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Long-term earthquake prediction in the Hellenic trench-arc system
Tectonophysics, 86 (1982) 3- 16 Elsevier Scientific Publishing Company, Amsterdam-Printed
LONG-TERM
EARTHQUAKE
TRENCH-ARC
PREDICTION
3 in The Netherlands
IN THE
HELLENIC
SYSTEM
B.C. PAPAZACHOS and P.E. CO~~AKIS Geoph,&ul
Luhorcrtoty, Aristoteliun
Seismolagicul
Institute,
University of Thessoloniki,
Nutional Ohservuto~
Thessuloniki
(Greece]
of A them, A them (Greece)
(Received December 19, 198I)
ABSTRACT Papazachos, B.C. and Comninakis, P.E., 1982. Long-term earthquake prediction in the Hellenic trench-arc system. In: X. Le. Pichon, S.S. Augustithis and J. Mascle (Editors), Geodynamics of the Hellenic Arc and Trench. Tectonophysics, 86: 3- 16. Data concerning seismic sequences which have occurred in the Hellenic trench-sedimentary arc tectonic system have been used to investigate the time variation of the difference, D,, between the magnitude of the main shock and that of the largest aftershock. The data are well fitted by an harmonic function with period of 29 years for both the western seismic zone (Ionian islands-S Peloponnesus-W Crete) and the south-southeastern zone (Crete-Karpathos-Rhodos). It has been observedthat the largest earthquakes, in both zones, are associated with low values of the D,(t) function. Based on these observations, we suggest that a strong seismic activity with earthquakes of Ma7.0 will break out soon in the Hellenic trench-arc system. The largest of these earthquakes are expected between 1983 and 1987. A quiescence in seismicity which started fifteen years ago in the southern part of the western zone also indicates the generation of a very strong earthquake in this area during the next five years or so.
INTRODUCTION
Several types of seismicity patterns have been suggested as possible long-term of large earthquakes (Mogi, 1969; Kelleher et al., 1973; Wyss and Habermann, 1979; Yamashina, 1980; Keilis-Borok et al., 1980; Wyss et al., 1981). However, no premonitory seismicity regularity has been firmly established as yet. Since the earthquake prediction is a problem of primary importance for areas of high seismicity, such as Greece, it is necessary to search for additional seismicity patterns which could prove to be precursors to large earthquakes. Papazachos (1981) has shown that the time variation of the difference D,, between the magnitude, (M, 3 6.4) of the main shock and the magnitude, M,, of the largest aftershock can be considered as a premonitory pattern of large earthquakes (M Z=6.5) in the Hellenic trench-sedimentary arc system. In the present paper, this precursors
0040- 195l/82/~-~/$02.75
Q 1982 Elsevier Scientific Publishing Company
4
method-here
called
earthquakes quiescence western
D,(t)
with Ma
method-is
further
7.0 in this system.
is also applied
for long-term
developed
Furthermore, prediction
and applied the method
of large
to predict
of seismicity
earthquakes
in the
part of the area.
The Hellenic the whole
trench-arc
Mediterranean
system is seismically and
surrounding
the most active tectonic
area.
Geophysical
and
system in
morphological
data show that this system has the main properties of the well-known trench-arc systems (Papazachos and Comninakis, 1971; Comninakis and Papazachos, 1980). For this reason, this is considered as an area of interaction between the eastern Mediterranean lithosphere (front part of the African lithospheric plate) and the Aegean lithosphere (front part of the Eurasian lithospheric plate) where the former is subducted
under the latter in an about NNE direction
Papazachos
and Comninakis,
can consider
1978; Le Pichon
this area tectonically
homogeneous
(McKenzie,
and Angelier,
1970; 1972, 1978;
1979). Therefore,
to a considerable
we
degree.
HYPOTHESIS ON WHICH THE D,(r) METHOD IS BASED
The difference, D,, in magnitude between the main shallow shocks and their largest aftershocks in an area can vary largely and only its mean value is equal to about 1.2 as suggested by Bath (Richter, 1958). Papazachos (1974), for example, used the homogeneous and complete data of 2 19 seismic sequences in the Aegean area to show that D, is independent of Ma; it varies between 0.1 and 2.5 or more and has an average value equal to 1.1. Okada (1979), based on data concerning aftershock sequences in Japan and Greece,
has shown
that D, is related
to the parameter
b of the Gutenberg-Richter
magnitude-frequency relation. There are also several examples which show that D, is negatively correlated to b (when b decreases, D, increases). It is known, for example, (Magi,
that for earthquake
swarms
(0,
very small)
the value of b is very large
1963).
On the other hand, it has been shown that b depends
on the stress condition
and
on the homogeneity of the material in the focal region (Mogi, 1963; Scholz, 1968). This indicates that D, must also depend on the same factors. Plotting of D, on a map of the Aegean area shows some geographical dependence of D,, but this dependence is rather weak. We can conclude that D, depends mainly on the.stress conditions. Since stress in a region varies with time due to the generation of large earthquakes (build-up and release of stress) an analogous time variation, D,(t) must apply also to D,. Therefore, some relation must exist between D,(Z) and the time of occurrence of the large earthquakes in a tectonically homogeneous region. Such a relation can be used for the achievement of long-term prediction of large earthquakes. This is the basic hypothesis on which the D,(t) method is based.
5
DATA
USED TO DETERMINE
D,(r)
Table I, based on existing catalogues (Papazachos et al., 1967; Papazachos, 1975, 198Oa), contains all the data which have been used to determine D,(t) and its relation to the time of occurrence of the strong earthquakes in the Hellenic trench-sedimentary arc system. It includes parameters (dates, origin times, focal TABLE
I
Information between
on the main
shocks
and their largest
aftershocks
which
occurred
in the Hellenic
1920 and 1979 Origin
Date
time
WN)
YE)
h (km)
MO
Ml
1921, Sept. 13
08:59:53
38.7
21.2
n
6.0
5.4
1922, June
5
04:31:05
35.0
22.5
n
5.9
5.5
1922, Aug.
13
00:09:50
36.0
28.0
n
6.8
5.9
1922, Nov.
4
04:20: 12
37.0
20.5
n
5.9
5.6
1925, Feb.
7
12: 14:58
37.0
19.0
n
5.6
4.8
1925, Apr.
5
03:04:25
35.5
29.0
n
5.9
5.6
1926, Feb.
26
16:08:23
37.8
21.1
n
5.1
4.2 5.7
1926, Mar.
18
14:06:06
35.5
29.0
n
6.8
1926, June
26
19:46:34
36.5
27.5
100
8.0
1926, Aug.
30
11:38:12
36.8
23.3
120
1.2
1926, Sept. 19,
01:03:57
36.0
22.0
n
6.3
1927, Mar.
24
14:46:35
35.0
26.0
n
5.6
5.0
1927, June
30
22~59~36
38.7
21.0
n
5.6
1927, July
1
08: 18:54
36.7
22.7
4.5 _
1932, June
29
02:30:06
35.5
27.6
n
5.6
5.4
1932, Sept. 30
06:12:16
36.0
22.7
n
5.6
3.5
1932, Oct.
23
13:36:43
35.5
27.6
n
5.6
4.7
1934, Feb.
21
11:37:20
34.5
22.5
n
5.6
4.3
1938, Feb.
IO
20:37:53
34.8
26.2
n
5.6
3.8
1938, Mar.
13
17:45:32
38.8
20.6
n
5.8
3.8
1940, Feb,
29
16:07&l
35.5
25.5
n
6.0
3.8
1942, June
16
04:47:30
33.8
26.5
n
5.8
3.9
1942, Sept.
1
09:42: 15
36.4
27.4
n
5.8
4.0
1943, Feb.
14
07:28: 14
38.0
20.0
n
5.8
5.0
1943, June
27
10:05:27
35.0
26.0
n
5.1
3.9
1943. July
22
07:09:28
38.8
20.6
n
5.6
5.0
1944, May
27
23:52:25
36.0
27.5
n
6.2
4.8
80
7.1
5.3
1944, July
20
10:37:20
35.5
26.5
n
5.6
5.2
1944, July
30
04:00:35
36.7
22.5
n
5.8
4.6
1946, July
16
05~26~26
33.8
25.3
n
5.9
4.0
1947, June
1
11:18:35
36.6
21.5
n
5.7
4.2
1947, Aug.
30
22:21:31
35.1
23.4
n
6.3
5.0
1947, Sept.
