earthquake prediction studies in japan

E A R T H Q U A K E P R E D I C T I O N S T U D I E S I N J A P A N

T S U N E J I R I K I T A K E

Earthquake Research Institute, University of Tokyo, Tokyo, Japan

Abstract. A long-term research program on earthquake prediction in Japan, officially launched in 1965, has made progress. Many of the developments achieved in recent years in various disciplines are outlined. Some of the important findings include: detection of land-deformation by intensified levelling and geodimeter surveys, empirical relations between the extent of a premonitory /and- deformation and the magnitude and occurrence-time, and the growth and decay of earthquake swarms accompanied by occurrences of large-scale earthquakes. Operations research on selection of survey areas for levelling and the location of crustal deformation observatories has been made.

To process data for earthquake predictions, three centers for different disciplines were set up: in the Geographical Survey Institute, the Japan Meteorological Agency, and the Earthquake Research Institute (University of Tokyo). A newly established committee, called the Coordinating Committee for Earthquake Prediction, which consists of about 30 specialists, analyzes the data flowing into these three channels. The committee issues a warning of earthquake danger, whenever possible.

A tentative strategy for achieving earthquake prediction is proposed. An attempt is made to evaluate ratings of earthquake threats on the basis of probability theory.

An anomalous land-deformation was found in the South Kanto district, an area south of Tokyo, in 1969. Following the strategy, an intensive effort, called Operation South Kanto, aiming at a possible prediction of large earthquakes is now under way. Judging from the results of various earthquake prediction elements, the probability of having an earthquake of magnitude 7 or there- abouts within a period of about 10 yr can not be low if anomalous land-deformation is related to the probability of earthquake occurrence.

1. Introduction

Japan has been suffering f rom ea r thquake disasters t h roughou t her history. The wors t

was the K a n t o ea r thquake (Kan to means the distr ict a round Tokyo) , which occurred

on September 1, 1923. More than 100 000 lives were lost, mos t ly by fires s tar ted soon

after the shock. The popu la t i on of Tokyo has ever been increasing since tha t t ime,

exceeding 10000000 in recent years. A l though cons t ruc t ion o f e a r t h q u a k e - p r o o f

bui ldings is now becoming popula r , i t is feared tha t the funct ioning o f the wor ld ' s

largest city would comple te ly s top if an ea r thquake of s imilar magni tude should be

repeated .

I f the occurrence of a large-scale ea r thquake could be foretold, i t wou ld at least be

possible to save the lives o f inhabi tants , even though it wou ld be difficult to save

p roper ty . Social demands for the pred ic t ion of ea r thquakes have therefore been very

s t rong in Japan, a l though no systematic effort t oward ea r thquake p red ic t ion was

begun unti l a g roup of seismologists p roposed a p lan abou t 10 yr ago. Tsubo i et al.

(1962) publ i shed a report , 'P red ic t ion of Ear thquakes - Progress to Da te and Plans

for Fu r the r Deve lopment ' . This was the very s tar t of the na t ion-wide research project

for ea r thquake predict ion. This now-famous repor t summar ized what Japanese seis-

mologis ts bel ieved was the best for achieving pred ic t ion of ear thquakes . The r epo r t

Geophysical Surveys 1 (1972) 4-26. All Rights Reserved Copyright �9 1972 by D. Reidel Publishing Company, Dordrecht-Holland

EARTHQUAKE PREDICTION STUDIES IN JAPAN 5

has been called the 'blueprint' of the earthquake prediction program. Developments in the following years were strongly influenced by the blueprint.

A government-supported, long-term plan of earthquake prediction research was launched in 1965, after much debate, by the Science Council of Japan, the Geodetic Council, and other governmental agencies. The plan was tested by a timely seismic event, i.e., the 1965-7 swarm of earthquakes around the town of Matsushiro, in the central part of Japan. The seismic activity was so high that more than 600 earthquakes a day were felt in April, 1966. Observations along the lines of the blueprint led to an outstanding success, so that warnings (or long-range forecasts) of possible occurrences of moderately destructive earthquakes were officially issued to the locality.

Northern Japan was violently shaken by the Tokachi-Oki earthquake on May 16, 1968. In consequence, the necessity of earthquake prediction was then discussed at the cabinet level, and a drastic intensification of earthquake prediction research was proposed. It is now the practice to undertake actual prediction, whenever possible, rather than prediction research, although the methodology of earthquake prediction has not yet been firmly established.

