Premonitory burst of seismicity and its significance for predicting large Aegean earthquakes

Tecza~ophysics,

257

156 (1988) 257-265

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Premonitory Hurst of seis~icity and its significance for predicting large Aegean earthquakes GERASSIMOS Section of S’eirmolu~ and ~a~tbq~ke

A. FAPADOPOULOS

Resistant Stmctures,

*

Ministry of Env~~o~~e~t and Public works,

71 Louisis Riankour szr., 11.523 Athens (Greecej

(Received February 29,1988; revised version accepted May 9.1988)

Papadopoulos, quakes,

G.A., 1988. Premonitory burst of seismicity and its significance for predicting large Aegean earth156: 257-265.

~eczonop~ysics,

The time variation of strain energy release prior to large t&f, > 6.9) Aegean shocks indicates that they are commonly preceded by a premonitory burst of seismicity in their focal regions several years prior to their occurrences. A linear relations~p, independent of the type of stress-field in the focal region, is established, implying that the higher the shock magnitude, MS, the longer the premonitory time. The fact that there is no correlation between the size of the premonitory burst and the magnitude of the approaching earthquake is attributed to the great inhomogeneity of crustat material from place to place. A second model, showing a long-term quasi-exponential increase of the strain energy release towards the time of the main shock, is applied in two cases. The usefulness of tbe two models for predicting large earthquakes is discussed.

The identification of seismicity anomalies is considered as a tool of primary importance in stud~ng seismicity and seismotectonics. In particular, research into anomalies occurring prior to large earthquakes has proved very useful for understanding the process of earthquake preparation and ~ons~uently for the prediction of such events. Reviews on this subject can be found in Kanamori (1981), McNally (1982), R&take (1982a}, and Magi (1985). One of the most interesting anomalies is the burst of seismic activity which takes place some

* Mailing address: Greece.

98 Mavromicbali

str., 11472 Athens,

@ 1988 Elsevier Science Publishers B.V.

years prior to large shocks at both the ~onsu~ng plate boundaries (see the case of the Kanto earthquake in Suyehiro and Sekiya, 1972; Aggarwal and Perez, 1980; Acharya, 1984) and within the continental crust (Kristy and Simpson, 1980). The relationships obtained from these studies provide a very promising means of predicting large earthquakes and, therefore, deserve to be further explored. The Aegean area provides an excellent geophysical en~ronment for two main reasons. Firstly, it is a seismically very active area, with crustal shocks having surface-wave magnitudes of up to about 7.5. Secondly, the seismotectonic complexity of the area (see review in B&h, 1983) offers an opportunity to examine seismic anomalies prior to the large earthquakes associated with different types of faulting.

258

Data and methodology The term indicate

“large

Fourteen

earthquake”

a seismic

event

is used here to

of surface-wave

magni-

large

(M, a 6.9)

shallow

earthquakes

took place in the Aegean

and surrounding

during

1, Fig. 1). The catalog of

that period (Table

Comninakis

and

Papazachos

region

(1986) was used as

tude, MS L 7. To investigate the types of long-term seismicity changes prior to a large earthquake, it is

data source. In this catalogue the epicenters are accurately located to within 30 km while the errors

essential

involved

to examine

a particular decades

area

before

The rupture such

the variation

around the

earthquake

number

zones

actually

However,

of Aegean

well-defined.

occurs.

fined as the area within

The most region

rates,

function

of 100 km of the

several

of time. problems,

epicenter of each event. This simple technique has been applied by other authors studying spatio-

difficulty However,

temporal

quantitative

variations

Rikitake,

in

foreshock

activity

estimates

are less than

(e.g.

presentations

for studying

of the seismicity

in a particular

are seismicity

seismicity

zone is here de-

a radius

common

the time variation

in a very

are the rupture

The rupture

in the magnitude

0.3.

area for

only

events

in

for some

zones is the most appropriate

an examination.

limited

in seismicity

its epicenter

and

maps, seismic

Usage

time-distance energy

as a

of the first two involves

the most

important

of including quantitative the other two presentations evaluations

plots,

release

of

changing

being

the

evaluations. do permit seismicity

patterns.

1982b).

The Aegean seismological data seem to be complete for M., a 5.2 from 1911 onwards. To ensure

Studying seismicity changes on the basis of changes in the seismicity rate has the disadvantage

that

that seismic events with very different

all relevant

cluded,

changes

in

seismicity

are

in-

be as large as possible. I, therefore, decided to examine events occurring from 1945 onwards.

