Seismic and volcanic activities and aseismic movements as plate motion components in the Aegean area

Tectonophysics,

31

167 (1989) 31-39

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Seismic and volcanic activities and aseismic movements as plate motion components in the Aegean area GERASSIMOS

A. PAPADOPOULOS

*

of Civil Engineering, Hellenic Air Force Academy, Dekelia, Attika (Greece) and Department of Natural Disasters,

Department

Ministry of Environment,

Physical Planning, and Public Works, 2 Panormou str., 11523 Athens (Greece)

(Received

August

8,1988;

revised version

accepted

December

29,1988)

Abstract Papadopoulos, Aegean

G.A., 1989. Seismic and volcanic area. Tectonophysics,

Slip rates varying fore-arc

and back-arc

between regions.

0.22 and 2.18 cm yr-’ The rate along

slip rate, equal to 2.5 or 0.5 cm yr-’ plate motion

is accommodated

per year/1000 aseismic

The eruption

(= 0.83 or 0.5) implying high aseismic

that the Hellenic

depending

reinforce

subduction

are determined

with respect

the suggestion

anomalously

is not necessarily

of the plate motion

that related

zone is of intermediate

type between

studying interrelationships between these components in several plate consuming boundaries mostly of the Pacific area. Such interrelationships are very useful in studies of seismicity, assessment of seismic and volcanic hazards, and deep tecton-

Mavromichali

str.,

11472

active

volcanism

low with respect

indicates

while aseismic

that at least 50% of the

eruption

(= 0.13 eruptions

in subduction

zones

to the degree

of plate decoupling

to the degree of decoupling seismicity.

the Marianas

of the Aegean

0.5 cm yr-’

is related

and suggesting

A tentative

hypothesis

and Chile subduction

to

that the is made

types.

zones of the world is tested as well. a preliminary approach was made by

the present author (Papadopoulos, 1987a), here more thorough, accurate and conclusive results are presented. The data The most complete and homogeneous data of the earthquake focal parameters in the Hellenic arc for the entire period 1800-1985 are listed in

Athens

(Greece). 0 1989 Elsevier Science Publishers

in several segments

is about

rate assumed,

subduction Although

Some tectonic associations between earthquakes and volcanism have been reported in the Hellenic arc (see short reviews in Bath, 1983a and Papadopoulos, 1986). The main aim of this paper

0040-1951/89/$03.50

in the

the Aegean and surrounding regions. The consistency of the results with those found in other

ics.

98

components

is to determine the seismic and volcanic activities and aseismic movements in the Hellenic arc as plate motion components and the degree of plate decoupling as a subduction parameter. For this purpose the slip rates from seismicity have been calculated in several seismotectonic segments of

Tectonic earthquakes, aseismic movements, and volcanism constitute components of the tectonic activity in subduction zones. The plate tectonics theory provided a suitable framework toward

address:

as plate motion

to the rate of volcanic

is taken up by subcrustal

Introduction

* Mailing

movements

from seismicity

on the plate motion

rate is, however,

that volcanism

slip component

and aseismic

the main zone of plate interaction

by creep. These values,

km of arc length)

subduction.

activities

167: 31-39.

B.V.

32

<;.A. PAPADOPOULOS

Fig. 1. Spatial intermediate occurred

during

Papazachos, dashed

distribution depth

direction

1800-1900

1980);

double

shocks dashed

of earthquake and 1901-1986, straight

in the Hellenic

arc. Circles

depict

M, 3 6.5 for 19OlL1986);

open

respectively.

Thick line-boundary

line-Eastern-Western

curves define the regions

of plate motion.

epicenters

(M, > 7.0 for 1800-1900,

Volcanoes:

Hellenic

of Patraikos-Corinthiakos Kr-Krommyonia.

MI-Methana,

two earthquake catalogs (Papazachos and Comninakis, 1982a; Comninakis and Papazachos, 1986). Each catalog is complete for certain lower magnitude thresholds and corresponding time intervals. Magnitude-frequency diagrams indicate that these data are complete for strong (MS z 6.5) shallow (h < 70 km) shocks of the outer (fore-arc) (Papadopoulos, 1987b) and inner (back-arc) (Papadopoulos and Voidomatis, 1987) seismic

between

arc boundary

Gulf (PC) Me-

shallow and

(after

and North

shocks

solid

(MS 2 6.5). triangles

symbols

the inner

and outer

Papazachos Aegean-Marmara

Melos. Su-Santorini,

represent

and

depict

events

which

seismic zones (after ComniAakis,

1982b);

Sea. Arrows

show the

Ko-Kos,

Ni-Nisyros.

zones (Fig. 1). The Bulletin of the Seismological Institute of the National Observatory of Athens has been used for the year 1986. The advantage of using data for a relatively long time span, such as the 1800-1986 one, is the incorporation of large events with long repeat times. The drawback, however, is the low accuracy of historical data with respect to instrumental data particularly of the few last decades. For these reasons two overlap-

SEISMIC-VOLCANIC

ACTIVITIES

AND

ASEISMIC

MOVEMENTS

ping data sets are used to determine rates in several segments Patraikos-Corinthiakos

seismic

with the exception

IN AEGEAN

slip

of the

Gulf segment.

