~ns. Fault plane solutions or focal mechanism solutions yield information on the present-day tectonic motions. Based on seismograph records, such solutions define the orientation of the fault plane and the direction of slip along the fault plane. In other words, the type of fault motion will be defined (strike slip, dip slip, etc.; cf. Bath, 1979a. p. 173). In addition, these solutions determine the
188
directions
of the stress axes, i.e., the compressional
results will then have to be combined covering
the whole area of interest,
both between
and tensional
and with all other
relevant
cially that which can be derived from geological investigations. It is noteworthy that practically all the Greek fault plane exclusively
on the first motion
(compression
or dilatation)
Such
directions.
a large number
of earthquakes,
information, solutions
espe-
are based
of the P wave, exception-
ally also of the PKP wave. On the one hand, this information is most easily available and usually the most reliable one; on the other hand. it constitutes only a small fraction of the data contained in a seismic record. More readings could give significant contributions Thus, Hodgson and Cock PPP and PcP, obviously and/or polarization) are 10s and Delibasis (1974)
to the solutions and (1957) use first motion to a great advantage. used by Delibasis and in addition to P waves.
P, is also reported by Shirokova (1967). A striking feature is that sometimes divergent
solutions,
as investigated
the
by different
restrict ambiguities to a minimum. not only of P, but also of PP, PKP, S waves (direction of first motion Drakopoulos (1974) and DrakopouSome use of S waves. in addition to same
earthquakes
authors.
are
given
quite
This fact is emphasized
by
Gertig (1972a) and Drakopoulos and Delibasis (1974). One example is offered by Di Filippo and Marcelli (1954) compared to Hodgson and Cock (1957). Such divergences may depend on a number of factors, such as on the data used (long period vs short period recordings), on the analysis methods and to some extent also on personal judgment. It is obvious that the divergent results deserve to be critically reexamined, using more information whenever available. Additional earthquake mechanism solutions for the Greek area are presented in the following papers: Papazachos ( 196 1, 1974f. 1975d,e, 1976a, 1977). Constantinescu et al. (1966) Petrescu et al. (1967), Delibasis ( 1968). Karnik ( 1968, 197 1), Papazachos and Delibasis (1969), Radu (1969), Delibasis and Drakopoulos (1972) Gertig (1972b), McKenzie (1972, 1978) Ritsema (1974, 1975a), Leventakis (1975), Carver
and
Henrisey
(1978)
Le Pichon
and
Angelier
(1979),
Papazachos
et al.
(1979), D’Ingeo et al. (1980) Carver and Bollinger ( 1981) Maamoun and Allam (1981) Soufleris and Stewart (198 1), Soufleris et al. (1982). As a contrast to the numerous studies of P (compression, dilatation), modern spectral methods (cf. Bath, 1974) and waveform analysis, which permit an estimation of source dimension, seismic moment, average displacement and stress drop, have been applied in relatively few cases. Although some of the numerical results so far appear to be rather vague or unreliable, there is no doubt that the modern methods deserve to be examined and applied much more than hitherto. It is particularly to be noted that considerably more information can be extracted from the seismic records by the modern methods than by the traditional ones. Relevant papers are given by North (1977), Kulhanek and Meyer (1979), Barker and Langston (1981, 1982) Soufleris and Stewart (1981), Langston (1982), Soufleris et al. (1982). Large scale tectonics. The tectonic structure of the eastern Mediterranean ranges
189
among
the most complicated
to fully understand Ocean
before
the huge but generally
embarking
that the tectonics
ones in the world, and it would be a good advice to try
on Greece.
of the eastern
understood
by simple
considered,
such as subsidence
Morelli
simpler
around
the Pacific
et al. (1975) and Morelli
(1978) state
area is too complicated
to be fully
Mediterranean
seismotectonic
models.
