Tectonophysics, 51 (1979) 261-283 267

@ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

PATTERN OF INTRAPLATE SEISMICITY IN SOUTHWEST JAPAN BEFORE AND AFTER GREAT INTERPLATE EARTHQUAKES

TETSUZO SENO

Geophysical Institute, Faculty of Science, University of Tokyo, Tokyo 113 (Japan)

(Submitted August 9, 1977; revised version accepted July 14, 1978)

ABSTRACT

Seno, T., 1979. Pattern of intraplate seismicity in southwest Japan before and after great interplate earthquakes. Tectonophysics, 57: 267-283.

A fairly complete set of data on intraplate seismicity in southwest Japan during the past 170 years reveals that the seismicity before great interplate earthquakes along the Nankai trough is high over the land area adjacent to the rupture zones of the great inter- plate earthquakes and the seismicity after the interplate earthquakes is high in the mar- ginal zones that border the preseismically active area. This change of seismicity distribu- tion before and after great interplate earthquakes can be explained by the two modes of horizontal deformation in the continental-plate margin; that is, the contraction of the land area adjacent to the rupture zone of great interplate events before their occurrence and the blockwise extension of the area seaward at the time of these interplate shocks.

One of the characteristic features of intraplate seismic energy release during historic times is that it is large in the narrow zones which border the land areas adjacent to the specific rupture zones of historic great interplate earthquakes. These zones must have been exposed to the shearing stress due to the blockwise extension of the areas adjacent to the specific rupture zones at the time of interplate shocks and this may provide a rea- son for the large seismic energy release within these marginal zones in historic times.

Recent intraplate seismicity in southwestern Japan shows that intraplate earthquakes tend to cluster in the area adjacent to the expected rupture zone of a future great event off the Tokai district. A simple statistical test shows that this clustering of intraplate events in the area is significant within a 96% confidence level. The level of seismic activity in this area is 18 times larger than the normal level of activity between interplate earth- quakes. This high level of activity provides another piece of evidence for a possibility of occurrence of a great interplate event off Tokai. The land area adjacent to the rupture zone off Tokai deserves high priorty for instrumentation of various types to record in the near field the destructive intraplate earthquakes which may occur over several decades before and after the future great Tokai event.

INTRODUCTION

In this study the interaction between the oceanic and continental plates of lithosphere along the Nankai trough (Fig. l), a consuming plate boundary

268

36’

32’

Sagan% Trough

Nankai Trough

Philippine Sea

130” E 135O 140°

Fig. 1. Map of southwestern Japan. The area treated in this study is indicated by the broken line.

between the Philippine Sea and the Eurasian plates, is investigated in terms of spatial distribution of intraplate seismicity before and after historic great interplate earthquakes along this plate boundary. The stress states of the con- tinental-plate margin along consuming plate boundaries may be significantly affected by the occurrence of great interplate earthquakes (Mogi, 1969; Utsu, 1974a, b; Shimazaki, 1976b, 1978; Seno, 1978). Intraplate earthquakes occurring in the vicinity of the plate boundary are supposed to have a tec- tonic origin closely related to the plate interaction. Southwest Japan may be one of the most appropriate plate margins for studying the plate interaction in terms of intraplate seismicity because the plate boundary along the Nankai trough has been broken regularly in a series of great interplate earthquakes at lOW200-year time intervals at least for the past lo5 years (Yoshikawa et al., 1964; Yonekura, 1968; Fitch and Scholz, 1971; Ando, 197513; Seno, 1977a, b) and because fairly complete historic seismicity data for the past hundreds of years are available in southwest Japan (e.g., Shimazaki, 1976a). The great historic interplate earthquakes along the Nankai trough are herein called Nankai trough earthquakes or Nankai trough events, for brevity.