Seismic slip rates According to the classical method (1968) and Davies and Brune (1971) megafault) slip rates tained from: u=

EM,

from

seismicity

of Brune fault (or can be ob-

sin+

CLLW

0)

where ii = total seismic slip, ZM,, = sum of earthquake moments, (p = fault dip, p = rigidity constant, L = fault length, W = thickness of the seismogenic layer. This method has been applied to determine seismic slip rates in several tectonic segments of the Hellenic arc. Only strong, shallow shocks are considered. The most fundamental segments in the region under examination are the outer (fore-arc) and inner (back-arc) seismotectonic zones as defined by Papazachos (1980) (Fig. 1). Although the outer zone seems to be considerably homogeneous from a seismotectonic point of view, there is some difference between its western and eastern segments (Papazachos and Comninakis, 1982b). The inner zone, however, is much more complicated, consisting of several hypozones of mainly normal or strike-slip faulting (e.g., Papadopoulos et al., 1986). These segments (Fig. 1) associated with shallow earthquakes were considered in the determination of the seismic slip rate. Table 1 supplies information on the several data sets used. As for the Patraikos-Corinthiakos Gulf segment (PC in Fig. l), it must be noticed that only one strong shock occurred there during the present century (Feb. 24, 1981; MS = 6.7). In contrast, six earthquakes of MS ranging from 6.5 up to 7.3 took place in the 19th century. Thus, the seismic slip rate in this segment has been calculated only for the entire period 1800-1986 in order to obtain a reliable slip-rate value. The conversion of earthquake magnitude, MS, to earthquake moment, M,, is based on the relationship proposed by Hanks and Kanamori (1979): log M, = 1.5M, + 16.05

(2)

AREA

34

GA.

The fault dip across the fore-arc

zone assumed

be equal to the dip of the South Aegean Benioff zone. Although pointed

Kondopoulou

out that the central

considerably

shallower

(36 o ) and northwest

part

(25 o ) than

angle is about

solutions

that a reliable

indicate

Aegean-Marmara

Corinthiakos ity constant cm-2

et al. (1985) of this zone is the southeast

(42” ) ones, we can say that

the mean dipping North

to

Wadati-

35O. Fault

plane

fault dip in the

Sea and the Patraikos-

Gulf is 45” as an average. The rigidassumed to be p = 3.3 X 10” dyne

for crustal

conditions.

PAPADOPOULOS

6.4 has been considered. The number, N, of events is estimated from the log N versus MS curves. Different

values of the parameters

been considered

for different

a and

segments

b have

(see Table

1). The

last

two columns

in Table

seismic slip rates determined observed

moments,

M,,‘. It is evident fore-arc

1 show

MO, and the theoretical that the rates

zone (segments

obtained

1 and 2) from

sets, covering the periods 1901-1986. are almost identical.

the

from the sum of the ones, for the two data

1800-1986 and The same is nearly

Large thrust-type earthquakes along a subducting plate boundary tend to occur at depths shallower than about 40 km (McCann et al., 1979). Across the fore-arc zone of the Hellenic arc the majority of shocks with MS = 5.5-5.9 and all of the shocks with MS >, 6.0 which occurred in the

valid for the eastern (segments 3 and 4) and western (segments 5 and 6) sectors of the arc. The consistency of the results obtained for these data confirms that the reporting of seismic events of Mh 2 6.5 is satisfactory for the previous century. The slip rate of the North Aegean-Marmara

post-1963

Sea stands considerably above the rates of the fore-arc zone and the Patraikos-Corinthiakos Gulf. However, it is obviously lower than the slip