13
15:11:17
37.4
20.0
n
5.6
4.9
1947, Oct.
6
19:55:34
36.9
22.0
n
7.0
5.0
trench
6
TABLE
I (continued)
Date
1948, Feb.
Origin
9
time
UN)
WE)
h (km)
b&LI,,
M,
12:58: 13
35.5
27.2
n
7.1
5.8
1948, Mar. 29
10:22:40
35.1
23.4
n
5.8
4.6
1948, Apr.
22
10:42:45
38.5
20.2
n
6.4
6.2
1948, Oct.
10
17:43:01
35.1
23.4
n
5.8
4.8
1951, Aug.
31
12~29~37
35.5
22.8
n
5.7
5.2
1952, Oct.
5
10:54:56
37.5
20.8
n
5.8
5.3
1952, Dec.
17
23:03:57
34.4
24.5
n
6.8
1953, Aug.
11
03:32:22
38.3
20.8
n
6.8
5.7 _
1953, Aug.
12
09:23:52
38.3
20.8
n
7.2
6.4
1954, Dec.
23
16:27: I8
37.9
21.1
n
5.8
5.5
1956, July
30
09: 14:57
35.9
26.0
n
6.0
5.3
1957, Feb.
19
07:43:59
36.2
21.6
n
5.9
1957, Apr.
24
19:10:17
36.4
28.6
n
6.9
4.5 _
1957, Apr.
25
02:25:42
36.5
28.6
n
7.2
6.3
1957, Oct.
30
01:42:59
35.2
27.2
n
5.6
5.4
1958, Jan.
2
02:08: 14
36.2
22.3
n
5.7
4.7
1958, Aug.
27
15:16:34
37.4
20.7
n
6.4
5.6
1959, May
14
06:36:56
35.1
24.6
n
6.5
5.9
1959, Nov.
15
17:08:43
37.8
20.5
n
6.8
6.1
1961, Oct.
2
07:21:39
36.6
21.9
n
5.7
4.2
1962, Jan.
26
08: 17:40
35.1
22.8
n
6.3
3.8
1962, Apr.
10
21:37:18
37.6
20.1
n
6.3
5.3
1962, Apr.
28
11:18:53
36.1
27.0
n
5.8
5.5
1962, July
6
09:16:15
38.0
20.2
n
6.1
4.6 3.6
1963, Dec.
16
13:47:59
37.3
20.9
n
5.9
1964, Aug.
25
11:11:49
35.6
28.8
n
5.8
5.2
1965, Apr.
5
03:12:50
37.4
21.9
n
6.1
4.0
1966, May
9
00:42:55
34.5
26.0
n
5.8
5.0
1966, Sept.
1
14:22:54
37.4
22.1
n
5.9
4.0
1968, Mar. 28
07:40:02
38.1
20.8
n
5.9
4.3
1968, May
30
17:40:25
35.4
28.0
n
6.0
5.6
1969, Jan.
14
23: 12:09
36.2
29.1
n
6.3
4.3 5.3
1969, May
1
18:02:16
35.3
27.8
n
5.6
1969, June
12
15:33:33
34.5
25.0
n
6.1
5.5
1969, July
8
08:09:15
37.6
20.3
n
6.0
4.8
1972, May
4
21:40:03
35.3
23.6
n
6.4
4.3
1972, Sept.
17
14:07:16
38.2
20.4
n
6.2
5.4
1973, Jan.
5
05:49: 17
35.1
21.9
n
5.7
4.8
1973, Nov.
4
15:52:14
38.8
20.6
n
5.9
4.9
1973, Nov. 29
10:57:42
35.2
23.6
n
6.0
4.4
1976, Jan
18
15:10:28
38.8
20.5
n
5.6
4.8
1976, May
11
16:59:45
37.4
20.4
n
6.5
5.7
1977, Sept. 11
23:19:19
34.9
23.0
n
6.2
5.0
1979, May
06:59:21
34.6
24.5
n
5.8
4.8
15
Fig.
I. Epicenters of the main shocks which occurred in the Hellenic trench-arc
-
-
-
system during
the period
1920- 1979.
8
WESTERN
ZONE
(IonIan- S. Peloponnesus- W. of Crete) l
M=7.2
I
I
1920
Expected Earthqake
M=7.2
1930
I
1940
I
1950
SOUTH-SOUTHEASTERN
I
1960
, I
I
1970
1
1980
1990
ZONE
(Crete -Karpdthos-Rhodes) l l
2.0 -
1.5 -
lo-
MO-Ml 0
M=7.2
M = 8.0
I
,
1920
Fig. 2. Time variation (dots=observed;
1930
I
1940
1950
and the times of expected
earthquakes system.
trench-arc
1960
1970
1m
1, 1990
in magnitude between the main shock and the largest
lines=calculated),
of the Hellenic
,
i
1
1
of the difference
sinusoidal
Expected Earthquake
the times of occurrence
of similar
magnitudes
of the earthquakes
in the western
aftershock
with Ma7.2
and south-southeastern
part
9
coordinates, magnitudes) of the main shallow shocks with MO ) 5.6, and the magnitudes of their largest aftershocks. It also includes parameters for all foreshocks and aftershocks with M> 6.5 and for all earthquakes of intermediate focal depth (h > 60 km) with M 3 7.0 which occurred in the Hellenic arc during the time period 1920- 1980. The errors in magnitude are in the range of kO.2. The time difference between the main shock and the largest aftershock is less than two months except for very few cases, while their epicenter separation is less than 50 km. The epicenters of the main shocks listed in Table1 are shown on the map of Fig. 1. Four symbols have been used to denote four magnitude and two focal depth ranges. Circles indicate epicenters of shallow (h < 60 km) main shocks and triangles epicenters of three intermediate (h = 80-120 km) shocks. Small, medium and large circles are used for shallow main earthquakes in the ranges 5.6-6.4, 6.5-7.1, and 7.2, respectively. The numbers included in the symbols of the earthquakes with M> 6.5 are the last two digits of their year of occurrence. A thick line separates the western (Ionian islands-S. Peloponnesus-W. of Crete) from the south-southeastern (Crete-Karpathos-Khodos) zone. This separation is based on several differences in their seismotectonic properties. The b value is relatively low (- 0.8) in the western zone and high (- 1.1) in the south-southeastern zone (Papazachos, 1980b). The western zone is characterized by thrust faulting, while the faulting is normal in the southern part of the other zone but it probably changes to strike-slip in its eastern part. Because of this last property we initially separated this zone in two parts but when we found a similar behavior of Dl(t) in both parts, we decided to consider the two parts as one zone for the purpose of the present paper. THE TIME PATTERN OF D, AND THE TIME OF OCCURRENCE QUAKES IN THE HELLENIC TRENCH-ARC
OF THE STRONG EARTH-
SYSTEM
To investigate the pattern of the time variation of the difference, D, = MO - M,, between the magnitude of the main shock and that of the largest aftershock, we only used differences with M, < 6.4 because of the fact that the generation of the larger earthquakes affects the stress field. However, calculation of D,(t) with different values for the upper bound of M,, shows that the pattern of D,(r) determined here is not sensitive to the upper bound of M, and therefore stability is secured. Values of the differences in magnitude between the main shocks and the largest aftershocks for M, G 6.4 are plotted versus time of occurrence in the upper and lower part of Fig. 2, for the western and south-southeastern zone, respectively. For the years in which two or more D, values are available, the average value is plotted. The data show almost harmonic variations for both zones. For this reason relations of the form: D,=E,
+D,sin
(F1fp)
(1)
are assumed to fit the data. fi, is the average of the D, values, D, is the mean of the four largest absolute values f), - n, for each zone, T is the period, and 9, is the phase determined in such a way as to have a best fit in the least-squares sense. The following two relations have been found, with the time t measured from the year 1920. Westernzone:
D, = 1.15 -l-O.96 sin z,-96
South-southeastern
zone: D, I= 1 .I0 + 0.95 sin gf
(2) - 174
(3)