Ozawa (1973) noticed that since the year 887, destructive earthquakes in the vicinity of Kyoto (Fig. 1) have occurred more frequently during the

269

thirty years before Nankai trough events than during other periods. Utsu (1974a, b) showed that for the past 1000 years the frequency of occurrence of earthquakes accompanied by damage in southwest Japan is almost four times larger during the fifty years before and the ten years after the Nankai trough events than during other periods. Shimazaki (1976b) confirmed the results by Utsu (1974a, b) for the past 400 years and further pointed out that the seismicity prior to the Nankai trough-events is high along the inland belt of southwest Japan and that after these events it is high along the Japan Sea and the Philippine Sea coasts.

In this study, the spatial distribution of intraplate seismicity before and after the Nankai trough events is investigated especially in connection with the spatial extent of the rupture zones of the Nankai trough events. Charac- teristic features of intraplate seismicity distribution before and after the Nankai trough events may provide a useful piece of information for instru- mentation to record various phenomena associated with severe intraplate earthquakes.

The intraplate seismicity during the 50 years before and the 10 years after Nankai trough events is called preseismic and postseismic activity, respec- tively. The seismic activity during other periods is called interseismic activ- ity.

DATA

The seismicity data used in this study are: (1) the “Descriptive Catalogue of Disaster Earthquakes in Japan” (Usami, 1975) for the period before 1926, (2) the “Catalogue of Major Earthquakes in and near Japan” (Japan Meteo- rological Agency, JMA, 1958, 1966, 1968) for the period 1926-1967, and (3) the “Seismological Bulletin” of the Japan Meteorological Agency for the period 1968-1976.

The time period treated in this study includes the past 422 years (1555- 1976). In this period, four seismic cycles of great Nankai trough events are contained (one seismic cycle means a series of pre-, post- and interseismic activity in this order for one Nankai trough event). They are: the 1605 Keicho event (M = 7.9), the 1707 Hoei event (M = 8.4), the 1854 Ansei I (il4 = 8.4) and Ansei II (1M = 8.4) events, and the 1944 Tonankai (M = 8.0) and the 1946 Nankaido (A4 = 8.1) events. The completeness of the seismicity data is not guaranteed in general, especially in dealing with such a long his- toric time as the past 400 years. Shimazaki (1976a) examined the detect- ability of earthquakes in this period in Usami’s catalogue and obtained a qualified guarantee for the detectability of earthquakes of magnitude 6.4 and above. We treat in this study earthquakes of magnitude 6.0 and above. Although a certain number of historic events of magnitude 6.0-6.3 might not be listed in Usami’s catalogue for the period before 1926, events in this magnitude range are used supplimentarily in this study. The Central Meteo- rological Observatory (the former JMA) began the systematic collection of

270

reports on felt earthquakes all over Japan in 1884 (Usami and Hamamatsu, 1967); thus, seismicity data since 1884 are reliable enough for the present study, although this has caused a remarkable change in detectability of earth- quakes in Usami’s catalogue (Shimazaki, 1976b). Fortunately, however, this discontinuity in data corresponds approximately to the start of the seismic cycle of the 1944 and 1946 events. Since intraplate seismicity is investigated for each seismic cycle in this study, the discontinuity around the years 1884 will not affect our study too seriously.

The area indicated by the broken line in Fig. 1 will be roughly termed “southwest Japan” and those intraplate earthquakes occurring within this area are treated in this study.

To select only intraplate earthquakes, we exclude the following earth- quakes: apparent aftershocks of the Nankai trough events, those with foci deeper than 50 km, and those that occurred in the sea near the rupture zones of the Nankai trough events. Although the focal depths of the earthquakes in Usami’s catalogue are not known, we regard them as shallow, because deep earthquakes rarely cause damage (e.g., Shimazaki, 1976a).

INTRAPLATE SEISMICITY BEFORE AND AFTER NANKAI TROUGH EARTHQUAKES

1944 Tonankai and 1946 Nankaido earthquakes

The Tonankai event (M = 8.0) of December 7, 1944 and the Nankaido event (M = 8.1) of December 21, 1946, are the most recent Nankai trough events. The former event ruptured zone C and the latter one zones A and B (Fig. 2; Ando, 1975b; Ishibashi, 1977). Since the latter event occurred only two years after the former one, intraplate seismicity before and after these events should be treated at the same time.