TABLE

magnitudes

are given the same weight, even though they release vastly unequal amounts of energy. On the other hand, since the sum of the energies in each

the time span prior to each event needs to

1

Burst of seismic activity prior to large earthquakes in the Aegean area Burst of seismic activity

Large earthquakes No.

Date

M,

Region

max( %!I&

x&

( x 10” ergP

)

Date

log f

,,,f”‘? SB

D(km)

(t in days) )

1

1947-10-06

7.0

Messinia

0.6673

34.92

1944-07-30

3.07

5.9

2

1948-02-09

7.1

Karpathos

2.55

45.49

1922-08-13

3.91

6.9

72

3 4

1952-12-17 1953-03-18

7.0 7.4

S. Crete a Yenice

9.036

35.93

1919-11-18

4.09

7.0

96

5

1953-08-12

7.2

Cephalonia

7.27

18.57

1915-08-07

4.14

7.0

24

6

1954Jw30

7.0

Sophades

1.535

31.01

1941-03-01

3.68

6.3

42

7

1955-07-16

6.9

samos

2.13

34.98

1933-04-23

3.91

6.6

86

8

1956-07-09

7.5

Amorgos

0.456

78.06

1919-10-25

4.13

6.1

18

9

1957-04-25

7.2

Dodecanese

1.67

22.93

1941-12-13

3.78

6.2

80

10 11

1964-10-06 1968-02-19

6.9 7.1

Manyas a Ayios

2.17

34.33

1967-03-04

2.55

6.6

12

1981-12-19

7.2

Lesvos b

13 14

1982-01-18 1983-01-17

7.0 7.0

Lemnos b Cephalonia

4.87

21.97

1959-11-15

3.93

6.8

Efstratios

a Not considered. ’ No shocks of M, 2 5.2 in the period examined.

42

259

2l'

20.

19.

22.

23.

24.

25.

27.

26'

26'

29.

3y' 2'

as

200

21.

22'

23'

Fig. 1. Locations

i?*

2s

of the earthquakes

26'

listed in Table

time window is completely dominated by the shocks with the largest magnitude, the smaller

tance of 100 km from its epicenter.

events are neglected when the seismic energy release as a function of time is examined.

parameter WC@, that is the yearly percentage of the total strain Cfi released in the whole period

Benioff (1951) introduced the term “strain” for the sum of the square roots of the energies (C@). The use of this parameter alleviates the problem mentioned above, while its time variation de-

examined, is plotted as a function of time. Energy

scribes the variation of the tectonic deformation. This parameter has also been used by other authors (e.g. Acharya, 1984) for the detection of seismicity changes prior to large earthquakes. For these reasons, I studied strain release as a function of time using the following technique. The parameter @ is calculated for each year from as far back as 1911 (in most cases) up to the occurrence of each large earthquake, taking into account all the shocks

of MS 2 5.2 which have taken place within a disNext,

the

released by a particular event is computed from the formula log E = 12.24 + 1.44 MS (Bath, 1973). The time period examined extends from January 1, 1911, up to the date on which each large earthquake occurred with the following exceptions: (1) The time period of the Yenice 1953 event starts after the MS = 7.6 August 9, 1912 shock, which occurred in the same area as the Yenice event. (2) In the area of the Lesvos 1981 and Lemnos 1982 events, the Ayios Efstratios earthquake occurred in 1968. Consequently, only the period after this earthquake is examined. (3) Similarly, the time

260

period

of the Cephalonia

after the occurrence area. The Manyas

1983 earthquake

and the occurrence

between

its occurrence

of the Yenice 1953 event in the

same area is too small to identify

reliable

seismic-

The South Crete 1952 event has been

examined,

but the result may not be reliable

the shock

took place

graphic events

network in

guaranteed

this

outside

and area

the Greek

the complete with

since

seismo-

The data release rence

imply

of the

than

the time,

strain

large

that in 1926,

energy

earthquake,

energy

was

t, between

the

and the occurdepends

according

on

with a correlation crlog

,

= 0.25 and

associated

either

the

to the linear rela-

log f = 0.33 + 0.51 MS

of

period.

