Recent seismic activity in the Tokai (Japan) region where a large earthquake is expected in the near future

Tectonophysics, Elsevier

255

138 (1987) 255-268

Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Recent seismic activity in the Tokai (Japan) region where a large earthquake is expected in the near future KIYOO MOGI Earthquake Research Institute, (Received

June 30,1986;

University of Tokyo, Tokyo, 113

revised version

accepted

September

(Japan)

23,1986)

Abstract Magi, K., 1987. Recent future.

A large shallow and various

seismic activity

Tectonophysics,

earthquake

intensive

is expected

observations

there have been no reports this area, but the following

in the Tokai (Japan)

the Nankai

Trough,

that have occurred the changes

to occur in the Tokai

are being carried

of any changes noteworthy

is expected

which includes

precursors

before

of central

as precursors

have been observed

Honshu,

Japan

have migrated

in a wide area surrounding

downward

other large shallow

of a large earthquake.

of the expected

in the seismic activity

decline in seismic activity

the focal region of the anticipated

in the Tokai district

that have appeared

region

out in the region with the goal of predicting

of M 6.5-7

(2) There has been a significant

that they are long-term

where a large earthquake

which could be regarded

changes

There has been a spate of large earthquakes Tokai earthquake.

region

in the near

138:255-268.

from shallow

Future

changes

in

over the past decade.

(1)

the focal region (3) The hypocenters

depths.

along the trench

So far

large earthquake

of the expected

in the area along the Suruga

earthquake.

earthquakes

in the near future,

its occurrence.

Trough

and

of earthquakes

Since these changes

resemble

in the past, it is conceivable

in seismic activity

in this area demand

close

attention.

1. The earthquake prediction problem in the Tokai region

Shizuoka Prefecture, and there were increased calls for the establishment of a system for predicting this earthquake. Observations in this area were

The possibility of a large shallow earthquake occurring in the Tokai region of central Honshu, Japan (Fig. 1) was first pointed out in 1969 (Mogi, 1970), and the Coordinating Committee for Earthquake Prediction designated this area as a region

reinforced, and a system was established whereby the major data collected are telemetered to an office in the Japan Meteorological Agency (JMA) in Tokyo, where they are monitored on an around-the-clock basis. If any anomalies are

for special observation. In 1973 the area was declared a region for intensified observation, and observations of crustal movements, seismic activity etc. were increased. In 1976 Ishibashi presented a hypothetic fault model of the expected Tokai earthquake and emphasized that its focal region would occur behind Suruga Bay, possibly inflicting great damage on the coastal region (Ishibasbi, 1976). This report had an enormous social impact on the Tokai district centering on

discovered, the Earthquake Assessment Committee is summoned. If the Committee decides that the anomalies are the immediate precursors of a large earthquake, the director general of the JMA is to report these results to the Prime Minister, who will immediately announce them to the public. Below is a summary of the reasons why it is currently thought that a large earthquake may occur in the Tokai region (Mogi, 1985):

0040-1951/87/$03.50

0 1987 Elsevier Science Publishers

B.V.

256

/ 1340

/ 1

I

136’

Fig. 1. <;eographical

1400

138”

142--E

map of the Tokai region and the surrounding

area

(1) Great low-angle thrust-type shallow earthquakes have occurred repeatedly at quite regular intervals of 100 to 150 years along the Nankai Trough-Suruga Trough. These earthquakes are caused by the subduction of the Phillippine Sea plate beneath the Eurasian plate. Figure 2 shows the focal regions of large earthquakes in this area since 1700. In both the 1707 and 1854 earthquakes the rupture zone extended over the whole region from the Nankai Trough to the Suruga Trough (Ando, 1975; Ishibashi, 1976; Utsu, 1977). In the 20th century the 1944 Tonankai earthquake (M * 7.9; IV,, 8.1) and the 1946 Nankaido earthquake (M 8.1; MW 8.1) occurred along the Nankai Trough, but the zone along the Suruga Trough, which is the eastward extension of this, remains unruptured. The Tokai region along the Suruga Trough is therefore a seismic gap of the first kind (Magi, 1979) and has a high potential as the site of the next large earthquake. Since there are no records of a large earthquake having occurred independently in the Suruga Trough, however, there are problems in making this simple in-