in both cases the period is 29 years. The average values of D, for the two zones as well as the amplitudes, Do, are almost identical but there is a phase shift of 78”, which corresponds to a time difference of 6 years. The curves based on eqs. 2 and 3 are also drawn in Fig. 2. The root mean squares of the differences between the observed and calculated values of D, are equal to 0.32 and 0.55 for the western and south-southeastern zone, respectively. The fitting is satisfactory, if we consider the errors involved and the possibility that D, depends on other factors too. In Fig. 2, the times of occurrence of the largest shallow and intermediate earthquakes are shown. There are four earthquakes, two in each zone, which have magnitudes M ;2 7.2. They are repeated in each of the two zones approximately every thirty years and are clearly associated with the minimum values of L+(t) curves. Of these events, two were shallow with M = 7.2 and the first occurred in the western zone (Ionian islands, 1953) and the second in the south-southeastern zone (Dodecanese, 1957). Although, they are located almost at the end points of the Hellenic arc, their focal mechanisms are typical for the western (thrust-type) and eastern (strike-slip type) parts, respectively, of the convex side of the arc. In the western zone there is a delay in the time of occurrence of both earthquakes in respect to the corresponding ~nimum of the Dl(t) curve. The first earthquake (S Peloponnesus, August 30, 1926, M = 7.2) occurred 6.2 years after the first minimum and the second earthquake (Ionian islands, August 12, 1953, it4 = 7.2) occurred 4.1 years after the second minimum. The calculated O,(t) shows that its third minimum has been already reached in 1979 and, therefore, if the periodicity is to be repeated, the next earthquake with M 2 7.2 should be expected in the western zone between 1983 and 1985. Since we assume an error of the order of 0.2 in the magnitude we must examine the time of occurrence of the earthquakes with magnitudes 7.0 and 7.1 with respect to the minima of L),(t). During this period an intermediate earthquake of M = 7.1 occurred in July 1927, that is 7.1 years after the first minimum of Di(t > and a shallow earthquake of M = 7.0 in October 1947, that is I .7 years before the second minimum of Di( t). Therefore, even these earthquakes are associated with the minimum values of D,( t ). This is the reason why we included all intermediate earthquakes with M zz 7.0 in Table I. Furthermore, these earthquakes are produced
11
by the same mechanism (thrust) as the shallow earthquakes along. the convex side of the Hellenic arc. In the south-southeastern zone the times of occurrence of the two largest earthquakes are close to the times of the minima of the o,(t) curve. The first earthquake (Dodecanese, June 26, 1926, M = 8.0) occurred 0.2 years before the first minimum and the second earthquake (NE of Rhodos, April 25, 1957, M = 7.2) occurred 1.6 years after the second minimum. If periodicity is repeated, the third minimum will be in 1985 and the next earthquake with Ma 7.2 will occur in the south-southeastern zone (Crete-Karpathos-Rhodos) between 1985 and 1987. An earthquake of M = 7.1 occurred in 1948, that is 7.7 years before the second minimum of D,(r). If we take into consideration the above observations and hypotheses as well as the fact that in the past, these largest earthquakes were preceded and followed by other strong earthquakes (see Table I), we can conclude that a period of strong seismic activity of several years is expected to break out soon along the Hellenic arc-trench system (Ionian islands-S Peloponnesus-Crete-Rhodos) with earthquakes of magnitude up to 7.2 or larger during the period 1983-1987. It is understood that this result must be considered as a working hypothesis. Taking into account the observation that the largest earthquakes occur close to the minimum of the D,(t) curve we can assume that, while the subperiods (14.5 years) of increase of o,(t) are mainly times of stress build-up, the subperiods of decrease of D,(f) are mainly times of preparation for the generation of the largest earthquakes. QUIESCENCE OF SEISMICITY IN THE WESTERN ZONE
Seismicity quiescence, that is, decrease in the rate (number of earthquakes per year) of smallearthquakes, has been observed for some years prior to the occurrence of some strong earthquakes (Mogi, 1969; Wyss and Habermann, 1979). In order to identify such anomaly in a region it is useful to know the background seismicity rate. For this reason, Papazachos (1980b) divided the whole Aegean area in nineteen regions of constant background seismicity rates. Region 2 covers the northern part of the Hellenic trench (Ionian islands) and region 3 the southwestern part of the trench (S Peloponnesus-W of Crete). In region2 a quiescence period before the earthquake of August 12, 1953 (M = 7.2) and in region 3 a quiescence period before that of October 7, 1947 (M = 7.0) have been observed (Papazachos, 1980b). Based on the cumulative plots of the earthquakes with M > 4.9 which occurred between 1950 and 1977 in a region which approximately coincides with the region (3) Wyss and Baer (1981) observed a 80% drop in the seismicity rate during the period 1962-1977 with respect to the rate of the period 1950-1962 in the same region and to the rate in the adjacent eastern regions of the Hellenic arc. Further-
12
more, they have made the assumption-not -that
the great earthquakes
in the Hellenic
documented
in our opinion
arc, in both the present
sufficiently
and the previous
century,
which have been listed as intermediate
authors
(Gutenberg
shallow.
Based on their observations
that an earthquake
and
Richter,
of magnitude
focal depth
1948; Galanopoulos, and assumptions,
earthquakes
by several
1963; Karnik,
1971), are
Wyss and Baer concluded
7.75 * 0.5 is expected
in this region between
1980
and 1991. For the reasons time variation
mentioned
of the seismicity
above, it is of great interest
to investigate
rate in region 3 which is bounded
in detail the
on a map by four
straight lines defined by the points (36.0 N, 23.2 E), (35.0 N, 22.4 E), (36.7 N, 20.6 E) and (37.7 N, 21.4 E). Data used here have been taken from a catalogue of earthquakes for the Aegean area which is being prepared for publication by the present authors. This catalogue
TABLE
II
Catalogue
of the earthquakes
1919 and 1979. Information for the period
which occurred
in the southwestern
for the earthquakes
with M r4.9
part of the Hellenic
for the period
1919-1949
trench
between
and with
M 24.8
1950- 1979 is given.
Date
Origin
time
1919, Feb. 24
01:56:00
36.7
21.0
n
6.3
1919, Aug. 18
11:23:56
37.2
21.3
n
4.9
9 (“N)
X (‘E)
h (km)
M
1922, July
12
05: 12:Ol
37.3
21.5
n
4.9
1923, Apr.
3
07:07:20
37.2
21.1
n
4.9
1925. Oct.
13
09:29:39
36.7
22.0
n
5.0
1926. Sept. 19
01:03:57
36.0
22.0
70
6.3 5.3
1926, Sept. 19
14:37:48
36.0
22.0
70
I
16:50:50
37.0
21.7
n
5.1
1930, Sept. 23
20:35:38
36.7
22.0
n
4.9
1931, Nov. 23
23:32: IO
36.5
21.5
n
5.5
1932, Sept. 30
06:12:16
36.0
22.7
n
5.6
1936, Oct.
24
14:06: I5
36.0
22.7
n
5.3
1938, Nov.
4
03:50: 13
36.5
21.5
n
5.1
1944. July
30
04:00:35
36.7
22.5
n
5.9
II 1947, June I
17:05:30
36.7
22.0
n
5.2
I l:I8:35
36.6
21.5
n
5.8
1947, July
21
09:36:21
36.7
22.5
n
5.3
1947, Oct.
6
19:55:34
36.9
22.0
n
7.0
1947, Oct.
7
19:15:28
36.9
22.0
n
5.0
1948, July
22
06:46:55
36.9
22.0
n
5.1
1948. Sept. 21
17:53:53
36.6
21.5
n
5.5
1928, July
1947, Mar.
1951, May 30
13:43:49
35.5
22.R
n
4.8
1951. Aug. 24
10:27:29
36.9
21.6
”
5.4
1951, Aug. 31
12:29:37
35.5
22.8
”
5.6
13
TABLE
II (continued) Origin
Date
time
+ (“N)
A (‘E)
h (km)
M
1951, Aug. 31
20: 18:34
35.5
22.8
n
5.3
1952, Sept.
2
23~20107
37.2
21.6
n
4.9
1953, Feb.
5
22:42:05
35.7
22.7
n
5.2
1953, Mar. 30
13:54:43
37.6
21.3
n
4.9
5
19:34:15
37.3
21.0
n
4.8
1954, Mar. 30
04:30:35
36.7
22.2
n
4.9
1954, May
3
05~24~57
36.0
22.0
n
5.1
1954, May
3
08:51:19
36.0
22.0
n
5.2
1954, May 15
12:24:34
36.2
21.7
n
5.1
1953, Dec.
1954, Dec. 30
11:05:56
36.1
21.7
n
5.4
1956, Aug. 15
14:38:06
36.0
21.7
n
5.1
1956, Aug. 16
00:38:34
36.2
21.8
n
5.4
1957, Jan.