interval

were

located

in

the

upper

seismogenic layer (O-40 km approximately) (Papadopoulos and Pavlides, 1984). Moreover, in the broad Aegean region nearly 50% of the earthquake frequency derives from the depth range O-35 km, over 50% of the energy from the range O-25 km and the largest magnitudes occur in the crust (O-30 km) (Bath, 1983b). Hence, the thickness, W, of the seismogenic layer was established to be 30 km. The seismic experience of the last decade, when several earthquake sequences occurred in the inner Aegean seismic zone, showed that strong (M, > 6.0) shallow events take place as a rule at the uppermost crustal layer at depths no more than about 15 km. The data of the last decade or so prove the most accurate, therefore it seems to be a realistic proposition to consider the value W = 15 km for the seismogenic layer of the inner zone. It is common knowledge (e.g. Brune, 1968) that, even though the number of events increases as magnitude decreases, the contribution of the smaller events to the total moment sum is still small and that the largest earthquakes are responsible for the majority of the seismic slip rate. However, the lower magnitude 6.5 of events analyzed here is relatively high and, therefore, in this case, smaller events could contribute considerably to slip. For this reason the sum of seismic moment, Md, due to earthquakes with 3.0 < M,’ <

rate determined for the North Anatolian fault zone ranging from 3.9 to 12 cm yr- ’ depending on the time interval considered (Brune, 1968; North, 1974; Toksiiz et al., 1979). The East Anatolian fault zone has a much lower slip rate, less than half of that of North Anatolian fault, although the exact value of that slip is difficult to calculate because of sparsity of events (Toksiiz et al., 1979). The slip rate in the Patraikos-Corinthiakos Gulf seems to be almost equal to that of the fore-arc zone for W = 30 km but it is certainly higher than that for W = 15 km. In the Gulf of Corinth, the pattern of deformation caused by the seismic slips shows a N-S and E-W extension at 0.3 and 0.6 cm yr-‘, respectively, together with a downwards movement of the northern side of the Gulf relative to the south at about 1 mm yr’ (Tselentis and Makropoulos, 1986). One of the most important results indicated by Table 1 is the low seismic slip rate in the Hellenic arc with respect to the rates found in several circum-Pacific and Indonesian subduction zones varying between 1 .l and 6.4 cm yr- ’ (Brune, 1968; McNally and Minster, 1979; Acharya, 1981). In the Japan arc a rate as much as 15.1 cm yr-’ is determined (Brune, 1968). North (1974) estimated slip rates in Mediterranean from seismic events of

SEISMIC-VOLCANIC

ACTIVITIES

M, > 6 which found sent

occurred

for regions study

AND

ASEISMIC

during

under

1910-1970.

examination

are as follows:

MOVEMENT3

Cretan

IN AEGEAN

Rates

in the prearc (fore-arc

35

AREA

reliable average rate of plate motion is about 3 cm yr-‘. This notion is justified by the independent calculations

of Van6k

et al. (1987)

Northern Greece (which zone) 0.23 cm yr-i, largely coincides with the North Aegean-Marmara

poulos

Sea area) 1.1 cm yr-‘.

position

of the South

nection

with

Studies

of instantaneous

These results

ment with those presented

are in agree-

in this paper.

Aseismic slip rate and volcanism

(1989) which indicated

subduction

rate of about Eurasian

The and

extensive

volcanic

spatial

zones

coincidence

in active

rate of 2-3 cm yr-’

trench-arc

of seismic systems

and

aseismic

1 cm yr-i

and African

Jordan,

Aegean

the geometry

1978).

Papadopresent

from the age and volcanism

in con-

of the Benioff plate

motions

zone.

imply

a

for the convergence

plates (Chase, As

and

an average

already

of

1978; Minster mentioned

the

slip rate, u’, is u’ = u - U. Thus, the value

supplies evidence that seismic and volcanic activity may be related. In many areas, however, there is no clear association of volcanic and seismic activity, and therefore there is no necessary causal link between the two phenomena (Carr, 1977, 1983). Honda and Uyeda (1983) emphasized that the problem of heat source and material source of

of u’ in the arc depends on the assumed rate of plate motion. Mantle shocks almost certainly do not represent motion between two adjacent lithospheric plates. Such events can be explained by rupturing within a single descending slab of lithosphere (Isacks and Molnar, 1971). By this means even

arc volcanism are two major unresolved problems associated with volcanism of this type. Thus, it seems that in subduction zones the relationship

large mantle strain along

earthquakes do not relieve tectonic the main zone of plate interaction.

between volcanism and plate motion is not properly understood. However, according to Acharya’s

Therefore, u’ must be estimated only from the seismic slip rate associated with shallow shocks. By considering that the seismic slip rate along the