I

I

I

I

i’

/

,

groups

(1703

and 1707. 1854, and 1923. 1944 and 1946) that occurred

Fig. 2. Focal regions of the three large earthquake

along

the Nankai-Suruga * M in this paper is the JMA magnitude the Richter’s

magnitude.

which corresponds

M, is the moment

magmtude.

to

that

the Tokai

(Magi,

1981).

Trough region

forms

and the Sagami a seismic

Trough,

showing

gap of the first kind

257

ference. Mogi (1981) has studied this issue in further detail. (2) The results of leveling surveys and triangulation surveys (trilateration surveys nowadays) by the Geographical Survey Institute over several decades have revealed a marked subsidence of the west coast of Suruga Bay and a reduction of the horizontal distance across the Suruga Trough by as much as one meter (Geographical Survey Institute, 1977, 1978, 1980). The marked subsidence along the trough, where the Philippine Sea plate is subducting, and the simultaneous reduction in the horizontal distance across the trough indicate that a considerable amount of elastic strain energy is accumulating in this area. The leveling route between Omaezaki and Kakegawa (see Fig. 1) is approximately perpendicular to the trough axis, and leveling surveys have been repeated here at frequent intervals in recent years. These have revealed that subsidence at Omaezaki, which is closer to the trough, is proceeding constantly (Geographical Survey Institute, 1986), and that strain energy is steadily building up in this area. The fact that this area where large shallow earthquakes have occurred repeatedly in the past has remained unruptured for a long period and that a considerable amount of elastic strain energy is building up there indicates that a large earthquake is expected to occur in the Tokai region in the future. Attempts have been made to forecast the occurrence of the anticipated earthquake on the basis of the average recurrence interval of large earthquakes and the amount of strain accumulated, but these attempts have only provided a rough guideline. The period of 100 to 150 years that is regarded as the recurrence interval of large earthquakes in this area is for large earthquakes covering the whole Nail-Su~ga Trough area, and it is unclear whether or not this interval also applies to the earthquake, which is expected to occur independently in the Suruga Trough region. Even if it does apply, the interval at which large earthquakes recur in this area varies widely, so it is difficult to infer the occurrence of the next earthquake merely from the time that has elapsed since the previous large earthquake in this area (though

to a certain extent it is possible to forecast the occurrence of earthquakes in places where the time-predictable model is applicable, such as in the case of the Nankaido earthquake-Shimazaki and Nakata, 1980). Rikitake (1977) attempted to estimate the occurrence probability of the predicted Tokai earthquake from the amount of accumulated strain, based on the viewpoint that earthquakes occur when crustal strain reaches its ultimate value. The ultimate value of crustal strain is highly uncertain, however, and this estimate too is no more than a general guideline. The most effective method of forecasting the time of occurrence of earthquakes is by observing precursory phenomena. The main purpose of the various observations in the Tokai region is to apprehend any long-term or short-term precursory phenomena so as to forecast the time of occurrence of the expected Tokai earthquake. So far, however, there have been no reports of any phenomena that are thought to be precursors of the predicted Tokai earthquake. In this paper we will discuss how an investigation of seismic activity in the Tokai region and the surrounding areas has revealed several noteworthy changes over the past decade or so, and how it is conceivable that these changes are long-term precursors of the anticipated Tokai earthquake. Data are taken from the JMA earthquake catalogue. 2. Seismic activity scolding

the Tokai region

Utsu (1977) and Mogi (1981) have pointed out that seismic activity in the belt running in a NW-SE direction through the Tokai region may have increased somewhat over recent years. The direction is that of the motion of the oceanic plate. Since then several large earthquakes have occurred in this area. Figure 3a shows a temporal and spatial dist~bution of shallow earthquakes (less than 60 km deep) of M 6.0 or larger that have occurred in this belt, with the large circles indicating earthquakes of M 7.0 or larger, the medium-sized circles earthquakes of 6.5 or larger and the small circles indicating earthquakes of 6.0 or larger. This belt-like area lies between the focal region of the 1944 Tonankai earthquake and the

258

134”

138’

142” E

M 6.0 l

1900-

0

0

.

l

I

65

1 ZO I

1920 -

. .