23
17:26:51
36.7
21.6
n
5.3
1957, Feb,
19
07:43:59
36.2
21.6
n
5.9
1958, Jan.
2
02:08: 14
36.2
22.3
!I
5.7 5.2
1958, May
3
20: 18:20
36.2
21.7
n
1959, Jan.
9
01:55:03
36.1
21.8
n
5.2
1959, Dec. 23
21:39:14
36.5
21.6
n
5.0 4.9
1961, Feb. 21
03:01:51
36.3
23.0
n
1961, Oct.
2
07:21:39
36.6
21.9
n
5.7
1962, Jan.
IO
12:36:33
35.8
22.5
90
4.9
1963, Jan.
31
15:07:04
35.9
21.9
60
5.1
1963, Dec. 16
13:47:53
37.0
21.0
”
5.9
1965, May 29
01~47~48
35.2
22.6
56
5.0
1965, May 29
04: 16:56
35.2
22.6
43
4.3 5.2
1966, May 24
09~39126
37.3
21.9
34
1966, June
4
06: 16:57
36.6
21.0
82
5.0
1966, June
I1
12:05:03
37.4
21.1
47
4.8
1966, July
12
02:56:22
35.5
22.5
7
5.2
1967, Feb. 28
14:21:51
37.5
21.2
46
4.8
1967, July
5
00:53:17
36.7
21.5
50
5.0
1968, Jan.
9
23:15:43
i5.5
22.5
46
4.8
1972, Mar. 3 I
02:58:01
36.4
21.3
20
4.8
1973, Jan.
5
05:49:18
35.8
21.9
34
5.6
1973, Jan.
10
03:24: 12
37.7
21.4
45
5.1
1973, Jan.
26
07:50:11
35.7
22.
I
41
5.0
1973, Mar. 12
20:30:44
35.9
21.8
45
4.9
1975, Sept. 12
13: IO:20
36.3
21.9
43
5.4
1977, July
30
19:51:37
36.8
21.6
49
5.0
1978, Jan.
4.8
12
20:08:38
35.8
22.3
59
1978, Mar. 16
05:51:56
36.7
21.6
49
5.1
1979, Mar. 26
08:05:58
37.6
21.4
n
4.8
1979, May 27
03:20:50
36.6
21.7
n
4.8
13:34:38
37.2
21.7
n
5.1
1979, Dec.
1
14
will contain homogeneous data, based on already published catalogues of the present authors as well as on new epicenter and magnitude determinations. Table II includes all earthquakes
with Ma
4.9 which occurred
all those with M 3 4.8 which occurred The frequency
distribution
the corresponding The cumulative
in region 3 between
between
of the magnitudes
1950 and
1919 and 1980 and
1980 in the same region.
shows that these data are complete
for
periods. number
of the earthquakes
with M 2 4.9 as a function
of time is
shown in Fig. 3. Aftershocks and foreshocks have not been plotted but the result remains the same if we plot these shocks too. Four subperiods with different rates (number rates
of earthquakes
are also shown
with M 2 4.9 per year) are observed. in the same figure.
The second
These subperiods
subperiod
( 1932
and
1947) is a
14.4-years period of quiescence which preceded the earthquake of October 6, 1947 with M= 7.0. Another quiescence period started in 1967 and it continues till now. This is an additional evidence that seismic activity to occur in the next few years.
of large earthquakes
is expected
Therefore, our data agree with Wyss and Baer’s result that this region is at quiescence now, but their plots indicate that this quiescence started in 1962. To check this point further, we made several cumulative plots with varying lower
Region
3
(S. Peloponnesus - W. of Crete) 50 Period
40 -
dN/dt
1919 -1932
:
0.6
1932-1947
: : :
0.3
1947-1967 1967-1980 30-
M ~4.9
I
1920
Fig. 3. Time variation southwestern
1.2 0.5
I
1930
I
L
I
I
I
I
1940
1950
1960
1970
1980
1990
of the yearly
part (S Peloponnesus-W
number
of earthquakes
(cumulative
of Crete} of the Hellenic trench-arc
plot) with Ma4.9 system.
in the
magnitude
values. Some of these plots indicate
recognized observed
as having
started
earlier
but
that some decrease
the most
prominent
of the rate can be decrease
is always
after 1967.
It is probably
interesting
1967- 1981) almost coincide zone (Fig. 2). Although
the
to note
both
quiescence
periods
of decrease of
(1932-1947,
D, in the western
D,(t) is referred to a broader zone than the zone which is
at quiescence
now, this observation
evidence
the subperiods
that
that
with the two subperiods
can be probably
generation of very strong earthquakes. A clear discontinuity of the western
considered
as an additional
D, are times of preparation
of decreasing
seismic zone is observed
for the
(Fig. 1) in the area
southwest of Peloponnesus (around 37ON, 21’E). It is known, however, that a great destructive earthquake occurred in this area in 1886.Therefore, this area can be considered as a “seismic gap” and it is probable that one of the expected large (M 2 7.0) earthquakes will occur there. REFERENCES
Comninakis,
P.E. and Papazachos,
earthquakes
in the Hellenic
Galanopoulos, Gutenberg,
1963. On mapping
B. and Richter,
Appl., Kamik,
A.G.,
CF.,
of seismic activity
1948. Deep-focus
of the intermediate
focal depth
70: 35-47. in Greece.
earthquakes
Ann. Geofis.,
16: 37- 100.
in the Mediterranean
region. Geofis.
Pura
12 (3-4).
V., I97 I. Seismicity
Keilis-Borok,
of the European
V.J., Knopoff,
strong earthquakes. Kelleher,
B.C., 1980. Space and time distribution
arc. Tectonophysics,
Area, Part 2. Reidel, Dordrecht,
L. and Rotvain,
Nature,
Le Pichon,
to major plate boundaries
X. and Angelier,
evolution
of the eastern
long-term
precursors
of
283: 259-263.
J., Sykes, L. and Oliver, J., 1973. Possible criteria
application
2 I8 pp.
I.M., 1980. Bursts of aftershocks,
J., 1979. The Hellenic Mediterranean
D.P., 1970. The plate tectonics
McKenzie,
D.P.,
1972. Active
D.P.,
1978. Active tectonics
of the Mediterranean of the Mediterranean
earthquake
J. Geophys.
arc and trench
area. Tectonophysics,
McKenzie,
tectonics
for predicting
of the Pacific and Caribbean.
system:
locations
and their
Res., 78: 2547-2585.
a key to the neotectonic
60: I-42. region. region.
Nature,
226: 239-243.
Geophys.
J. R. Astron.
Sot.. 30:
109-185. McKenzie,
region. Geophys.
J. R. Astron.
Mogi, K., 1963. Some discussions semi-infinite Earthquake
aftershock.
Papazachos,
foreshocks
and earthquake
swarms,
to the earthquake
the fracture
of a
phenomena.
Bull.
B.C.,
of recent seismic activity
in and near Japan
(2). Activity
before and after
Res. Inst. Univ. Tokyo, 47: I I72- I 186.
distribution
of the difference
in magnitude
between
the main shock and its
J. Seismol. Sot. Jpn., 32: 462-476.
1974. On certain
aftershock
and foreshock
parameters
in the area of Greece.
Ann.
27: 497-515.
Papazachos,
B.C., 1975. Aftershock
I91 I-1973. Papazachos,
on aftershocks,
Bull. Earthquake
M., 1979. Statistical
Geofis.,
Sea and surrounding
Res. Inst., 41: 615-658.
great earthquakes. largest
belt: the Aegean
body caused by an inner stress origin and its relation
Mogi, K., 1969. Some features Okada,
of the Alpine-Himalayan
Sot., 55: 217-254.
and foreshock
sequences
in the area of Greece
during
the period
sequences
in the area of Greece
during
the period
Bull. Sci. Cr. Space Res., 3: l-44.
B.C., 1980a. Aftershock
1974- 1979. Publ. Geofis.
and foreshock
Lab. Univ. Thessaloniki,
IO: I-IS.
16
Papazachos, Geod.,
B.C., 1980b. Seismicity
Papazachos,
B.C., 198 1. The time variation
largest aftershock Papazachos,
Papazachos,
pattern
in magnitude
of strong
P.E., 1971. Geophysical
in the Aegean
between
earthquakes.
and Tectonic
P.E., 1978. Deep structure
Delibasis,
N.,
Liapis,
of some large earthquakes
C.F., 1958. Elementary C.H.,
area. Quat.
the main shock and its
Quat. Geod.,
features
N.,
Seismology.