Fig. 2A shows the preseismic activity during the 50 years before these events. The direction of plate convergence under southwest Japan (Seno, 1977b) is indicated by the white arrows. In this activity, the well-known destructive intraplate events: the 1905 Geiyo (M = 7.1), 1909 Anegawa (M = 6.4), 1925 Kita-Tajima (M = 6.5), 1927 Kita-Tango (M = 7.5), 1936 Kawa- chi-Yamato (M = 6.4), and 1943 Tottori (M = 7.4) events are included. It can be seen that the seismic activity is distributed over the entire region of south- west Japan, which is adjacent to the rupture zones of these Nankai trough events (hatched area in Fig. 2A).

Fig. 2B shows the postseismic activity during the 10 years after these events. In this activity, the 1945 Mikawa (M = 7.1), 1948 Fukui (M = 7.3), 1948 Hidakagawa (M = 7.0), and the 1952 Daishoji-Oki (M = 6.8) events are included. The level of this activity is not necessarily lower than that of the preseismic activity, as it appears, because the duration of postseismic activity is one fifth of that of preseismic. It can be seen from this figure that earth- quakes tend to cluster in the hatched zones adjacent to the edge of the rup- ture zones of these interplate events.

A PRESEISMIC ( 50yrs before 194&,1946 events)

36"

I -n 0 200km

B POSTSEISMIC (10yrs after 1964,1946 events > 34"

36 '

34"

370

I;

30" .*

0 2OOkm I t ,

, I I I

130°C 132" 13‘2" 136" 1380 1400

0 ?!>7.0 0 ?.O>It16.5 0 6.i>M~6.0

Fig. 2. A. Preseismic activity of the 1944 Tonankai and the 1946 Nankaido events. The rupture zones of these events are indicated by A, B and C. The preseismically active area is hatched. White arrows show the direction of the relative plate motion along the Nankai trough. B. Postseismic activity of the 1944 and 1946 events. Marginal zones adjacent to the edge of the rupture zones of these events are hatched.

272

A PRESElSMIC ( SOyrs before Ansei events )

36"

34"

3.x n

N 10"

B POSTSElSMlC (10yrs after Ansei events )

0 MZ3.0 0 7.037HZ6.5 0 G.5>M26.0

Fig. 3. A. Preseismic activity of the 1854 Ansei I and Ansei II events. The rupture zones of these events are indicated by A, B, C and E. Preseismically active area is hatched. B. Postseismic activity for the Ansei events. Marginal zones adjacent to the edge of the rup ture zones of these events are hatched.

273

INTERSEISMIC (1865-1894 )

Fig. 4. Interseismic activity between the Ansei events and the Tonankai and Nankaido events during the period 1865-1894.

1854 Ansei I and Ansei II earthquakes

The Ansei I event (A4 = 8.4)bf December 23, 1854, and the Ansei II event (M = 8.4) of December 24,1854, are the Nankai trough events preceding the 1944 and 1946 events. The Ansei I event ruptured zones C and E and the Ansei II event zones A and B (Ando, 197513; Ishibashi, 1977). The latter occurred only 32 h after the former; thus, the preseismic and postseismic activity of these events should be investigated as if the two events made one rupture zone A + B + C + E.

Figures 3A and 3B show the preseismic and postseismic activity of these events, respectively. The land area adjacent to the rupture zones of these events is hatched in Fig. 3A. Although the number of the intraplate events is scarce, the postseismic activity seems to cluster around the edge of the rup- ture zone of the Ansei events (hatched area in Fig. 3B).

Figure 4 shows the interseismic activity between the Ansei events and the 1944 and 1946 events for 30 years (1865-1894). There are found only two main shocks of magnitude 6.0 and above; they are the 1872 Hamada event (M = 7.1) and the 1891 Mino-Owari event (M = 8.0). It may be possible to include the Mino-Owari event in the preseismic activity of the 1944 and 1946 events because it occurred 53 years before the 1944 event.