1960 -

.

..e l

1980 -

t’. ..

I

1923

W

Kanto

l

Earthquake

x 22w IO

Fig. 3. a. Space-time

distribution

numing in a NW-SE

direction

time on the vertical b. Temporal

variation

axis. Arrow: of released

of shallow through

earthquakes

(less than 60 km deep) of M 6.0 or larger

the Tokai region (the dotted

1923 Kanto

earthquake

of M 7.9.

seismic energy in the area.

area in the top figure).

that have occurred

Longitude

on the horizontal

in the belt axis and

259

((1)1950 - 1960

i I I ys. 1

M

5.5

1360

tb)

140’ E

138”

1961 1972 ( 1’2~s 1 -

N

36”

0

I

0

6.0 t 6.5

northeastern Kanto region where moderate-size earthquakes are continually occurring. This reveals the high level of seismic activity prior to the great Kanto earthquake in 1923 (M 7.9). This is regarded as long-term precursory activity. Activity was also high for some time after the Kanto earthquake, but the 40 years between 1930 and 1970 were a period of relative quiescence, and the only earthquake above M 6.5 was the Boso-oki earthquake (M 7.5) in 1953. Since the 1969 Gifuken-Chubu earthquake (M 6.6), however, eight earthquakes of M 6.5 or larger have occurred, and recent years have seen increased activity in this area. The recent high seismic activity is also recognized in Fig. 3b which shows the temporal variation in released seismic wave energy. Figure 4 shows the distribution of the shallow earthquakes (up to a depth of 40 km) of M 5.5 or larger that occurred in the Tokai region and the surrounding area during the three periods of 1950-1960 (eleven years), 1961-1972 (twelve years) and 1973-1985 (thirteen years). This reveals that recently many quite large earthquakes have been occurring in the area around the Suruga Trough. 3. Seismic activity along the Suruga Trough

32“l

tcI 1973

136”

- 1985

1380

( 13~s

I 140” E

I

1

Let us now look at recent changes in seismic activity in the area along the Suruga Trough where the large earthquake is expected to occur. Figure 5 shows the dist~bution of shallow earthquakes (up to a depth of 60 km) of M 4.2 or larger during the same three periods as in Fig. 3-1950-1960, 1961-1972 and 1973-1985. Focusing on the area along the Suruga Trough and Nankai Trough, it is evident that whereas there was a certain degree of activity before 1972, during the 13 years since 1973 no earthquakes of M 4.2 or larger have occurred, and the area has been extremely quiescent. Figure 6 shows the results of section 2 and the above results of an M-T graph (denoting M on

Fig. 4. Epicentral

distribution

of the shallow

earthquakes

than 40 km deep) of M 5.5 or larger that occurred

32”[

136’

136

l40”E

J

region

and the surrounding

(a) 1950-1960;

(b) 1961-1972;

area for successive (c) 1973-1985.

(less

in the Tokai three periods:

I 32”

i 134”

tbj

/’

e

0 136”

1961 - 1972

ccl I973 - 1985

‘38”

( 12~s

I

i 13~5

I

. Fig. 4.

distribution

of shallow earthquakes

/ 142”E

140”

i38”

Fig. 5. Epicentral

/

.

. -5

140”

l’

I b

.

142” E

(less than 60 km deep) of M 4.2 or larger during

the same three periods

as in

261

1; ,I,

60 1950

!