of crustal
blocks,
Other
structures
contributing
flow of material
agents
must
in the mantle
be and
bending of certain tectonic elements. Bornovas et al. (1974) emphasize that the geophysical data alone are insufficient and that geological and neotectonic data must be included in an interpretation. Cf. also Berckhemer and Hsii (1982). The Balkan region is like a “crushed zone” pressed between two major plates, the African plate, moving northwards, and the Eurasian plate. Several authors talk about “buffer plates” between the two major plates. Cf. McKenzie (1972) Roman (1973) and Karnik (1975). From simple rock dynamics. one would expect that the rocks pressed between two blocks moving northhsouth would be fractured in NW-SE and NE-SW directions. This is also what has apparently happened. These fracture directions produce a mosaic pattern, dominating a large part of the Balkan peninsula. But the plate motion is more complicated than this, and we have not only a compressed and crushed zone, but obviously also subduction of the African plate. This subduction appears in places where the African plate experiences resistance and the earthquake belt is concave towards the north, i.e., north of Crete as well as in the Calabrian arc in southern Italy, possibly also in southern Spain. In other places, where the African plate moves more freely northwards and creates a belt concave towards the south, e.g., in the Adriatic Sea, there is no subduction. But all this is controversial
and problematic.
As to the large scale tectonics of the eastern general agreement about the dominant influence
Mediterranean of the African
area, there is fairly and Eurasian plates.
approaching each other. But even in this respect, there are some divergent opinions. Ritsema (1975a, b) emphasizes that throughout the Mediterranean area, at shallow as well as at deeper levels, the prevalent
direction
of tectonic
transport
is in the E-W
or W-E directions, i.e.. in clear contrast to the generally assumed N-S collision of the African and Eurasian plates. As a consequence, the northward movement of the African plate would only play a subordinate role for the Mediterranean tectonics. A similar idea is expressed by Makris (1973), referring the compression in the Hellenides to the opening of the Atlantic, while Makris (1977) refers to the relative movement between Europe and Africa as the most probable initiator of the tectonic system. Including quantitative estimates, Le Pichon and Angelier (1979) explain the Hellenic trench and arc as a result of rotation around a pole in the southern Adriatic Sea, while McKenzie (1970) mentions a rotational movement of the African plate around a center in the Atlantic. Several of these suggestions are interesting in view of a possible eastward migration of the seismic activity in the entire Mediterranean belt (Bath,
1979a, p. 306). Fig. 4 gives a recent example.
Though
this migration
pattern
190
12 -
se
_
1000
6 _
60
/
a7
km
EASTWARD MIGRATION
/
04 /
4-
34 km/day
7” l2 0
0
I
3 100 days
o-
l 1’
I 0
I
Fig. 4. Suggested
I
I
2
eastward
I
I
I
4
migration
6
of seismic activity
in the Mediterranean
belt in 1980- 1981 (number
I is the origin of the coordinate system with distances measured along the belt): I. AroresIslands,Jun1,1980,M=7.0 5. Pakistan.Mu~2, 1981.M=6.7 6. Irun.JuneII. 1981.M=6.9 z AIgeria, Oct. 10. 1980, M = 7.5 3.
Iia!y. Noo. 23, 1980. M = 7.1
7.
Zrun,Ju!r~28.
4.
Greece, Feb. 24. 1981. M = 6.7
8.
Pakrstun.
198l.M=7.3
Sep. 12. 1981. M= 5.8
has appeared in clear form only a few times and therefore must still be considered hypothetical, it is important enough to be seriously taken into acc:‘ymt in any effort to explain the large scale tectonics of the whole belt. Moreover, it suggests that earthquake prediction should be extended over the entire Mediterranean belt, including interrelations between all parts of the belt, and not be restricted to Greece (Bath, 1979b). The Mediterranean resembles superficially it is agreed
ridge with its sinusoidal shape in the eastern Mediterranean an oceanic ridge, like the Atlantic rift, but quite unanimously
that the Mediterranean
ridge is of a different
nature
and origin.
This
conclusion is based on a variety of observations. such as the absence of a central rift. horizontal compression and not tension as in a rift zone, an unsymmetrical epicenter location around the ridge, existence of earthquakes deeper than normal, magnetic field properties, etc. (Galanopoulos, 1968a; Wong et al., 1971). The Mediterranean ridge consists of thick sedimens which have been uplifted due to compressive forces between the African and Eurasian plates (Lort, 1972; Finetti and Morelli. 1973: Finetti, 1976; Woodside, 1977) or due to vertical faulting (Sancho et al., 1973; Hinz, 1974). In summary, east-west, north-south of dominant tectonic importance-a whole region.
and vertical directions are all claimed to be good illustration of the complexity of this
191
Greek
there
is
tectonics.
fairly
controversial. arrangement
Passing
good
now to the more specific
agreement
Seismicity
about
some
Greek
results.