2: 11 l-l 17.
of the Aegean
arc. J.
of the eastern
Mediterranean.
Moumoulidis,
Freeman,
G.
and
San Francisco,
relation
Purcaru,
Ann. Geofis..
G.,
1967. Aftershock
20: l-93.
Calif.. p. 69.
of microfracturing
in rock
and its relation
to
Bull. Seismoi. Sot. Am., 58: 399-415.
K., 1980. Some empirical
rules on foreshocks
M. and Baer, M., 1981. Seismic
earthquakes.
Nature,
M., Klein,
F.W.
and earthquake
in the western
R.E., 1979. Seismic quiescence
Pure Appl. Geophys.,
M= 7.2 earthquake.
quiescence
prediction.
Hellenic
Manuscript.
arc may foreshadow
large
289: 785-787.
Wyss, M. and Habermann, earthquake.
and tectonics
in the region of Greece,
1968. The frequency-magnitude
earthquakes.
Wyss,
prediction
46: 285-296.
B.C.,
sequences
Wyss,
of the difference
premonitory
B.C. and Comninakis,
Tectonophysics,
Yamashina,
earthquake
Res., 76: 8517-8533.
Papazachos,
Scholz,
as long-term
B.C. and Comninakis,
Geophys.
Richter,
rates and long-term
3: 171-190.
and Johnston, Nature,
117: 1195-121 A.C.,
289: 23 I-234.
precursory
to a past and a future Kurile island
I.
1981. Seismicity
pattern
precursory
to the 1975 Hawaii
LONG-TERM
EARTHQUAKE
TRENCH-ARC
PREDICTION
3 in The Netherlands
IN THE
HELLENIC
SYSTEM
B.C. PAPAZACHOS and P.E. CO~~AKIS Geoph,&ul
Luhorcrtoty, Aristoteliun
Seismolagicul
Institute,
University of Thessoloniki,
Nutional Ohservuto~
Thessuloniki
(Greece]
of A them, A them (Greece)
(Received December 19, 198I)
ABSTRACT Papazachos, B.C. and Comninakis, P.E., 1982. Long-term earthquake prediction in the Hellenic trench-arc system. In: X. Le. Pichon, S.S. Augustithis and J. Mascle (Editors), Geodynamics of the Hellenic Arc and Trench. Tectonophysics, 86: 3- 16. Data concerning seismic sequences which have occurred in the Hellenic trench-sedimentary arc tectonic system have been used to investigate the time variation of the difference, D,, between the magnitude of the main shock and that of the largest aftershock. The data are well fitted by an harmonic function with period of 29 years for both the western seismic zone (Ionian islands-S Peloponnesus-W Crete) and the south-southeastern zone (Crete-Karpathos-Rhodos). It has been observedthat the largest earthquakes, in both zones, are associated with low values of the D,(t) function. Based on these observations, we suggest that a strong seismic activity with earthquakes of Ma7.0 will break out soon in the Hellenic trench-arc system. The largest of these earthquakes are expected between 1983 and 1987. A quiescence in seismicity which started fifteen years ago in the southern part of the western zone also indicates the generation of a very strong earthquake in this area during the next five years or so.
INTRODUCTION
Several types of seismicity patterns have been suggested as possible long-term of large earthquakes (Mogi, 1969; Kelleher et al., 1973; Wyss and Habermann, 1979; Yamashina, 1980; Keilis-Borok et al., 1980; Wyss et al., 1981). However, no premonitory seismicity regularity has been firmly established as yet. Since the earthquake prediction is a problem of primary importance for areas of high seismicity, such as Greece, it is necessary to search for additional seismicity patterns which could prove to be precursors to large earthquakes. Papazachos (1981) has shown that the time variation of the difference D,, between the magnitude, (M, 3 6.4) of the main shock and the magnitude, M,, of the largest aftershock can be considered as a premonitory pattern of large earthquakes (M Z=6.5) in the Hellenic trench-sedimentary arc system. In the present paper, this precursors
0040- 195l/82/~-~/$02.75
Q 1982 Elsevier Scientific Publishing Company
4
method-here
called
earthquakes quiescence western
D,(t)
with Ma
method-is
further
7.0 in this system.
is also applied
for long-term
developed
Furthermore, prediction
and applied the method
of large
to predict
of seismicity
earthquakes
in the
part of the area.
The Hellenic the whole
trench-arc
Mediterranean
system is seismically and
surrounding
the most active tectonic
area.
Geophysical
and
system in
morphological
data show that this system has the main properties of the well-known trench-arc systems (Papazachos and Comninakis, 1971; Comninakis and Papazachos, 1980). For this reason, this is considered as an area of interaction between the eastern Mediterranean lithosphere (front part of the African lithospheric plate) and the Aegean lithosphere (front part of the Eurasian lithospheric plate) where the former is subducted
under the latter in an about NNE direction
Papazachos
and Comninakis,
can consider
1978; Le Pichon
this area tectonically
homogeneous
(McKenzie,
and Angelier,
1970; 1972, 1978;
1979). Therefore,
to a considerable
we
degree.
HYPOTHESIS ON WHICH THE D,(r) METHOD IS BASED
The difference, D,, in magnitude between the main shallow shocks and their largest aftershocks in an area can vary largely and only its mean value is equal to about 1.2 as suggested by Bath (Richter, 1958). Papazachos (1974), for example, used the homogeneous and complete data of 2 19 seismic sequences in the Aegean area to show that D, is independent of Ma; it varies between 0.1 and 2.5 or more and has an average value equal to 1.1. Okada (1979), based on data concerning aftershock sequences in Japan and Greece,
has shown
that D, is related
to the parameter
b of the Gutenberg-Richter
magnitude-frequency relation. There are also several examples which show that D, is negatively correlated to b (when b decreases, D, increases). It is known, for example, (Magi,
that for earthquake
swarms
(0,
very small)
the value of b is very large
1963).
On the other hand, it has been shown that b depends
on the stress condition
and
on the homogeneity of the material in the focal region (Mogi, 1963; Scholz, 1968). This indicates that D, must also depend on the same factors. Plotting of D, on a map of the Aegean area shows some geographical dependence of D,, but this dependence is rather weak. We can conclude that D, depends mainly on the.stress conditions. Since stress in a region varies with time due to the generation of large earthquakes (build-up and release of stress) an analogous time variation, D,(t) must apply also to D,. Therefore, some relation must exist between D,(Z) and the time of occurrence of the large earthquakes in a tectonically homogeneous region. Such a relation can be used for the achievement of long-term prediction of large earthquakes. This is the basic hypothesis on which the D,(t) method is based.
5
DATA
USED TO DETERMINE
D,(r)
Table I, based on existing catalogues (Papazachos et al., 1967; Papazachos, 1975, 198Oa), contains all the data which have been used to determine D,(t) and its relation to the time of occurrence of the strong earthquakes in the Hellenic trench-sedimentary arc system. It includes parameters (dates, origin times, focal TABLE
I
Information between
on the main
shocks
and their largest
aftershocks
which
occurred
in the Hellenic
1920 and 1979 Origin
Date
time
WN)
YE)
h (km)
MO
Ml
1921, Sept. 13
08:59:53
38.7
21.2
n
6.0
5.4
1922, June
5
04:31:05
35.0
22.5
n
5.9
5.5
1922, Aug.
13
00:09:50
36.0
28.0
n
6.8
5.9
1922, Nov.
4
04:20: 12
37.0
20.5
n
5.9
5.6
1925, Feb.
7
12: 14:58
37.0
19.0
n
5.6
4.8
1925, Apr.
5
03:04:25
35.5
29.0
n
5.9
5.6
1926, Feb.
26
16:08:23
37.8
21.1
n
5.1
4.2 5.7
1926, Mar.
18
14:06:06
35.5
29.0
n
6.8
1926, June
26
19:46:34
36.5
27.5
100
8.0
1926, Aug.
30
11:38:12
36.8
23.3
120
1.2
1926, Sept. 19,
01:03:57
36.0
22.0
n
6.3
1927, Mar.
24
14:46:35
35.0
26.0
n
5.6
5.0
1927, June
30
22~59~36
38.7
21.0
n
5.6
1927, July
1
08: 18:54
36.7
22.7
4.5 _
1932, June
29
02:30:06
35.5
27.6
n
5.6
5.4
1932, Sept. 30
06:12:16
36.0
22.7
n
5.6
3.5
1932, Oct.