1605 Keicho and 1707 Hoei earthquakes

The Keicho event (M = 7.9) of February 3,1605, and the Hoei event (M = 8.4) of October 28, 1707, are the two Nankai trough events preceding the

274

Ansei events. There remains much ubiquity about the spatial extent of the rupture zones of these former events. Although the level of preseismic and/ or postseismic activity of these events is significantly higher than that of interseismic activity, it is difficult to find any significant difference in spatial distribution between pre- and postseismic activity (figures for the pre- and postseismic activity of these events are not presented).

Recent seismic activity in southwest Japan

Figure 5 shows the intraplate seismicity for the past 20 years (1957- 1976), that is, the activity after the postseismic period for the 1944 and 1946 events. Many Japanese seismologists (Mogi, 1969; Utsu, 1974a, b; Rikitake, 1974; Ando, 1975a; Ishibashi, 1977) noted a possibility of the occurrence of a &eat interplate event off the Tokai district, southwest Japan (see Fig. 1). Recently, Ishibashi (1977) estimated a fault zone for this predicted event, which is indicated by E in Fig. 5. It is very interesting to notice that intra- plate earthquakes for the past 20 years tend to cluster in the area adjacent to the expected rupture zone E. The other three intraplate events during this period are distributed in Shikoku and Kyusyu. They occurred a few years after the 1968 Hyuganada interplate event (M = 7.5) in the vicinity of its rupture zone (Fig. 5); thus, these events may be regarded as the postseismic activity of the Hyuganada event.

RECENT SElSMlClTY 11957 -1976 1

la00

32”

1 .30°E 7 730 1340 1’5” 13?0 l‘i00

Fig. 5. Recent (X957-1976) intraphte seismicity (M 2 6.0). The area adjacent to the expected rupture zone of a future great Tokai event is hatched. The rupture zone of the 1968 Hyuganada event is shaded.

275

36'

N 32'

130'E 134" 13a"

Fig. 6. Southwest Japan divided into 17 compartments from a to q parallel to the direc- tion of the relative plate motion. Compartments n,o,p adjacent to the rupture zone E are shaded.

The spatial distribution and the level of seismic activity in the vicinity of the Tokai district may suggest a future rupture of zone E as was first noted by Utsu (1974b). In this section, we attempt to examine whether this activ- ity is really high or not. First, the spatial distribution of intraplate earth- quakes is examined as follows. We divide the whole area of southwest Japan by equally spaced parallel lines into seventeen strips a, b, c, . . . and q in the direction of relative plate motion (Fig. 6); these strips are called “com- partments” and each compartment is approximately 40 km wide as mea- sured perpendicular to the direction of relative plate motion. The compart- ments II, o, p, which are adjacent to the rupture zones E in the direction of relative plate motion is shaded in Fig. 6. We disregard here the compart- ments m and q for convenience, although it may be better to shade also a part of these compartments. We calculate the probability that more than a certain number of intraplate events fall into compartments n, o, p or the assumption that each event occurs randomly and independently in the j-th compartment with a probability of 0. Pj is defined by the relative frequency of intraplate events in each compartment during the period 1555-1956. Figures 7A and 7B show the cumulative number of intraplate events in each compartment during 1555-1956 with magnitude 6.4 and above, and with 6.0 and above, respectively.

Out of the eleven events of magnitude 6.0 and above during 1957-1976, seven occur in compartments n, o, p; the probability of occurrence of seven or more events in these compartments in eleven Bernouill’s trials gives 0.95%.

276

1555 - 1956

z A:?.4364 Y

u

z

abcdefghijklmnopq

I

~

8:~*60

210

abcdefghi j k lmnopq

Fig. 7. Cumulative number of historic (1555-1956) intraplate events in each compart- ment: A. for the events of magnitude 6.4 and above; 3. for the events of magnitude 6.0 and above.