(1981) model the number of volcanic eruptions per year in a convergent zone is linearly related to the aseismic slip component of plate motion. If the aseismic slip rate, u’, is low (coupling between converging plates is strong), then the primary manifestation of tectonic activity is the occurrence of large, shallow shocks with only infrequent volcanic activity. If, however, u’ is high (coupling

main zone of plate interaction yr-l (see rates in segments 1 aseismic slip rates of 2.5 cm yr-’ are alternatively obtained for u u = 1 cm yr-‘, respectively. This 83% or 50% of the plate motion aseismic slip motion. This result predicted by North (1974) when

is weak), then there are few large earthquakes, and volcanism is the principal manifestation of tectonic activity. This relationship may be valid for small sections of plate boundary as well. The previous model has been tested in the Hellenic arc by examining the extent of volcanic activity and aseismic slip rate since 1800. As u’ is the difference between rate of plate convergence, u, and seismic slip rate, U, we face the problem of estimating the value of u. The rate of plate convergence across the Hellenic arc-trench system has been estimated by several authors on the basis of geotectonic and seismic considerations. From the estimations of Le Pichon (1968) Papazachos and Comninakis (1971), McKenzie (1972, 1978) and Le Pichon and Angelier (1979) I concluded that a

most

plausible

explanation

is about 0.5 cm and 2, Table l), and 0.5 cm yr-’ = 3 cm yr-’ and means that either is consuming in was qualitatively he stated that the

of the

considerable

discrepances between theory and observation is that a major proportion of the deformation in this area takes place in viscoelastic processes such as creep. From the last 70 years seismicity in the Mediterranean, Jackson and McKenzie (1987) concluded Anatolian

that in the Aegean Sea and North fault zone all or most of the deforma-

tion is probably taken up seismically while in the Hellenic Trench probably 10% or less of the upper crustal deformation is seismic and the rest must be accommodated by creep. Values of u’ in circumPacific convergent zones are generally higher varying between 2.2 and 6.9 cm yr-’ (McNally and Minster, 1979; Acharya, 1981). An effective way of estimating the rate of

36


volcanic activity is to calculate amounts of ash or lava products. The existing data, however, do not allow

us to do this in the Hellenic

other hand, if we wish to compare those

obtained

in other

to follow Acharya

aseismic

slip rate with volcanism

the eruption

rate,

arc. On the

our results with

subduction

essential

zones

10

it is

06

(1981) who correlated by calculating

n, that is the number

tions per year. He counted

a 12 -

one eruption

06

0.4

for each

year in which there is a reliable

report of eruption,

regardless

time or number

of length of eruption

T

of erup-

n

02

of I

active periods in that year. This procedure has been applied in the South Aegean. The standard catalog of Georgalas (1962) shows that normal eruptions occurred only in Santorini (Fig. 1) since 1800. Phreatic eruptions and strong detonating solfatara, occurring in Nisyros (Fig. 1) during the 19th century, have not been considered. The last normal eruption in Santorini occurred during 1950. Finally, the number of eruptions counted over the period 1800-1986 is 12. After normalization to 1000 km of arc length the eruption rate is 0.13 eruptions per year per 1000 km of arc length. If we also consider the 1707-1711 eruption cycle in Santorini, n is being 0.12. In other convergent plate boundaries n varies between 0.22 and 1.28 (Acharya, 1981).

20

30

u’

Fig. 2. Linear

regression

eruption

n (per

rate,

presented

40

50

--+

between

aseismic

(1981)

(dashed

lines / and 2 show the modified

u’ and

n by considering

indicated

slip rate,

1000 km of arc length

by Acharya

arc (alternative

3

6.0

(cmlyr)

Straight

line;

as

solid

symbols).

relationship

between

also observations

values of 2.5 cm/yr

u’, and

per year).

from the Hellenic

and 0.5 cm/yr

for u’ are

corresponding

to lines I

by open circle and triangle

and ,7. respectively).

the two plates are strongly coupled (low u’). the frequency of volcanic activity is low. This relationship is expressed

by the equation:

n = 1.14d - 0.05

(6)

with r = 0.8. Assuming plate

The equation: n = 0.18~’ - 0.15

1.0

motion

rate

that in the Hellenic

arc the

d is being

is u = 3 cm yr-‘.