23
13:36:43
35.5
27.6
n
5.6
4.7
1934, Feb.
21
11:37:20
34.5
22.5
n
5.6
4.3
1938, Feb.
IO
20:37:53
34.8
26.2
n
5.6
3.8
1938, Mar.
13
17:45:32
38.8
20.6
n
5.8
3.8
1940, Feb,
29
16:07&l
35.5
25.5
n
6.0
3.8
1942, June
16
04:47:30
33.8
26.5
n
5.8
3.9
1942, Sept.
1
09:42: 15
36.4
27.4
n
5.8
4.0
1943, Feb.
14
07:28: 14
38.0
20.0
n
5.8
5.0
1943, June
27
10:05:27
35.0
26.0
n
5.1
3.9
1943. July
22
07:09:28
38.8
20.6
n
5.6
5.0
1944, May
27
23:52:25
36.0
27.5
n
6.2
4.8
80
7.1
5.3
1944, July
20
10:37:20
35.5
26.5
n
5.6
5.2
1944, July
30
04:00:35
36.7
22.5
n
5.8
4.6
1946, July
16
05~26~26
33.8
25.3
n
5.9
4.0
1947, June
1
11:18:35
36.6
21.5
n
5.7
4.2
1947, Aug.
30
22:21:31
35.1
23.4
n
6.3
5.0
1947, Sept.
13
15:11:17
37.4
20.0
n
5.6
4.9
1947, Oct.
6
19:55:34
36.9
22.0
n
7.0
5.0
trench
6
TABLE
I (continued)
Date
1948, Feb.
Origin
9
time
UN)
WE)
h (km)
b&LI,,
M,
12:58: 13
35.5
27.2
n
7.1
5.8
1948, Mar. 29
10:22:40
35.1
23.4
n
5.8
4.6
1948, Apr.
22
10:42:45
38.5
20.2
n
6.4
6.2
1948, Oct.
10
17:43:01
35.1
23.4
n
5.8
4.8
1951, Aug.
31
12~29~37
35.5
22.8
n
5.7
5.2
1952, Oct.
5
10:54:56
37.5
20.8
n
5.8
5.3
1952, Dec.
17
23:03:57
34.4
24.5
n
6.8
1953, Aug.
11
03:32:22
38.3
20.8
n
6.8
5.7 _
1953, Aug.
12
09:23:52
38.3
20.8
n
7.2
6.4
1954, Dec.
23
16:27: I8
37.9
21.1
n
5.8
5.5
1956, July
30
09: 14:57
35.9
26.0
n
6.0
5.3
1957, Feb.
19
07:43:59
36.2
21.6
n
5.9
1957, Apr.
24
19:10:17
36.4
28.6
n
6.9
4.5 _
1957, Apr.
25
02:25:42
36.5
28.6
n
7.2
6.3
1957, Oct.
30
01:42:59
35.2
27.2
n
5.6
5.4
1958, Jan.
2
02:08: 14
36.2
22.3
n
5.7
4.7
1958, Aug.
27
15:16:34
37.4
20.7
n
6.4
5.6
1959, May
14
06:36:56
35.1
24.6
n
6.5
5.9
1959, Nov.
15
17:08:43
37.8
20.5
n
6.8
6.1
1961, Oct.
2
07:21:39
36.6
21.9
n
5.7
4.2
1962, Jan.
26
08: 17:40
35.1
22.8
n
6.3
3.8
1962, Apr.
10
21:37:18
37.6
20.1
n
6.3
5.3
1962, Apr.
28
11:18:53
36.1
27.0
n
5.8
5.5
1962, July
6
09:16:15
38.0
20.2
n
6.1
4.6 3.6
1963, Dec.
16
13:47:59
37.3
20.9
n
5.9
1964, Aug.
25
11:11:49
35.6
28.8
n
5.8
5.2
1965, Apr.
5
03:12:50
37.4
21.9
n
6.1
4.0
1966, May
9
00:42:55
34.5
26.0
n
5.8
5.0
1966, Sept.
1
14:22:54
37.4
22.1
n
5.9
4.0
1968, Mar. 28
07:40:02
38.1
20.8
n
5.9
4.3
1968, May
30
17:40:25
35.4
28.0
n
6.0
5.6
1969, Jan.
14
23: 12:09
36.2
29.1
n
6.3
4.3 5.3
1969, May
1
18:02:16
35.3
27.8
n
5.6
1969, June
12
15:33:33
34.5
25.0
n
6.1
5.5
1969, July
8
08:09:15
37.6
20.3
n
6.0
4.8
1972, May
4
21:40:03
35.3
23.6
n
6.4
4.3
1972, Sept.
17
14:07:16
38.2
20.4
n
6.2
5.4
1973, Jan.
5
05:49: 17
35.1
21.9
n
5.7
4.8
1973, Nov.
4
15:52:14
38.8
20.6
n
5.9
4.9
1973, Nov. 29
10:57:42
35.2
23.6
n
6.0
4.4
1976, Jan
18
15:10:28
38.8
20.5
n
5.6
4.8
1976, May
11
16:59:45
37.4
20.4
n
6.5
5.7
1977, Sept. 11
23:19:19
34.9
23.0
n
6.2
5.0
1979, May
06:59:21
34.6
24.5
n
5.8
4.8
15
Fig.
I. Epicenters of the main shocks which occurred in the Hellenic trench-arc
-
-
-
system during
the period
1920- 1979.
8
WESTERN
ZONE
(IonIan- S. Peloponnesus- W. of Crete) l
M=7.2
I
I
1920
Expected Earthqake
M=7.2
1930
I
1940
I
1950
SOUTH-SOUTHEASTERN
I
1960
, I
I
1970
1
1980
1990
ZONE
(Crete -Karpdthos-Rhodes) l l
2.0 -
1.5 -
lo-
MO-Ml 0
M=7.2
M = 8.0
I
,
1920
Fig. 2. Time variation (dots=observed;
1930
I
1940
1950
and the times of expected
earthquakes system.