that

earthquake magnitude tionship (Fig. 3):

be

for the whole of the 1911-1952

significant

of maximum

cannot

(1)

coefficient

of 0.63 and where

t = time in 24 hrs. Each peak is with

a single

event

or with

a

group of events occurring in the same year. Thus, t is measured either from the single event or from the largest event of the group. In the case of the

Figure 2 shows the plot of the parameter WC@ of time, prior to the earthquakes

as a function

with code numbers

1, 2, 4, 5, 6, 7, 8, 9, 11 and 14

1. The straight

line represents

the mean

of the parameter %oC@ during the period covered by each diagram. The dashed line represents the 95% confidence level which is computed as 1.65 x u + WI, assuming

standard

is more

since a considerable amount of strain released during 1942 and 1943.

coverage

MS 2 5.2

Results

in Table

December,

1964 event has not been examined,

since the time that elapsed

ity changes.

starts

of the 1953 event in the same

deviation

normal

distribution,

and

m = mean.

where The

time

u = of

Dodecanese 1957 earthquake (see above) the time is measured from the December 13, 1941 event. It is noteworthy that the relationship (1) is consistent with that found by Acharya (1984) for large earthquakes in the circum-Pacific area. In addition, a similar exponential length of the premonitory time creasing reported

increase in the period with in-

magnitude of the main shock has been by Aggarwal and Perez (1980) and Kristy

pattern is present. More precisely, in the particular areas of the events with code numbers 2, 4, 5,

and Simpson (1980). Another important result seems to be that the earthquakes mentioned above represent three different main types of faulting, namely, reverse

6. 7, 8, 9 and 14 a peak (= WC@) in strain release in prominent several years before the oc-

(Yenice, 1953; Cephalonia, 1953), normal (Karpathos, 1948; Samos, 1955) and strike-slip

currence of the respective earthquake (see also Table 1). This means that some important tectonic

(Amorgos, 1956; Dodecanese, 1957; Cephalonia, 1983). Hence, the time/magnitude relationship

deformation, reflected as a burst of seismic activity, took place in the vicinity of the forthcoming large earthquake several years prior to its occur-

described earlier probably does not depend upon the type of stress-field prevailing in the particular

occurrence of each large earthquake is shown. It is evident that in most cases a common

rence. The mean

distance,

D, of 57 + 30 km be-

tween the large earthquake and the single event, or the largest event of the group (for data see Table l), leaves no doubt that the premonitory deformation takes place in the focal region of the approaching earthquake, or very close to In the post-1910 period preceding canese 1957 earthquake, the seismicity characterized by two peaks in 1926 and equal to max%C@ = 22.93 erg’12. I gest that the peak in 1941, connected event of MS = 6.2 which occurred on

it. the Dodevariation is 1941, both would sugwith the the 13th of

area of the large earthquake. Besides this, it is likely that the size of the burst is independent of the magnitude, MS, of the subsequent large event. The plot of the MS versus the size of the burst, M&o, does not show any correlation (see Table 1); Ms..Q is either the magnitude of the single event, if the burst is associated with a single event, or the equivalent magnitude corresponding to the total seismic energy released in the burst, if it is associated with a group of events. The plots t/SC@ of the Lesvos 1981 and Lemnos 1982 earthquakes are not included in Fig. 2 since no shocks of IUs > 5.2 occurred in those

261

Papazachos and Comninakis (1982) and Comninakis and Papazachos (1986), seem to be complete for the whole period from 1800 to the present for MS > 6.5 (Papadopoulos, 1987). Thus, we may discuss the problem as follows: Karpathos 1948, iIf, = 7.1. The burst is associated with an event of MS = 6.8 which occurred in 1922. No other events of MS > 6.5 have been reported since 1800 in this area. It is, therefore, likely that the 1922 event is the only one within the 150-yr time interval preceding the Karpathos earthquake.

particular areas during the period examined. In other words, these areas were in seismic dormancy for about 13 years prior to the occurrence of the large earthquakes. This result is compatible with the model of strain energy release discussed above. One problem which must be clarified is whether we can be sure that only one burst of activity occurs prior to the large earthquake. To answer this question it is essential to know the repeat time for large earthquakes in each place, that is to have complete data over a long time-span. The data for Aegean shallow shocks, listed in the catalogs of

40 :j-C2) __-_

--_---

20 7. I &

0 I910

20

40

30

I910

40

----____---_ (4’

t

20

30

40

50

I

60

1910

20

-0

40

30

50

_

------

-_7.2

t~&yl,A,,~yyy 1910 t

Fig. 2. Time variation parameter

W&k magnitude

-z

of the strain

3o

4o

release prior

and the 95% confidence of the large earthquake,

So

to large Aegean

level, respectively. respectively.

earthquakes.