(3)

expresses the relationship between n and u’, with a correlation coefficient r = 0.82 (Acharya, 1981). By considering the alternative values of n = 0.13, U’ = 2.5 cm yr-’ and n = 0.13, U’ = 0.50 cm yr-’ for the Hellenic arc, eqn. (3) is being: n = 0.19U’ - 0.20

(4)

or: n = 0.16~’ - 0.06

T

n

(5)

with r = 0.83 in both cases (Fig. 2). These results are consistent with Acharya’s suggestion that active volcanism in subduction zones is related to aseismic subduction. Moreover, he suggested that the degree of plate decoupling, d, where d = u’/u, varying between 0.26 and 0.9 in circum-Pacific subduction zones, is linearly related to n. Thus, where the degree of decoupling is high, volcanic activity along the subduction zone is high. Where

1 L I 0.0 02 04

I

1

0.6

Ob

1

d----+ Fig. 3. Linear regression eruption

rate,

presented Straight

n (per

by Acharya

between

degree of decoupling,

1000 km of arc (1981)

(dashed

lines I and 2 show the modified

d and n by considering (alternative

also observations

length line;

corresponding

as

solid

symbols).

relationship

between

from the Hellenic

values of 0.83 and 0.5 for d are indicated

circle and triangle

d, and

per year)

arc

by open

to lines I and 2, respectively).

SEISMIC-VOLCANIC

ACTIVITIES

0.83. By linear

regression

AND

ASEISMIC

MOVEMENTS

eqn. (6) is modified

n = 0.70d + 0.11

to: (7)

with r = 0.47 (Fig. 3). If, however, u = 1 m yr-t,

IN AEGEAN

d is reduced

we assume

that

to 0.50 and eqn. (6) is

37

AREA

that

the

mediate

Hellenic type

types corresponding Japan or Kanamori’s

case, more thorough

n = 0.96d - 0.02

(8)

subduction

with r = 0.7 (Fig. 3). In any case, these results

do

not fit the main trend described to the anomalously

low eruption

the degree of decoupling.

by (6). This is due rate in respect

to

Note that eqn. (7) is not

probably

zone

is of inter-

Marianas

and

Chile

to the Kurile-North

investigation

of the Hellenic

In the Acharya’s of the downgoing

on the

the plate under-

along the upper surface

slab and this results

The extent

arc therefore

is needed

in any

arc.

(1981) model

leads to shearing

production. island

the

the Sanriku type of subduction (1977) evolutionary model. In

being:

thrusting

subduction

between

of volcanic

appears

in magma

activity

in an

to be a function

of

significant, while eqns. (3)-(6) and (8) are. In conclusion, data from the Hellenic arc support a possible correlation between active volcanism and aseismic subduction, and, on the other hand, indicate that the rate of active

arc, however, it seems that the degree of decoupling does not correlate with the rate of active volcanism. Here modified explanations may be

volcanism is not necessarily related to the degree of decoupling as suggested by Acharya (1981).

given suggesting that the high aseismic slip component of the plate motion is finally accommo-

Discussion According to Uyeda and Kanamori (1979) trench-arc systems (subduction zones) can be classified into two types depending on whether or not actively opening back-arc basins are associated with them. Earthquake studies suggest that there is a significant difference in the mode of plate motion at interplate boundaries between the two types of trench-arc systems. Extreme cases are Chile, where plate motion is seismic, and the Mariana arc, where it is aseismic. The work of Uyeda and Kanamori (1979) indicated that the nature of the contact zone between the upper and lower plates changes from tight coupling (Chile) to decoupling (the Marianas) through the evolutionary process of subduction suggested by Kanamori (1977). Comparative studies of subduction have not been made for the Hellenic arc. However, the high decoupling degree and the lack of great, thrust-type interplate earthquakes supply evidence that this system represents an example rather of Marianas than of Chile types. Other observations, however, concerning the stress-field (tensional in the back-arc region; e.g. Papadopoulos et al., 1986) volcanic types (series of rhyolites-dacitesandesites in the South Aegean volcanic arc) and dip of the Wadati-Benioff zone (- 35”) imply

the frequency of underthrusting or in other words of the degree of plate decoupling. In the Hellenic

dated by large mantle earthquakes leading to magma formation. The possibility for magma formation from fracturing associated with large intermediate depth earthquakes has been discussed on the basis of time relations between volcanic and subcrustal seismic events in the South Aegean, the Calabrian, and Indo-Pacific subduction zones (Papadopoulos, 1986, 1987~). A geometric explanation for the concentration of volcanism in regions where earthquakes in the subducting slab reach a depth of 100-150 km has been given by Spiegelman and McKenzie (1987). From 2-D models for melt extraction they suggested that corner flow in the mantle wedge beneath island arcs causes melt to flow to the slab-plate junction.

Acknowledgements I am grateful to Professors M. Bath (Uppsala) and C. Sengor (Istanbul) for critically reading of the manuscript.

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