trench-arc
1960
1970
1m
1, 1990
in magnitude between the main shock and the largest
lines=calculated),
of the Hellenic
,
i
1
1
of the difference
sinusoidal
Expected Earthquake
the times of occurrence
of similar
magnitudes
of the earthquakes
in the western
aftershock
with Ma7.2
and south-southeastern
part
9
coordinates, magnitudes) of the main shallow shocks with MO ) 5.6, and the magnitudes of their largest aftershocks. It also includes parameters for all foreshocks and aftershocks with M> 6.5 and for all earthquakes of intermediate focal depth (h > 60 km) with M 3 7.0 which occurred in the Hellenic arc during the time period 1920- 1980. The errors in magnitude are in the range of kO.2. The time difference between the main shock and the largest aftershock is less than two months except for very few cases, while their epicenter separation is less than 50 km. The epicenters of the main shocks listed in Table1 are shown on the map of Fig. 1. Four symbols have been used to denote four magnitude and two focal depth ranges. Circles indicate epicenters of shallow (h < 60 km) main shocks and triangles epicenters of three intermediate (h = 80-120 km) shocks. Small, medium and large circles are used for shallow main earthquakes in the ranges 5.6-6.4, 6.5-7.1, and 7.2, respectively. The numbers included in the symbols of the earthquakes with M> 6.5 are the last two digits of their year of occurrence. A thick line separates the western (Ionian islands-S. Peloponnesus-W. of Crete) from the south-southeastern (Crete-Karpathos-Khodos) zone. This separation is based on several differences in their seismotectonic properties. The b value is relatively low (- 0.8) in the western zone and high (- 1.1) in the south-southeastern zone (Papazachos, 1980b). The western zone is characterized by thrust faulting, while the faulting is normal in the southern part of the other zone but it probably changes to strike-slip in its eastern part. Because of this last property we initially separated this zone in two parts but when we found a similar behavior of Dl(t) in both parts, we decided to consider the two parts as one zone for the purpose of the present paper. THE TIME PATTERN OF D, AND THE TIME OF OCCURRENCE QUAKES IN THE HELLENIC TRENCH-ARC
OF THE STRONG EARTH-
SYSTEM
To investigate the pattern of the time variation of the difference, D, = MO - M,, between the magnitude of the main shock and that of the largest aftershock, we only used differences with M, < 6.4 because of the fact that the generation of the larger earthquakes affects the stress field. However, calculation of D,(t) with different values for the upper bound of M,, shows that the pattern of D,(r) determined here is not sensitive to the upper bound of M, and therefore stability is secured. Values of the differences in magnitude between the main shocks and the largest aftershocks for M, G 6.4 are plotted versus time of occurrence in the upper and lower part of Fig. 2, for the western and south-southeastern zone, respectively. For the years in which two or more D, values are available, the average value is plotted. The data show almost harmonic variations for both zones. For this reason relations of the form: D,=E,
+D,sin
(F1fp)
(1)
are assumed to fit the data. fi, is the average of the D, values, D, is the mean of the four largest absolute values f), - n, for each zone, T is the period, and 9, is the phase determined in such a way as to have a best fit in the least-squares sense. The following two relations have been found, with the time t measured from the year 1920. Westernzone:
D, = 1.15 -l-O.96 sin z,-96
South-southeastern
zone: D, I= 1 .I0 + 0.95 sin gf
(2) - 174
(3)
in both cases the period is 29 years. The average values of D, for the two zones as well as the amplitudes, Do, are almost identical but there is a phase shift of 78”, which corresponds to a time difference of 6 years. The curves based on eqs. 2 and 3 are also drawn in Fig. 2. The root mean squares of the differences between the observed and calculated values of D, are equal to 0.32 and 0.55 for the western and south-southeastern zone, respectively. The fitting is satisfactory, if we consider the errors involved and the possibility that D, depends on other factors too. In Fig. 2, the times of occurrence of the largest shallow and intermediate earthquakes are shown. There are four earthquakes, two in each zone, which have magnitudes M ;2 7.2. They are repeated in each of the two zones approximately every thirty years and are clearly associated with the minimum values of L+(t) curves. Of these events, two were shallow with M = 7.2 and the first occurred in the western zone (Ionian islands, 1953) and the second in the south-southeastern zone (Dodecanese, 1957). Although, they are located almost at the end points of the Hellenic arc, their focal mechanisms are typical for the western (thrust-type) and eastern (strike-slip type) parts, respectively, of the convex side of the arc. In the western zone there is a delay in the time of occurrence of both earthquakes in respect to the corresponding ~nimum of the Dl(t) curve. The first earthquake (S Peloponnesus, August 30, 1926, M = 7.2) occurred 6.2 years after the first minimum and the second earthquake (Ionian islands, August 12, 1953, it4 = 7.2) occurred 4.1 years after the second minimum. The calculated O,(t) shows that its third minimum has been already reached in 1979 and, therefore, if the periodicity is to be repeated, the next earthquake with M 2 7.2 should be expected in the western zone between 1983 and 1985. Since we assume an error of the order of 0.2 in the magnitude we must examine the time of occurrence of the earthquakes with magnitudes 7.0 and 7.1 with respect to the minima of L),(t). During this period an intermediate earthquake of M = 7.1 occurred in July 1927, that is 7.1 years after the first minimum of Di(t > and a shallow earthquake of M = 7.0 in October 1947, that is I .7 years before the second minimum of Di( t). Therefore, even these earthquakes are associated with the minimum values of D,( t ). This is the reason why we included all intermediate earthquakes with M zz 7.0 in Table I. Furthermore, these earthquakes are produced
11
by the same mechanism (thrust) as the shallow earthquakes along. the convex side of the Hellenic arc. In the south-southeastern zone the times of occurrence of the two largest earthquakes are close to the times of the minima of the o,(t) curve. The first earthquake (Dodecanese, June 26, 1926, M = 8.0) occurred 0.2 years before the first minimum and the second earthquake (NE of Rhodos, April 25, 1957, M = 7.2) occurred 1.6 years after the second minimum. If periodicity is repeated, the third minimum will be in 1985 and the next earthquake with Ma 7.2 will occur in the south-southeastern zone (Crete-Karpathos-Rhodos) between 1985 and 1987. An earthquake of M = 7.1 occurred in 1948, that is 7.7 years before the second minimum of D,(r). If we take into consideration the above observations and hypotheses as well as the fact that in the past, these largest earthquakes were preceded and followed by other strong earthquakes (see Table I), we can conclude that a period of strong seismic activity of several years is expected to break out soon along the Hellenic arc-trench system (Ionian islands-S Peloponnesus-Crete-Rhodos) with earthquakes of magnitude up to 7.2 or larger during the period 1983-1987. It is understood that this result must be considered as a working hypothesis. Taking into account the observation that the largest earthquakes occur close to the minimum of the D,(t) curve we can assume that, while the subperiods (14.5 years) of increase of o,(t) are mainly times of stress build-up, the subperiods of decrease of D,(f) are mainly times of preparation for the generation of the largest earthquakes. QUIESCENCE OF SEISMICITY IN THE WESTERN ZONE
Seismicity quiescence, that is, decrease in the rate (number of earthquakes per year) of smallearthquakes, has been observed for some years prior to the occurrence of some strong earthquakes (Mogi, 1969; Wyss and Habermann, 1979). In order to identify such anomaly in a region it is useful to know the background seismicity rate. For this reason, Papazachos (1980b) divided the whole Aegean area in nineteen regions of constant background seismicity rates. Region 2 covers the northern part of the Hellenic trench (Ionian islands) and region 3 the southwestern part of the trench (S Peloponnesus-W of Crete). In region2 a quiescence period before the earthquake of August 12, 1953 (M = 7.2) and in region 3 a quiescence period before that of October 7, 1947 (M = 7.0) have been observed (Papazachos, 1980b). Based on the cumulative plots of the earthquakes with M > 4.9 which occurred between 1950 and 1977 in a region which approximately coincides with the region (3) Wyss and Baer (1981) observed a 80% drop in the seismicity rate during the period 1962-1977 with respect to the rate of the period 1950-1962 in the same region and to the rate in the adjacent eastern regions of the Hellenic arc. Further-
12
more, they have made the assumption-not -that
the great earthquakes
in the Hellenic
documented
in our opinion
arc, in both the present
sufficiently
and the previous
century,
which have been listed as intermediate
authors
(Gutenberg
shallow.
Based on their observations
that an earthquake
and
Richter,
of magnitude
focal depth
1948; Galanopoulos, and assumptions,
earthquakes
by several
1963; Karnik,
1971), are
Wyss and Baer concluded
7.75 * 0.5 is expected
in this region between
1980
and 1991. For the reasons time variation
mentioned
of the seismicity
above, it is of great interest
to investigate
rate in region 3 which is bounded
in detail the
on a map by four
straight lines defined by the points (36.0 N, 23.2 E), (35.0 N, 22.4 E), (36.7 N, 20.6 E) and (37.7 N, 21.4 E). Data used here have been taken from a catalogue of earthquakes for the Aegean area which is being prepared for publication by the present authors. This catalogue
TABLE
II
Catalogue
of the earthquakes
1919 and 1979. Information for the period
which occurred
in the southwestern
for the earthquakes
with M r4.9
part of the Hellenic
for the period
1919-1949
trench
between
and with
M 24.8
1950- 1979 is given.
Date
Origin
time
1919, Feb. 24
01:56:00
36.7
21.0
n
6.3
1919, Aug. 18
11:23:56
37.2
21.3
n
4.9
9 (“N)
X (‘E)
h (km)
M
1922, July
12
05: 12:Ol
37.3
21.5
n
4.9
1923, Apr.
3
07:07:20
37.2
21.1
n
4.9
1925. Oct.
13
09:29:39
36.7
22.0
n
5.0
1926. Sept. 19
01:03:57
36.0
22.0
70
6.3 5.3
1926, Sept. 19
14:37:48
36.0
22.0
70
I
16:50:50
37.0
21.7
n
5.1
1930, Sept. 23
20:35:38
36.7
22.0
n
4.9
1931, Nov. 23
23:32: IO
36.5
21.5
n
5.5
1932, Sept. 30
06:12:16
36.0
22.7
n
5.6
1936, Oct.
24
14:06: I5
36.0
22.7
n
5.3
1938, Nov.
4
03:50: 13
36.5
21.5
n
5.1
1944. July
30
04:00:35
36.7
22.5
n
5.9
II 1947, June I
17:05:30
36.7
22.0
n
5.2
I l:I8:35
36.6
21.5
n
5.8
1947, July
21
09:36:21
36.7
22.5
n
5.3
1947, Oct.
6
19:55:34
36.9
22.0
n
7.0
1947, Oct.