Out of the five events of magnitude 6.4 and above, four occur in these com- partments; the probability of occurrence of four or more events in these compartments in five trials gives 3.6%. In either case, the clustering of intra- plate events into compartments n, o, p is significant within a 96% confidence level.

Assuming a flat distribution for pj (i.e., equal pi for all j) reduces the prob- ability of earthquake clustering into these compartments even more. We recall that in the above calculation of the probability, comp~ments n, o, p were identified before calculation as the area adjacent to the expected rup- ture zone E for a future great event off Tokai. If the compartments are not identified, but, instead, we allow earthquakes to cluster in any series of three compartments, the probability of earthquake clustering becomes larger, i.e., between 10 and 20%.

Next, we examine whether the level of recent intraplate seismicity is high or not when compared with interseismic activity. It is difficult to tell whether seismicity at a particular time is high or not when the source of seis- micity data is not uniform. The discontinuity in data around the year 1884 is of particular concern. Fortunately, however, this discontinuity approxi- mately co~esponds to the time of onset of the preseismic activity of the 1944 and 1946 events; thus, we normalize the preseismic activity of each

277

TABLE I

Relative levei of pre-, post-, and interseismic activity in the whole region of southwest Japan

Frequency (M > 6.4)

Rate of se&- mic energy release

Period Preseismie Postseismic interseismic -- PPP

A 1555-1890 1.0 * 3.3 0.5 B 1891-19% 1.0 * 2.3 1.2

A 1555-1890 1.0 * 0.4 0.15 3 1891--1976 1.0 * 0.7 0.17

* The frequency and rate of seismic energy release are normaiised to 1.0 for the level of preseismic activity.

seismic cycle to 1.0 and compare the relative level of seismic activity for the past 20 years with that of interseismic activity for the former three seismic cycles of the 1605, 1707, and 1354 events. Intraplate earthquakes of magni- tude 6.4 and above are treated in these statistics because some of those of magnitude smaller than 6.4 may not be listed in Usami’s catalogue.

The frequency of intraplate events and the rate of seismic energy release, calculated by the energy-magnitude relation of Gutenberg and Richter (1956) during the periods A (1555-1390) and B (1891-1976) for a cycle of pre-, post-, and interseismic activity, are shown in Table I.

The preseismic values are normahzed to 1.0. For the period A, the value averaged over the three seismic cycles is presented. The period of the preseis- mic activity of the 1944 and 1946 events is extended to 53 years from 50 years to include the Mino-Owari event in 1891 which released great seismic energy. The level of the recent (1957 to 1976) seismic activity is shown con- ventionally in the last column (interseismic) of period B in Table I. The level of recent seismicity relative to that of preseismic activity of the 1944 and 1946 events is slightly higher than that of the interseismic activity of the for- mer three seismic cycles; however, the difference is small and considered not to be si~ific~t. It should, however, be noticed that the recent seismicity is especially high in compartments ft, o, p as was shown above. Thus, we can further investigate the level of the activity restricting the region to only the compartments Iz, 0, p,

Table II shows the normalized level of activity for each seismic cycle dur- ing the periods A and B when the region is confined to compartments n, o, p. In contrast, the relative level of the recent seismicity (last column for

period B in Table IX) is 18 times higher than that of the interseismic activity during period A, both in frequency and in rate of seismic energy release.

278

TABLE II

Relative levels of pre-, post-, and interseismic activity in compartments n, 0, p

Period Preseismic Postseismic Interseismic

Frequency A (M > 6.4) 1555-1890 1.0 * 2.5 0.2

B 1891-1976 1.0 * 5.3 3.5

Rate of seis- A mic energy X55-1890 1.0 * 0.13 0.013 release 3

1891-1976 1.0 * 0.82 0.23

* The frequency and the rate of seismic energy release are normalised to 1.0 for the level of preseismic activity.