maps of the Greek area usually
of earthquakes
The areas of compressional
around
the Aegean
and tensional
tectonics.
while
others
we note that remain
exhibit a practically
Sea with its markedly
stress, evidenced
quite circular
lower activity.
by fault plane solutions,
are quite well defined (Mercier et al., 1972. 1976: Ritsema. 1974). and corroborated by direct stress measurements in shallow boreholes (Paquin et al.. 1982). Compression prevails perpendicularly to the seismic belts in the western and southern Greece (Constantinescu et al., 1966: Petrescu et al.. 1967; Comninakis and Papazachos. 1980). however. contradicted by Hinz ( 1974), 1972; Gertig, 1972a; Le Quellec et al., while the Aegean Sea is an extensional area (Jongsma et al., 1975, 1977: McKenzie, 1978; Le Pichon, 1981). According to Makris (1975. 1976, 1977), the Aegean Sea is the site of ascending and expanding hot mantle material-a “hot plume”-while McKenzie (1970, 1972) talks in terms of an Aegean microplate moving towards SW, just as the minor Turkish plate moves west (Fig. 5). The dominating
tectonic configuration
on the Greek mainland
is oriented
NW -SE
(or NNW--SSE) vs NE-SW (or ENE- WSW). the two intersecting (conjugate) systems apparently both seismically active (Hodgson and Cock. 1957: Kiskyras, 1959; Galanopoulos, 1967a; Delibasis and Drakopoulos. 1974; Kgrnik, 1975; Drakopoulos, 1976b; Kronberg and Giinther. 1978). A controversial problem concerns the westward extension of the north Anatolian fault (C’omninakis and Papazachos,
1979a). While some seismologists
hold the idea that this fault continues
up to
Fig. 5. Plate boundaries and motions in the Aegean area. Double lines = extensional plate boundaries; single heavy lines = transform faults; solid line crossed by short lines at right angles = boundaries where shortening
is occurring;
Redrafted
after McKenzie
A = Aegean (1972).
plate;
T= Turkish
plate;
H = Hellenic
arc;
c =
Cyprean
arc.
192
the Ionian Kg&k,
Islands,
as evidenced
in the northeast
Greece,
POU~OS,1974; McKenzie, 198 1; Makropoulos There are great Hellenic
by epicenter
1968, 1971, 1972; Maley and Johnson, breaking
and
198 1). concerning
arc. While many researchers
a Benioff zone, sometimes said to Comninakis, 1969, 1971; Caputo 1972; Papazachos, 1973b. 1977; Strobach, 1978; Comninakis and there are others who do not find regular
(Galanopoulos,
Sengor, the nature
maintain
1967a. C;
1971) others mean that it terminates
up into several branches
1978; Dewey
and Burton, controversies
distributions
(Delibasis
1979; Carver
and Drakoand
of the Aegean,
that the hypocenters
Bollinger, Cretan
or
are arranged
in
be of an amphitheatrical shape (Papazachos and et al., 1970; Maley and Johnson, 1971; Karnik. Beuzart. 1975; Gregersen. 1977; Richter and Papazachos. 1980; Maamoun and Allam. 198 1) or who deny that the hypocenters define such a
structure
(Lort, 1971, 1972; Galanopoulos. 1953a, 1973b. 1975; Jongsma. 1977; Woodside, 1977; Makropoulos, 1978: McKenzie. 1978; 1975; Hinz et al., Makropoulos and Burton, 1981). In the northern Aegean Sea. there are also shocks somewhat deeper than normal, interpreted by some as defining a Benioff zone (Papazachos,
1976a, b, 1977) while others question
or deny such a zone (Maley and
Johnson, 1971; Makris, 1977; McKenzie, 1978). The conflicting opinions about Benioff subduction zones in the Aegean area are in part due to inaccuracies of hypocenter locations. As soon as more accurate determinations have become possible, a more well-defined Benioff zone seems to result, at least in the investigated areas (Leydecker. 1975; Leydecker et al.. 1975). On the other hand, it is quite clear that there are scattered, intermediate-depth earthquakes south of the Aegean arc, i.e., on its convex side. These have been used as the main argument against the Benioff zone hypothesis. However, advocates of the Benioff zone offer explanations even for these apparently irregular occurrences of shocks.