The arrow

Number

Straight

and the number

in parenthesis

and dashed

lines represent

near it indicate

is the earthquake

the mean of the

the time of occurrence

code number

in Table 1.

and

Yenice

shock

19.53, MS = 7.4. As the previous

took place

only

one

in 1912, the 1919 burst

to occur

between

the

large

in this

1912

event

area

and

is the

in the interval

the Yenice

earth-

quake. Cephalonia

19.53,

M, = 7.2.

large

earth-

quakes with M, = 7.3 and 6.9 were reported

in this

area

in 1867

notable

and

event

1870-1911

in

was

1869, this

the

Two

respectively.

area

during

The the

MS= 6.7

1893,

only

interval

event.

was the largest

1869 and Sophades

burst 1910

20

40

30

50

_I-

70

60

was

earthquake

1954,

of

1953

M, = 7.0. The size of the 1941

MsEQ = 6.3,

while

in this area

for 1894-1917.

the previous

occurred

M, = 7.0. No earthquakes

ported

The

burst

with

Although

large

in 1894, with

M, > 6.3 were re-

this suggests

that

the 1941 burst was the largest one to take place between 1894 and 1954, such a conclusion can only be tentative because of the incompleteness of data on events 1894-1910.

1950

-2

4

-2s

--

35

the 1904 aftershocks,

Amorgos

C-

in the Aegean,

-

t

7.1

G.9

MS Fig.

3. Log

maximum earthquake Straight sense.

of time

strain

(in 24 h) between

energy

as a function

number represent

release

approximation

lines represent

explanation

and

of magnitude

line is the linear Dashed

7.3

the occurrence

occurrence

of

of a large

of the large earthquake. in the least-squares

the 95% confidence

see code numbers

the Messinia

7.5

+

level. For

in Table 1. Open circles

1947 and Ayios Efstratios

1968 events.

1904 and

1956.

which ruptured

_9>----

@-

period

one may argue that the 1933

burst was the only one to occur between 1955.

0

4

;

G

the

As no shocks were reported between 1904 and 1911 in this area, with the exception of

-- ----I1

4.0

for

M, = 6.6.

Fig. 2 (continued).

--g7’

M, < 6.5

Samos 1955, M, = 6.9. The previous large earthquake in this area took place in 1904 (M, = 6.9). The 1933 burst is associated with an event of

go

‘O

of

M, = 7.5. This earthquake. one of the most aseismic regions

was preceded

by an M, = 6.1 event

in 1919. No shallow shocks were reported in this area between 1800 and 1910, with the exception of a volcanic M, = 6.0 event associated with the 1866 eruption of the Santorini volcano. Despite the incompleteness of the data on events of M, < 6.5, the 1919 burst was probably the largest one during at least the early instrumental period of 1899-1910. Dodecanese 1957, MS = 7.2. As explained above. this shock was preceded by the 1941 burst, which is associated with an M, = 6.2 event. The only notable event located in this area before 1911 was that of 1869, with MS = 6.8, which seems to have been the previous large earthquake in the area. However, the incompleteness of the data does not

263

permit us to suggest that, apart from the 1926 burst which has already been discussed, the 1941 burst was the only one to occur between 1869 and 1957. Cephalonia 1983, MS = 7.0. The 1959 burst is surely the only one preceding this earthquake, since our examination covers the time interval after the occurrence of the 1953 large earthquake. Figure 2 clearly indicates that the mode of strain energy release prior to the Messinia 1947 and Ayios Efstratios 1968 earthquakes is similar in both cases, but completely different from the above described model. Here the burst of seismic activity took place relatively near the time when the large earthquake occurred, and does not follow the relationship shown in Fig. 3 (see also Table 1). Moreover, in the case of both the Messinia and Ayios Efstratios events there is a quasiexponential increase in the strain energy released with the approach of the next large earthquake. This model is further discussed in the next section. Conclusions