:

‘:;,

:,

1960

1970

1980

1960

1970

1980

B 6.0

40

1950

Fig. 6. M-T graphs of shallow earthquakes (less than 60 km deep) in Regions A and B. Dotted area in the left figure indicates the focal region of the 1944 Tonankai earthquake.

the vertical axis and time on the horizontal axis) (Mogi, 1986). We have presented an M-T graph

cal coupling among these areas along the troughs. Few earthquakes have occurred in the Suruga Bay

for the surrounding region (A) that includes the Tokai region and for the region B along the Suruga Trough-Nankai Trough area. In (A) we have de-

area alone, but looking

noted earthquakes of M 6.0 or larger and in (B) we have denoted ones of M 4.0 or larger. In region (A) large earthquakes of M 6.5-7.0 began occurring around 1970, and they have been increasing in frequency. By contrast, in area (B), which includes Suruga Bay, more or less steady activity continued since 1950 (though activity ceased in 1960), but since 1973 there has been a marked decline in seismic activity. In Fig. 5 the lower limit for the magnitude of the earthquakes was M 4.2, but Fig. 6 reveals that the results vary little even if this limit is raised slightly. That is, Fig. 6 shows that M 4.2 is suitable as the threshold magnitude in Fig. 5. The 1944 Tonankai earthquake occurred in the western half of region B, so our interest nowadays lies in the eastern part that contains Suruga Bay. We have divided region B into three areas; Suruga Bay, the Zenisu ridge and the sea off the eastern Kii Peninsula which is the aftershock area of the Tonankai earthquake. Figure 7 shows M-T graphs for the activity in each of these areas. Note that the temporal changes in seismicity in these three areas are very similar, suggesting a close mechani-

.

Surugo

o Zenlsu

at the changes

in the whole

Boy Ridge

6

5

4 1950

1960

1970

-GE+--

Tonankw

“1 I

0

8

P

P

Ihi.‘!_I;lrj, i950

1960

1970

1980

Fig. 7. M-T graphs of shallow earthquakes (less than 60 km deep) in the Suruga Bay, Zenisu Ridge and Tonankai regions.

262

the troughs, including Suruga Bay where the Tokai earthquake is expected to occur, has declined remarkably since 1973, while many large earthquakes have occurred in the surrounding area. Research has shown that the focal region often becomes inactive prior to a large earthquake (appearance of a seismic gap of the second kind-Mogi, 1979), and conversely seismic activity in the su~oun~ng area increases, thus forming a doughnut pattern. It is conceivable, therefore, that the changes described above are long-term precursors of a large earthquake. However, there are occasions in which this pattern is unrelated to

of region B brings out in even clearer relief the features of the temporal course of seismic activity in this area. This approach resembles the method adopted by Ohtake et al. (1977) in discussing the quiescence prior to the 1978 Oaxaca (Mexico) earthquake of M 7.7. They concentrated on the fact that not only the focal region (the part that remained unruptured) of the expected large earthquake, but also the wide area containing the aftershock regions of the adjacent earthquakes in 1965 and 1968 became seismically inactive from about 1973. Collating these results, seismic activity along

I

/,

/I

/

134”

!36”

138” E

Tonankoi 1930

tb)

01.

Fig.

8. Focal

earthquakes

depths gradually

deepened

1935

ZP

of earthquakes before

(

in the Nankaido-Tonankai the Tonankai

1940

earthquake

$

+WW-

region

shown

as time passed.

in (a) are plotted Triangles

against

time, Hypocenters

in (a) show JMA’s seismic stations.

of

263

the occurrence of a large earthquake and is merely a fluctuation in seismic activity. In all events, future developments should be watched closely. 4, Downward mi~ation of seismic a&iv&y According to Mogi (1987), before the 1944 Tonankai earthquake seismic activity in the adjacent area (the area from the Kii Peninsula across to eastern Shikoku) to the land side of the focal region first increased at shallow depths, then gradually migrated downwards. The Tonankai earthquake occurred when the seismic activity reached its maximum depth (W-70 km) (Fig. 8). Magi (1973) pointed out examples in which a great shallow earthquake occurred along the Japan Trench one or two years after activity in the deep seismic plane had increased at a depth of 300-500 km. In this case, also, a downward migration pattern was recognized prior to a great shallow earthquake and the possible mechanism of this phenomena was discussed (Magi, 1987). Therefore, it is possible that a downward migration in seismic activity in a subduction zone may be a precursor of a large shallow earthquake, and so these phenomena merit attention. The author has examined the temporal changes in the depths of recent earthquakes in the Tokai