Many of the advances of earthquake prediction research in Japan have been re- viewed from time to time, by Rikitake (1966, 1968a), Hagiwara and Rikitake (1967), and Kanamori (1970). The aim of this article is to cover some of the most recent developments, along with the well-established results of earthquake prediction research in Japan.

2. Japanese Program on Earthquake Prediction Research

Researches on earthquake phenomena were initiated mainly by scientists invited to Japan from western countries at the beginning of the modernization of Japan about 100 yr ago. These scientists, who were surprised at the frequency of earthquakes, organized the Seismological Society, together with Japanese scientists. Since then, many data relating to earthquake phenomena have been accumulating.

As a result of observations made over a long time, Japanese seismologists began to recognize, although vaguely, what they should do to achieve earthquake prediction. All through the discussions made by the Earthquake Prediction Research Group, a number of important points had become clear, as pointed out in the blueprint.

Since the mechanism of earthquake generation is not definitely known, the methodology of earthquake prediction depends largely on experiences in association with geophysical observations over many years, rather than on physical theories of earthquake origin. However, it has become widely believed in recent years that most earthquakes are caused by the sudden rupture of underground rocks. Assuming that this theory is correct, it becomes possible, in some cases to imagine what would happen before an earthquake. The following geophysical factors are those which seismologists at present consider useful for predicting earthquakes. They are more or less an extension of those pointed out in the blueprint, supplemented by advances in recent years.

6 TSLrNEJI RIKITAKE

2.1. GEODETIC WORK

Historical occurrences of remarkable land deformations in association with earthquakes of large magnitude have often been reported. Such deformations are often detected now by means of precise levelling surveys or triangulation surveys, even at a considerable distance from the epicentral area. It is quite natural to think, therefore, that some anomalous land deformations, probably one order of magnitude smaller than that for the whole deformation throughout an earthquake event, might be monitored prior to an earthquake, provided that precise land surveys are carried out repeatedly over the area in question.

In the case of the 1964 Niigata Earthquake, with an estimated magnitude M = 7.5 in the Gutenberg-Richter scale, an anomalous land deformation was noticed by unusually frequent repetitions of levelling surveys along a route which passed near the epicenter (Tsubokawa et al., 1964). These levellings were originally planned for check- ing land subsidence due to the withdrawal of natural gases. As can be seen in Figure 1, the monotonic change in the height of bench marks since 1900 ended about 1955, being followed by an anomalous change which may be a premonitory effect of the earthquake. For the purpose of earthquake prediction it is important to detect the beginning of the anomalous phase.

According to the Japanese program on earthquake prediction research it was planned to resurvey all routes of first-order levelling surveys, with a total length of 20000 km, every 5 yr. More frequent resurveys, with an interval of 2 yr or less, were recommended for particularly important areas such as the Tokyo area. The Geographical

Earthquake

cmf I J0

s, SEA 0 - 1900 20 40 60~

~a A~A N crn

1900 2O 40 6O

10 /

- - Kashiw 0 40km ~> , , , 0 ~ _

19C0 20 40 60

\ ~ , ~ 20 4o 6o ~ o

Fig. 1. Anomalous changes in height at levelling bench marks before the 1964 Niigata earthquake (solid circles). The open circles indicate the heights as found by a levelling survey immediately after the earthquake. The land subsidence at the time of the earthquake as estimated from the tide-gauge

observation at Nezugaseki is marked by a solid triangle (after Tsubokawa et al., 1964).


Survey Institute of the Ministry of Construction, which is responsible for the nation-

wide geodetic work completed the levelling survey through intensified operations in a few years.

The wider the area of land deformation associated with an earthquake is, the larger is the magnitude of that earthquake. Figure 2 shows the empirical relation between

�9

21

'~ 17 o

15 5

0 0 0 0 0

�9 0 �9 �9

! i ~ l t l l I r l r l l l m 6 7 8

M

Fig. 2. Log r a versus M, where r is the effective radius, m cm, of the area over which an anomalous crustal deformation associated with an earthquake is observed (after Dambara, 1966).

the logarithm (to base 10) of the mean radius r of the land-deformation area and the magnitude M of the earthquake (Dambara, 1966). The relation is approximately

logr 3 = 8.18 + 1.53 M (1)

when r is measured in cm. I t is reasonable to suppose that relation (1) holds good even for a precursory deformation. In that case the probable magnitude of an expected earthquake can be guessed at if a premonitory anomalous land-deformation is detected by geodetic measurement.