Both the spatial distribution and the level of’ recent seismicity indicate that the activity in the vicinity of the rupture zone E is significantly high. This may provide another piece of evidence for the possibility of the occur- rence of a great Tokai event in the near future.

DISCUSSION

Horizontal deformation of Continental-plate margin along consuming plate boundary

In the former section, we have shown that the area adjacent to the rupture zone of impending great Nankai trough earthquakes is preseismically active and, in contrast, the marginal zones which border the preseismically active area seem to be postseismically active, although the latter feature is less evi- dent because of the limited data and the short time duration of post seismic activity. These features of spatial pattern of pre- and postseismic activity may be closely related to the mode of deformation in the continental-plate margin due to the interaction between the plates. One of the most character- istic features of the crustal deformation of the Japanese islands is that the pattern of horizontal deformation is significantly affected by the occurrence of great interplate shocks (Mogi, 1970; Fitch and Scholz, 1971; Ando, 1975b). Figures 8A and 8B show schematic~ly the mode of horizontal deformation of the Japanese islands after Mogi (1970). During the period until a great interplate event occurs, the crust of the islands adjacent to the rupture zone is compressed by the underthrusting movement of the oceanic plate and is contracted (Fig. 8A). In contrast, the contracted area will extend seaward at the time of the interplate shock (Fig. 8B). Mogi (1970) showed, using triangulation data, that the recent horizontal deformation of the Japa-

279

A

Fig. 8. Schematic figure which represents horizontal deformation of the Japanese islands (After Mogi 1970.) A. Pattern before a large interplate earthquake. B. Pattern at the time of the earthquake. The land area exposed to sudden shearing stress at the time of the shock is shaded.

nese islands really represents this mode of defo~ation for the past several tens of years. Before an interplate event occurs, compressive stress in the crust adjacent to the rupture zone will increase gradually and reach a maxi- mum just before the interplate event. This will cause the crust to fracture where the stress exceeds the crustal strength; this provides a mech~ism for preseismic activity, which is proposed by Shimazaki (19’7613, 1978). In eon- trast, once an interplate shock has occurred, the major portion of the stress in the crust of the area adjacent to the rupture zone will be relieved. How- ever, seaward, the extension of the compressed crustal block at the time of the shock will expose a sudden shearing stress to the zones bordering the area which extends seaward (shaded zone in Fig. 8B). This may provide an explanation for the postseismic activity, although further investigation on the detailed behavior of the cont~ent~ plate margin associated with great interplate events along the Japanese islands may be needed to substantiate this idea. It may be useful to notice that the change of maximum shear strain caused by the occurrence of Nankai trough earthquakes computed by the dislo- cation theory using the computer program by Sato and Matsu’ura (1974) reaches at most lo-’ in the inland belt of southwest Japan and 10m6 along the Japan Sea coast. Thus, shear strain change caused by the occurrence of

280

A

B POSTSEISMIC

IO”’ ENERGY RELEASE

erg 1556-1976

abcdefghijklmno

c

5*1& @ w

TOTAL SEISMIC

11 Pq

abcdrfghijklmnopq

Fig. 9. A. Marginal sheard zones in reference to the specific rupture zones of historic Nan- kai trough events. B and C. Postseismic and total seismic energy release in each compart- ment during the period 1555-1976, respectively.

281

interplate events is too weak to fracture the crust by itself and probably plays a role in triggering intraplate events, as was discussed for the Fukui event by Yamashina (1975). In contrast, the compression due to subduction of the oceanic plate may play an essential role in inducing preseismic activity because the contraction of the crust revealed by repeated geodetic surveys reaches 2-3 - 10e5 in 120 years in southwest Japan (e.g., Harada and Kassai, 1971). It is interesting to note that in Tables I and II the frequency of earth- quakes in postseismic activity is a few times larger than that in preseismic activity and, in contrast, the rate of seismic energy release of postseismic activity is less than that of preseismic activity. This means that postseismic events tend to have smaller magnitude than preseismic ones, that is to say, that the b-value of postseismic activity is larger than that of preseismic activ- ity. This is consistent with the small change of shear strain caused by the occurrence of interplate shocks in comparison with the large contraction of the crust before their occurrence.