region from this viewpoint. Since the accuracy with which the depths of earthquakes can be determined declines greatly outside the seismic observation network, we will focus on the land area (but including Suruga Bay) in the Tokai region, which is amply covered by the observation network on land. Figure 9 plots the earthquakes of M 4.0 or larger that have occurred in this region over the past four decades (1945-1985), with the vertical axis denoting the depth and the horizontal axis denoting the time. The lively activity up until 1953 is thought to be an after-effect of the 1944 Tonankai earthquake. There was no systematic pattern in subsequent activity until 1974, since when there has been a downward migratory tendency. Figure 10 shows a blow-up of this time, plotting earthquakes of M 3.5 or larger. A similar migratory tendency was also recognized in the Tokai area before the Tonankai earthquake in 1944 (Mogi, 1987). Figure 11 shows the temporal changes in the depth of the earthquakes in the same region as in Fig. 10. It is thought that the main rupture zone of the Tonankai earthquake, however, was located to the west of the Tokai region and did not reach so far. Nevertheless, it is noteworthy that a downward ovation of seismic activity occurred in the Tokai area before the Tonankai earthquake in the adjacent area.

M

4.0:

45 :

0.0

50 - 5.5 - 6.0 -

Fig. 9. Focal depths of earthquakes of A4 4.0 or larger in the Tokai district shown in the lower figure are platted against time for the period from 1955 to 1985.

264

1985

. , .

60-

70

Fig. 10. Focal depths of earthquakes period

Figure depths the

of M 3.5 or larger in the Tokai district

shown in the upper figure are plotted

against

time for the

from 1970 to 1985

Kii

Note

12 shows Peninsula

corresponding

migratory

pattern

area. This suggests

Figure course

of

earthquakes

that extends

13 uses an M-T activity

the

level of the figure shows earthquakes

of

recent in Fig.

one, but is an actual

over quite a wide area. graph

to indicate

the and

deeper than 20 km in the Tokai

area.

that occurred

at greater

area.

seen in Fig. 10 in the

shallow

that occurred

are shown

earthquakes

The earthquakes

of

to

and earthquakes

from

migration

that the pattern

10 is not merely an apparent phenomenon

in the

Shikoku

of the downward

activity,

downward

changes

in the area extending

to the eastern

the pattern

seismic Tokai

the temporal

of earthquakes

down to 20 km in

the region in Fig. 10 are shown in the middle level.

in the bottom

larger

that

which

borders

this figure occurred

occurred

it is evident until

they have occurred seismic

markedly. area,

The

depths

uppermost of M 6.0 or

Peninsula

area,

area to the east.

From

that shallow

about

1980,

earthquakes

but since

very infrequently,

then

and instead

activity at depths below this has increased Note that during the period when shal-

low earthquakes curred

in the Izu

the Tokai

often

level.

several in

Izu-Hanto-oki

occurred

large the

Izu

shallow

frequently

earthquakes

Peninsula

earthquake

in the Tokai

(M

also

area-the 6.9)

oc1974

the

1978

265

Tonankal Earthquake 1 1945

1940

7080-

Fig. 11. Focal depths before

of earthquakes

the 1944 Tonankai

Izu-~s~rna-~nk~

in the same region

as in Fig. 10 are plotted

earthquake

1980 Izu-Hanto-toho-oki

(M

7.0) and

earthquake

district

during

1980. Of particular activity of shallow

the

period

the

trict

(M 6.8). In

other words, shallow earthquakes occurred quently throughout the whole Izu Peninsula Tokai

from 1926 up until just

from

1974

to

Figure

interest is the fact that the earthquakes in the Tokai dis-

of earthquakes

in the Nankaido-Tonankai

7, 1944 to 1985. Open circles indicate

earthquakes

slightly

earthquake

in

advance

of

the

on May 9, 1974. These

facts suggest a close relationship between activity on the Izu Peninsula and that in the Tokai area. 14 shows

the epicentral

distribution

of

earthquakes of M 4.0 or larger that occurred during 1974-1979 and 1980-1985 at depths down

.