Hagiwara (1971) presented an at tempt based on operations research for seeking the most effective way of planning levelling surveys over Japan in order to monitor anomalous land-deformation preceding an earthquake. Assume that an anomalous land-deformation, of mean radius r, takes place in an area A. Let AL be the length of a survey along a levelling route within the area concerned. Then the probability of detecting the deformation is estimated as 2reAL~A, where r e is the effective radius of the deformed portion of the area. Since some finite length of levelling route is required for evaluating the result of a levelling survey, r e must be smaller than r. The area A being much larger than that of premonitory deformation, however, we may assume r e ~ r. The probability is estimated f rom the consideration that an anomalous deformation must be detected when the deformation area is overlapped by an area AL x re, on both sides of AL.

We now assume that a survey is conducted along a levelling route of length L within A. The probability for detecting a premonitory deformation is then 2reL/A. Dividing A into a number of sub-areas At (i = 1, 2 . . . . , n) and denoting the length of the levelling route by Li, we have

Li = L . (2) i = 1

8 TSUNEJI RIKITAKE

The expectancy of the number of earthquakes that can be detected by levelling survey is

r T

N= i=I ~ f Ni(r)(2reLi/A)dr (3) rS

where Ni(r) is the frequency of earthquake occurrence as a function of r. The limits of integration r s and r r are, respectively, the radii of deformation for the earthquakes of the minimum and maximum magnitudes considered. These can be estimated from the seismicity of the area concerned and also from the Gutenberg-Richter relation between the number of earthquakes and magnitude, along with the relation between earthquake energy and magnitude as given by Equations (5) and (6) in a later Section, 3.3, (Rikitake, 1969). The optimum proportion of levelling route length for the respective sub-areas is obtained by maximizing N under condition (2).

Hagiwara applied the theory to the Japanese Islands, which are divided into 7 districts of roughly equal area. It is striking that the conclusion is reached that we should concentrate levelling surveys mostly in the central part of Japan. For achieving effectiveness, it is of little use to carry out surveys in the northernmost and southern- most areas, i.e., Hokkaido and Kyushu. In forming the estimate, the seismicities in respective areas are determined from the catalog of earthquakes for M > 6 during the period from 1800 to 1968.

The above conclusion seems quite reasonable, judging from the low seismicities in the Hokkaido and Kyushu Islands. Attention is drawn to the fact that no consideration is taken into account in the theory of the existing distribution of levelling routes, so that the actual capability of detecting a forerunning land-deformation will be somewhat different from that estimated. It seems impractical to discuss further details of dividing Japan into many sub-areas of smaller extent because of insufficient seismic data.

The time-interval AT~ between the detection of anomalous land-deformation and an earthquake occurrence is also correlated to the magnitude M of the quake concerned. According to I. Tsubokawa (personal communication, 1968) the AT G vs M relation is approximately

log A T G = 0.75 M - 4.27 (4)

in which ATG is measured years. Relation (4) may not be accurate enough because it is deduced from scanty data. It should be improved, however, with the many data which will soon be accumulated through the earthquake prediction program. Empiric- al relations such as (1) and (4) play an important role in forming a theory of predicting the occurrence-time and magnitude of an expected earthquake, as will be discussed later.

In contrast to a levelling survey, a triangulation survey is much more laborious and expensive. Hence, the resurvey of the whole of Japan has been completed only recently, although local triangulation surveys have been conducted over portions of Japan


where great earthquakes have occurred in order to recover precise positions of triangulation stations. Comparisons between the first and second surveys disclosed marked displacements of stations, amounting to a few metres in some districts in Japan in about 70 yr. Almost all stations seem to have moved inland in an area along the Pacific coast of Central Japan. I t is suspected that this area has been subjected to a compressional force pointing inland from the Pacific. I f we think in terms of the current theory of plate tectonics, such a crustal force could be the result of an inter- action between the Philippine Sea Plate and the continental lithosphere. It is therefore believed that an appreciable amount of strain energy is stored in the crust there. Actually, no large earthquake has occurred in the vicinity of the area since 1854 when an earthquake of M = 8.4 occurred immediately off the Pacific coast there. Although nothing particularly anomalous has been found as yet, the area along the Pacific coast of Central Japan is ranked as an area of possible occurrence of a large earthquake. An area of similar character is found in eastern Hokkaido.