Mode of historic seismic energy release

It is well known that historic Nankai trough earthquakes have occurred repeatedly on specific rupture zones (Ando, 197513); for example, A and B make one rupture zone, although sometimes zone C and/or zone E are also involved. If this is also true in the geological time scale, the horizontal deformation of the crust associated with the occurrence of interplate events will bring shearing stress to specific marginal zones bordering the areas adja- cent to the rupture zones of historic interplate events; these zones will become weak after many cycles of loading and unloading. The marginal zones resulting from the specific rupture zones A + B, C, and E are hatched in Fig. 9A. Seismic energy will be released more effectively in these zones than other areas, especially in postseismic activity, provided that no creep occurs in the crust of southwest Japan. Figs. 9B and 9C show the postseismic and total seismic energy release, respectively, during the period 1555-1976. Ninety per cent of the postseismic energy is released in eight compartments b, c, i, j, n, o, p, q which cover the hatched zones in Fig. 9A. For the total seismic energy, 75% of it is released in these compartments. Although an energy release pattern such as that presented in Fig. 9 for a short historic time may be a transient one as was discussed by Shimazaki (1976a), we believe that it reflects, at least in part, the intrinsic efficiency of seismic energy release in southwest Japan.

CONCLaUSIONS

A fairly complete set of seismicity data in southwest Japan for the past 172 years (1805-1976) reveals that preseismic activity prior to great inter- plate earthquakes along the Nankai trough is high in the area adjacent to the rupture zone of these interplate events and that postseismic activity is high

282

in the maq$na.l zones which border the preseismically active area. This differ- ence in spatial distribution between preseismie and postseismic activity may be explained by the two modes of horizontal deformation of the continen- tal-plate margin along the consuming plate boundary associated with great interplate e~hquakes; these modes are the contraction of the margin adja- cent to the rupture zone of an impending great interplate earthquake before its occurrence and the seaward extension of the contracted block at the time of the shock.

The pattern of intraplate seismic energy release for the past 400 years shows that total seismic energy, and especially pos~eismic energy are released more effectively in the zones which border the areas adjacent to the specific rupture zones of historic Nankastrough events. This may be caused by the weakness of these zones due to many cycles of loading and unloading associated with ~terplate events in geologic time.

The hypothesis of the spatial distribution of preseismic and postseismic activity presented in this study can be used as a possible criterion for fore- casting not only the fault zone which is likely to be ruptured by an impend- ing interplate event, but also the land area of high seismic risk for severe intra- plate shocks. Recent intraplate events in southwestern Japan tend to cluster in the area adjacent to the expected rupture zone of a future great Tokai event and the level of activity is significantly higher than the normal level of activity between inte~la~ events. This probably indicates a possibility of the occurrence of a great Tokai event in the near future. Another application of the criterion proposed above to the earthquake prediction along the Japa- nese islands suggests a possibility of the occurrence of a large interplate earth- quake off the southern Sanriku coast, northern Japan, which will be dis- cussed in a separate paper (Seno, 19’78). From the viewpoint of earthquake prediction for severe intraplate events, the area adjacent to the rupture zone off Tokai deserves high priority for instrumentation of various types to record phenomena in the near field associated with imminent intraplate earth- quakes.

I wish to thank Dr. K. Shimazaki and Dr. P.G. Somerville for critical review of the manuscript and Prof. T. Rikitake for his encouragement, I benefitt~ from discussions with Dr. S. Ohnishi, Dr. K. Y~as~na, Dr. K. Ishibashi, Prof. H. Kanamori, and with the members of the Laboratory of Prof. H. Takeuchi. I also wish to thank Dr. 523, Krishna and Dr. D. Hadley for critical review of the manuscript at the summer school of geodynamics at Cargese, Corsica, 1977.

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