December

time for the period

commenced

Izu-Hanto-oki

freand

M

Fig. 12. Focal depths

against

earthquake.

9

45-50-55

l

*

- 60-

region shown in Fig. 8a are plotted which occurred

in the sea area.

against

time for the period

from

266 Izu Peninsula M

.1970

1975

1980

1985

Eostern bkoi

hr20km

/

h>ZOkm

ii, 1985

Fig. 13. M-T activity

graphs of earthquakes

of shallow

earthquakes

in the Izu Peninsula

and earthquakes

deeper

and the Tokai region. The middle and bottom

figures indicate

the course of

than 20 km in the Tokai area shown in Fig. 10, respectively.

to 20 km and deeper. Only earthquakes of M 5.0 or larger (open circles) are shown for the Izu Peninsula region. This figure reveals that shallow earthquakes occurred throughout the whole region during the period from 1974 to 1979, but that almost no deeper earthquakes occurred. Between 1980 and 1985 shallow earthquakes were confined to the northeastern part and occurred very infrequently, whereas deeper earthquakes occurred often, with larger earthquakes occurring particularly on the western side where the Philippine Sea plate gradually subducts. As the discussion above indicates, during the

past ten years in the Tokai area there has been a downward migration of seismic activity for the first time in recent decades. Considering that a downward migration of seismic activity occurred in the Kii Peninsula-eastern Shikoku area before the 1944 Tonankai earthquake and that a similar tendency was also present at the same time in the Tokai area, it is possible that this may be a long-term precursor of the expected Tokai earthquake. Since the years between 1974 and 1980 were a period in which there were several larger earthquakes of the M 7 class in the Izu Peninsula area, however, it is also conceivable that the effect

267

1980-

M Fig, 14. Epicentral figure)

dist~bution

at the focal depths

1985

h=O-20km

h=O-20km

h:21-50km

h: 21-50km

Amoco 40.45.50.55-60-

of earthquakes

of M 4.0 or larger

of O-20 km and 21-50

that occurred

during

1974-1979

(left figure) and 1980-1985

(right

km.

of the earthquakes in the 121.1Peninsula extended into the Tokai area to produce these changes. In either case, these changes must be followed up in the future. 5. Conclusion (1) Since 1970 large earthquakes of M 6.5 or larger have been occurring frequently in the region surrounding the expected focal region of the Tokai earthquake, thus suggesting increased stress in this region. (2) Seismic activity in the region along the Suruga Trough-Nankai Trough, which contains the expected focal region of the future Tokai earthquake, has declined significantly since 1973. This quiescence may indicate the appearance of a seismic gap of the second kind. (3) A pattern of downward migration of the depth of earthquakes can be observed in the land part of the Tokai area over the past ten years. This pattern resembles the pattern of seismic activity before the 1944 Tonankai earthquake, so it may

be a long-term precursor of a large earthquake. (4) It is also fully conceivable that the various changes indicated above have merely appeared by chance with no relation to the occurrence of a large earthquake. It is noteworthy, however, that these changes have appeared at virtually the same time in the past decade, and so future developments should be followed carefully. Acknowledgements I would like to express my appreciation to Prof. T. Yoshii and Dr. Y. Ishikawa for providing computer programs for the space-time distribution of earthquakes. References Ando,

M., 1975. Source

of historical Tectonophysics, Geographical Tokai

mechanisms

earthquakes

along

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siguificance

trough,

Japan.

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Coord.

(in Japanese).

1977. Crustal Comm.

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19: 96-98 Geographical strains

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in the Predict..

in horizontal

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K., 1976.

expected

1986. Crustal Comm.

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Earthquake

in the

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in the Tokai

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of the “Suruga

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seismic

earthquake

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(in

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(SuppI.):