A remarkable advance has currently been achieved in distance-measuring technique. Lengths of about 5 x 104 m can be measured to an accuracy of 10-2 m with an electro- optical instrument, the geodimeter. With a laser source it can be operated even in daytime. Geodimeter surveys are being used extensively in place of triangulation surveys. An extensive geodimeter survey covering the Kanto area south of Tokyo brought out a marked contraction of land in 40 yr, as will be mentioned later. Since geodimeter operation is less laborious than triangulation surveying, emphasis will be placed on geodimeter surveying in earthquake prediction work in the future.

2.2. TIDE-GAUGE OBSERVATION

There are a number of reports of changes in sea level just prior to an earthquake. One of the most marked examples is the Azikazawa earthquake, which occurred in the northern part of Japan in 1792. The quake occurred at about 14 h local time. The inhabitants, who had noticed an extraordinary retreat of the sea in the morning, were so afraid of tsunamis (high waves associated with an earthquake, formerly called tidal waves) that they ran up into the mountains where they felt a strong shock in the afternoon. The terrified people ran back to the seashore only to be washed away by the tsunamis which arrived some time later. This tragic story clearly indicates that an anomalous upheaval of the land relative to sea level occurred several hours before the earthquake.

Changes in sea level are observed nowadays with a tide-gauge. I t has become clear that changes in sea level of several centimetres may be due also to effects of winds, ocean currents, water temperature changes, etc., not related to earthquakes. An anomalously high tide, exceeding the normal level by about 0.2 m, was in fact observed along the Pacific coast of Japan in October, 1971. The event lasted about one week, but no earthquake occurred.

Although 92 tide-gauge stations will be set up at intervals of approximately 100 km along the coast line of the Japanese Islands according to the program, it seems un- likely that a small land-deformation of the order of centimetres could be detected by

10 TSUNEJI RIKITAKE

these stations until a technique for eliminating the effects of oceanographic origin is

established.

2.3. TILTMETER AND STRAINMETER

Geodetic surveys are essentially intermittent. However, to monitor land-deformation continuously, about 20 crustal deformation observatories will be set up throughout Japan, according to the earthquake prediction program.

Two types of tiltmeters are usually employed. One is the water-tube tiltmeter, which compares the heights of water surface at two reservoirs about 35 m apart. It is possible to measure ground tilt to about 0."1 in this way. The other filtmeter which has been widely used is of the horizontal pendulum type. The horizontal pendulum type is highly sensitive to local disturbances, whereas a water-tube measures the average ground tilt over its length.

One of the best examples of ground tilting accompanied by seismic activity is provided by the water-tube tiltmeters installed in a vault of the Matsushiro Seismolog- ical Observatory during the 1965-7 Matsushiro earthquakes. A tilting exceeding 10" was observed within about 1 yr. Moreover, the tilting rate was large when the seismic activity was high, so that the tiltmeter observations provided a means of foreseeing seismic activity. In addition to such long-term changes, anomalous tiltings, forerun-

A p r i l l 1

20 0 4 8 12 16 h.

N up

0.1" 1

" ' " -~ _.,"P o , ~ % ~ - - - M = 4.~ 0 " ~ I 0 4 h 5 8 r n . . . . . . . . .

~' "-'~ . . . . M : 4.5

W up

1

\ Fig. 3. A change in the ground tilting prior to a series of earthquakes. A sharp change in the direction of tilting, starting at about 3h on April 11, 1966, followed by three strong shocks, all occurring northeast of the observation point within a hypocentral distance less than 10 km (after Hagiwara

and Rikitake, 1967).


ning earthquakes of moderately large magnitude have often been observed. Figure 3 shows a change prior to a series of earthquakes which occurred within several km of the observation point.

Tilt observations are supplemented by strainmeter observations of extensions and contractions of the ground surface. Silica-tube strainmeters, which are affected very little by temperature changes, are commonly used. It is customary to use strainmeters installed in three directions: to determine the strains in a horizontal plane, and in two directions perpendicular to each other. These instruments are installed in an underground vault to minimize the effects of temperature fluctuations.

A few strain steps, i.e., permanent deformations accompanied by earthquakes, have been observed by the array of crustal deformation observatories in Japan, although systematic study has just been started. It is of the utmost importance to study such deformations of the Earth's crust, not only for clarifying what is occurring at the hypocentral region, but also for monitoring possible strain changes forerunning an earthquake.