According
to Papazachos
and
Papadopoulos
(1977) and Papazachos
Comninakis (1978b), the northern Aegean arc is presently dying out, while southern is active, and the intermediate-depth shocks south of the Aegean represent
the initial
stages of an oncoming
Benioff zone structure
and the arc
(cf. Galanopoulos,
1975). This means a gradual shift of this structure from north geological epochs. So far, this explanation is naturally hypothetical.
to south
over
An interesting solution is presented by Papazachos and Papadopoulos (1977). See Fig. 6; cf. also Makris (1975, 1976) and Papazachos (1977). In its essence, this involves a circulation cell with descending motion along the subducting slab in the Aegean arc, coupled with ascending motion further north, at the volcanic arc. This idea appears to be very promising, especially as it seems to combine several phenomena into a unified picture. Therefore, this suggestion deserves to be further explored by all available means. An alternative explanation of the seemingly irregular intermediate-depth shocks south of the Aegean arc is offered by Richter and Strobach (1978). They refer to the fact that the sharp curvature of the arc leads to tension along its direction. This
193
-..,
I Fig. 6. A lithospheric Aegean
area.
compression:
tension
model (vertical
M = Mediterranean t = tension.
is strong
section)
Sea;
Redrafted
enough
J interpreting
S = southern
after Papazachos
geophysical
Aegean
area;
and magmatic N = northern
and Papadopoulos
properties Aegean
of the
area;
c=
(1977).
to break up the arc into several pieces, responsible
for the
observed scatter of the hypocenters. For a better understanding of the Greek tectonics, it would be helpful if some other areas on the earth could be found with a similar tectonic structure. Areas of a lower seismic activity, like the Aegean Sea, surrounded on all sides by very active seismic belts, are found for example in the Caribbean and the Philippine areas. A detailed examination of similarities and dissimilarities of the tectonics among these and possibly
some other areas (marginal
seas around
the Pacific) would no doubt be
very informative. In addition to the papers quoted above in this section, also the following contain extensive information on the seismotectonics of the Greek area: Galanopoulos (1965a, 1973a, 1974a, 1979a), Papazachos and Delibasis ( 1969) Gertig (1972b), Hedervari (1973), Papazachos (1973c, 1974b, f, 1975e), Drakopoulos and Delibasis (1974), Weigel (1974b, 1978) BonEev (1975) Payo (1975, 1976), Agarwal et al. (1976) Karnik and Radu (1976), Papazachos and Comninakis (1976, 1978a), Dtirbaum
Leydecker
et al. (1978)
Makris (1978a), Mercier et al. (1979), D’Ingeo et al. (1980), Hovland ( 198 l), Sore1 et al. (198 l), King et al. (1982), Le Pichon et al. (1982).
et al. (1977),
Makris
et al. (1977),
Petrov
(1977),
and Husebye
CONCLUSION
If the reader of this review has got the impression that many results are controversial or inconclusive, then this is a correct impression. While there is a fairly unanimous agreement about fundamental results, such as the trends of the seismic belts, the controversies increase steadily as we proceed to more complicated problems in order to culminate in the interpretation of the tectonics of the whole area. Greece
occupies
a crucial
point
in the Mediterranean
earthquake
belt and
its
194
wheel-shaped seismicity pattern acts as a “rolling hoop” structures to the west and east, combined with subduction Although practical
the present
reasons,
the research
review is limited
it is necessary
in all directions,
(1) All other branches
solution
the more linear
zones.
to the seismology
for a complete
especially
between
of Greece,
mainly
of the problems
for
to extend
as follows:
of geophysics.
such as gravity,
geomagnetics.
geothermy,
vulcanology. moreover geodesy and geology should be involved in addition seismology. The problems are too complex for seismology alone to solve them. (2) The whole
Mediterranean
belt should
be involved.
as forming
to
a unity,
of
which Greece is only a part, let it be of crucial significance. (3) International groups from advanced institutes all over the world are needed for the solution of the problems, in cooperation with each other and with Greek institutions. ACKNOWLEDGEMENTS
I am grateful to the Ministry of Coordination, Athens. for the inspiration to make this report, and to the Scientific Research and Technology Agency, Athens, for the interest and concern shown for this work. I also like to express my gratitude and appreciation to libraries, institutes and numerous individual authors who provided me with books, reports and reprints of papers, indispensable for the preparation of this review of the Greek seismology. Professor Seweryn J. Duda provided a useful review of the manuscript.
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