7
19:15:28
36.9
22.0
n
5.0
1948, July
22
06:46:55
36.9
22.0
n
5.1
1948. Sept. 21
17:53:53
36.6
21.5
n
5.5
1928, July
1947, Mar.
1951, May 30
13:43:49
35.5
22.R
n
4.8
1951. Aug. 24
10:27:29
36.9
21.6
”
5.4
1951, Aug. 31
12:29:37
35.5
22.8
”
5.6
13
TABLE
II (continued) Origin
Date
time
+ (“N)
A (‘E)
h (km)
M
1951, Aug. 31
20: 18:34
35.5
22.8
n
5.3
1952, Sept.
2
23~20107
37.2
21.6
n
4.9
1953, Feb.
5
22:42:05
35.7
22.7
n
5.2
1953, Mar. 30
13:54:43
37.6
21.3
n
4.9
5
19:34:15
37.3
21.0
n
4.8
1954, Mar. 30
04:30:35
36.7
22.2
n
4.9
1954, May
3
05~24~57
36.0
22.0
n
5.1
1954, May
3
08:51:19
36.0
22.0
n
5.2
1954, May 15
12:24:34
36.2
21.7
n
5.1
1953, Dec.
1954, Dec. 30
11:05:56
36.1
21.7
n
5.4
1956, Aug. 15
14:38:06
36.0
21.7
n
5.1
1956, Aug. 16
00:38:34
36.2
21.8
n
5.4
1957, Jan.
23
17:26:51
36.7
21.6
n
5.3
1957, Feb,
19
07:43:59
36.2
21.6
n
5.9
1958, Jan.
2
02:08: 14
36.2
22.3
!I
5.7 5.2
1958, May
3
20: 18:20
36.2
21.7
n
1959, Jan.
9
01:55:03
36.1
21.8
n
5.2
1959, Dec. 23
21:39:14
36.5
21.6
n
5.0 4.9
1961, Feb. 21
03:01:51
36.3
23.0
n
1961, Oct.
2
07:21:39
36.6
21.9
n
5.7
1962, Jan.
IO
12:36:33
35.8
22.5
90
4.9
1963, Jan.
31
15:07:04
35.9
21.9
60
5.1
1963, Dec. 16
13:47:53
37.0
21.0
”
5.9
1965, May 29
01~47~48
35.2
22.6
56
5.0
1965, May 29
04: 16:56
35.2
22.6
43
4.3 5.2
1966, May 24
09~39126
37.3
21.9
34
1966, June
4
06: 16:57
36.6
21.0
82
5.0
1966, June
I1
12:05:03
37.4
21.1
47
4.8
1966, July
12
02:56:22
35.5
22.5
7
5.2
1967, Feb. 28
14:21:51
37.5
21.2
46
4.8
1967, July
5
00:53:17
36.7
21.5
50
5.0
1968, Jan.
9
23:15:43
i5.5
22.5
46
4.8
1972, Mar. 3 I
02:58:01
36.4
21.3
20
4.8
1973, Jan.
5
05:49:18
35.8
21.9
34
5.6
1973, Jan.
10
03:24: 12
37.7
21.4
45
5.1
1973, Jan.
26
07:50:11
35.7
22.
I
41
5.0
1973, Mar. 12
20:30:44
35.9
21.8
45
4.9
1975, Sept. 12
13: IO:20
36.3
21.9
43
5.4
1977, July
30
19:51:37
36.8
21.6
49
5.0
1978, Jan.
4.8
12
20:08:38
35.8
22.3
59
1978, Mar. 16
05:51:56
36.7
21.6
49
5.1
1979, Mar. 26
08:05:58
37.6
21.4
n
4.8
1979, May 27
03:20:50
36.6
21.7
n
4.8
13:34:38
37.2
21.7
n
5.1
1979, Dec.
1
14
will contain homogeneous data, based on already published catalogues of the present authors as well as on new epicenter and magnitude determinations. Table II includes all earthquakes
with Ma
4.9 which occurred
all those with M 3 4.8 which occurred The frequency
distribution
the corresponding The cumulative
in region 3 between
between
of the magnitudes
1950 and
1919 and 1980 and
1980 in the same region.
shows that these data are complete
for
periods. number
of the earthquakes
with M 2 4.9 as a function
of time is
shown in Fig. 3. Aftershocks and foreshocks have not been plotted but the result remains the same if we plot these shocks too. Four subperiods with different rates (number rates
of earthquakes
are also shown
with M 2 4.9 per year) are observed. in the same figure.
The second
These subperiods
subperiod
( 1932
and
1947) is a
14.4-years period of quiescence which preceded the earthquake of October 6, 1947 with M= 7.0. Another quiescence period started in 1967 and it continues till now. This is an additional evidence that seismic activity to occur in the next few years.
of large earthquakes
is expected
Therefore, our data agree with Wyss and Baer’s result that this region is at quiescence now, but their plots indicate that this quiescence started in 1962. To check this point further, we made several cumulative plots with varying lower
Region
3
(S. Peloponnesus - W. of Crete) 50 Period
40 -
dN/dt
1919 -1932
:
0.6
1932-1947
: : :
0.3
1947-1967 1967-1980 30-
M ~4.9
I
1920
Fig. 3. Time variation southwestern
1.2 0.5
I
1930
I
L
I
I
I
I
1940
1950
1960
1970
1980
1990
of the yearly
part (S Peloponnesus-W
number
of earthquakes
(cumulative
of Crete} of the Hellenic trench-arc
plot) with Ma4.9 system.
in the
magnitude
values. Some of these plots indicate
recognized observed
as having
started
earlier
but
that some decrease
the most
prominent
of the rate can be decrease
is always
after 1967.
It is probably
interesting
1967- 1981) almost coincide zone (Fig. 2). Although
the
to note
both
quiescence
periods
of decrease of
(1932-1947,
D, in the western
D,(t) is referred to a broader zone than the zone which is
at quiescence
now, this observation
evidence
the subperiods
that
that
with the two subperiods
can be probably
generation of very strong earthquakes. A clear discontinuity of the western
considered
as an additional
D, are times of preparation
of decreasing
seismic zone is observed
for the
(Fig. 1) in the area
southwest of Peloponnesus (around 37ON, 21’E). It is known, however, that a great destructive earthquake occurred in this area in 1886.Therefore, this area can be considered as a “seismic gap” and it is probable that one of the expected large (M 2 7.0) earthquakes will occur there. REFERENCES
Comninakis,
P.E. and Papazachos,
earthquakes
in the Hellenic
Galanopoulos, Gutenberg,
1963. On mapping
B. and Richter,
Appl., Kamik,
A.G.,
CF.,
of seismic activity
1948. Deep-focus
of the intermediate
focal depth
70: 35-47. in Greece.
earthquakes
Ann. Geofis.,
16: 37- 100.
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region. Geofis.
Pura
12 (3-4).
V., I97 I. Seismicity
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of the European
V.J., Knopoff,
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B.C., 1980. Space and time distribution
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L. and Rotvain,
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X. and Angelier,
evolution
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long-term
precursors
of
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J., Sykes, L. and Oliver, J., 1973. Possible criteria
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2 I8 pp.
I.M., 1980. Bursts of aftershocks,
J., 1979. The Hellenic Mediterranean
D.P., 1970. The plate tectonics
McKenzie,
D.P.,
1972. Active
D.P.,
1978. Active tectonics
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earthquake
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J. R. Astron.
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the fracture
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Bull.
B.C.,
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in and near Japan
(2). Activity
before and after
Res. Inst. Univ. Tokyo, 47: I I72- I 186.
distribution
of the difference
in magnitude
between
the main shock and its
J. Seismol. Sot. Jpn., 32: 462-476.
1974. On certain
aftershock
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Ann.
27: 497-515.
Papazachos,
B.C., 1975. Aftershock
I91 I-1973. Papazachos,
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Bull. Earthquake
M., 1979. Statistical
Geofis.,
Sea and surrounding
Res. Inst., 41: 615-658.
great earthquakes. largest
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Mogi, K., 1969. Some features Okada,
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Sot., 55: 217-254.
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sequences
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Bull. Sci. Cr. Space Res., 3: l-44.
B.C., 1980a. Aftershock
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IO: I-IS.
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Papazachos, Geod.,
B.C., 1980b. Seismicity
Papazachos,
B.C., 198 1. The time variation
largest aftershock Papazachos,
Papazachos,
pattern
in magnitude
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P.E., 1978. Deep structure
Delibasis,
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