and discussion

In the Aegean area, large crustal earthquakes associated with several types of faulting are commonly preceded by a burst of seismic activity in their focal regions several years prior to their occurrence. Each burst is associated either with a single event or with a group of events occurring around the same time. The relationship log t = 0.33 + 0.51 M has been established between precursor time, t, and magnitude, M. Similar relationships have been suggested by other investigators in regions located both at convergent plate boundaries and within intraplate tectonic environments. It is noteworthy that the relationship log t = - 1.01 + 0.60 M approximately holds good for a data set of world-wide precursors of several disciplines (Rikitake, 1982a), while the relationships log t = - 1.08 + 0.59 M and log t = - 0.89 + 0.38 M have been proposed (Rikitake, 1987) for Japanese precursors of the first kind (disciplines of land deformation) and the quasi-first kind (disciplines of electrical, geochemical and underground water anomalies), respectively. It is clear that the slope of the straight line for the precursory burst of seismicity is almost the same as

that for the precursors of the first kind. However, the precursor time for these is significantly smaller than that for the burst of seismicity, implying that the burst may occur at an earlier stage with respect to the precursors of the first kind. Evidence that the size of the burst generally increases with the magnitude of the main shock has been found in other regions (Kristy and Simpson, 1980; Acharya, 1984), but is not verified by the Aegean data. According to Acharya (1984) the fact that prior displacement does not strictly depend on displacement in a large earthquake suggests that other factors, such as the strength of the contact zone and the degree of homogeneity of this strength along the entire rupture zone, presumably also play a part in fault displacement. Factors of this type are crucial in the Aegean area, where the extreme complexity of the stress-field and tectonism (Papadopoulos et al., 1986) signify a high degree of inhomogeneity in the crustal material. For example, large differences in crustal composition and strength between the regions of Amorgos and Dodecanese (Papadopoulos, 1986a), as well as from place to place within the Dodecanese region (Papadopoulos, 1984), are suggested by studies of the seismicity. Now we may discuss the usefulness of the results obtained for predicting large earthquakes. The exponential increase of the premonitory time period with increasing magnitude of the main shock allows us to estimate the time of occurrence of coming earthquakes, assuming that their approximate location has been identified from other observations. As has already been mentioned, the size of the burst, MsiQ, does not depend on the magnitude, M,, of the large earthquake. Therefore, it does not seem to be of any use in estimating M,. However, a rough approximation of M, can be made on the basis of the average value of the ratio M,/MsiQ, which is 1.09 f 0.07 (see Table 1). I believe that the main problem with this kind of prediction is to distinguish the premonitory burst from the normal background variations in seismic activity. In the case of the Messinia and Ayios Efstratios main shocks, a different model for premonitory seismicity changes is needed; in these cases, there was a long-term quasi-exponential increase of

264

strain energy release towards the time that the main shock took place. This seems to be a promising model for estimating the focal parameters of a subsequent large earthquake. However, quantitative relationships between the parameters of increasing seismicity and the parameters of a main shock cannot be established on the basis of Aegean data only, due to the extremely small number of observations. It is of special interest to note that the long-term exponential increase in seismicity as a large earthquake draws near, determined by studying the number and occurrence of all “foreshocks” (in the broadest sense of the term), was recognized by the present author as a seismotectonic peculiarity of the Messinia and adjacent regions. This led to the successful prediction, accurate to within a few months, of the MS= 6.2 Kalamata 1986 destructive earthquake (Papadopoulos, 1986b, 1988a, b). The importance of this second model is also confirmed by observations made elsewhere. One of the best known instances is that of the large (MS = 7.9) 1923 Kanto earthquake in Japan. According to Rikitake (1976, p. 170), “No remarkable seismic activity took place within the area in which substantial crustal deformation and aftershock activity associated with the earthquake were observed, until about 1.5 years before the main shock. The increase in seismic activity in the area concerned accelerated tremendously from that point in time, eventually reaching the catastrophe. Should an e~thqu~e of a similar kind be repeated in the future, the pre-earthquake activities mentioned above would doubtless be monitored by the observation network provided by the earthquake prediction program in Japan”. More recently, Hagiwara (1983) analyzed the time-interval between moderately large earthquakes that hit the Tokyo area during the period between the Genroku (M$ = 8.2, 1703) and the Kanto earthquakes. Taking the year 1706 as the time origin he observed that the time-interval between two successive earthquakes becomes shorter as time goes on. Thus, the model of accelerating foreshock activity before large earthquakes deserves to be further explored by all available means.

I would like to express my sincere thanks to Prof. Markus Bath (Uppsala, Sweden) and to one anonymous referee for reviewing the paper. References

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