On the basis of the present-day seismicity, K. Shimazaki (personal communication, 1972) has estimated the probability of observing 3 or more precursory strain changes within 5 yr at a number of hypothetical stations covering Japan. It is assumed that the precursory change is as large as one-tenth of those for the whole course of the earthquake event. The probabilities calculated for three ranges of overall sensitivity of strainmeters are shown in Figure 4. It is striking that high probability areas are located along the Pacific coast of northeastern Japan. The location of a crustal deformation observatory has been selected, thus far, without consideration of the resulting weight of an observation. However, the accumulation of data for earthquake prediction could be accelerated very much by selecting appropriate locations for the observation points. On the basis of this kind of operations research, it is recommended

/ \

Fig. 4. Probabilities of observing three or more strain changes forerunning an earthquake within a future 5-yr period. The overall detectability limit of strain change is assumed to be 5 • 10 -7, 1 X 10 -7,

and 5 x 10 -8, respectively for the left, middle, and right drawings (after Shimazaki, personal communication, 1972).

12 TSUNEJI RIKITAKE

that the location of the crustal deformation observatory which has been planned for a location about 150 km northeast of Tokyo should be shifted about 50 km south- ward.

2.4. MICROEARTHQUAKE OBSERVATIONS

A knowledge of the seismicity of a certain area provides the fundamental knowledge for earthquake prediction over that area. The Japanese program plans to locate earthquakes with M > 3 through the permanent observatories which belong to the Japan Meteorological Agency. Microearthquakes with M < 3 are to be monitored by about 20 observatories, supplemented with accompanying stations. The construction of these observatories, mostly operated by university personnel, has now been almost completed. A number of field patrol teams for ultra-microearthquake observation have also been formed.

During the 1965-7 Matsushiro seismic activity a number of areas in which microseismic activity was concentrated were located either by fixed station observations or by mobile observations. A moderately strong shock of about M = 5 took place almost without exception in such an area within a few months. The relation between microseismicity and main shock provided a powerful means for warning a possible site of moderately large earthquakes during the Matsushiro activity.

On the other hand, it has been noted that there are places where we observe almost no microearthquakes now, although a large earthquake occurred there many years ago. One example is the area south of Tokyo, which will be discussed later. Seismologists generally believe that stresses are accumulating there. The present low seismicity in such an area is probably accounted for by high structural strength, possibly due to some locking mechanism.

We now encounter two different views. The Matsushiro experience indicates that the higher the microseismicity, the larger the danger of having a strong earthquake. However, in such an area, e.g., south of Tokyo, a large-scale earthquake may some day occur because of the accumulating crustal stresses. These are suspected from the present seismicity, although low. The paradox can be resolved only by taking the difference in the crustal conditions into account. It is a matter of urgency to classify earthquake areas from such a view-point.

When we observe earthquake activity, either as a single shock or in swarms, it is often suspected that a larger shock will follow. However, in spite of intensive studies, it is not yet possible to establish a criterion by which we can judge whether or not a series of earthquakes are foreshocks preceding a main shock. Mogi (1963) pointed out a number of districts in Japan in which foreshock activity has often been reported. It seems likely that foreshocks are apt to occur in a moderately fractured crust.

Earthquake occurrence is statistically random and stationary. If seismic activity, which is different from that of the steady state, takes place we may consider that the activity is anomalous. It is not clear, however, how to define such anomalous activity. K. Shimazaki (personal communication, 1971) proposed a method for detecting anomalous seismic activity with a given significance level. Suyehiro and Sekiya (1972)


examined frequency and magnitude relations for foreshocks and aftershocks. They reported that significant differences in the coefficient of the Gutenberg-Richter relation between both types of shocks are sometimes found.

Kanamori (1972b) pointed out an extremely interesting relationship between the occurrences of large-scale earthquakes in an extensive area having a length and width of several hundred km, off and along the Pacific coast in Central Japan, and temporal variations of earthquake swarm activity in the Wakayama district on Kii Peninsula, at the southwestern corner of the area considered. Enormous increases in the number of earthquakes, from a few 10's to 300 or more per year, were found in the Wakayama area prior to the 1923 Kanto earthquake (M = 8.3) and the 1953 Boso-Oki earthquake (M = 8.3) which occurred in the vicinity of the Kanto district. The increases seem to have been completed within a few years. According to a current theory of plate tectonics, these earthquakes occurred along the northeastern edge of the Philippine Sea Plate, which tends to move in a northwest direction, as inferred from the source mechanisms of the 1944 Tonankai earthquake (M =8.3) and the 1946 Nan- kaido earthquake (M =8.4). The 1944 and 1946 earthquakes occurred immediately off Kii Peninsula, where the Philippine Sea Plate is believed to interact with the continental lithosphere.

As discussed by Kanamori (1971, 1972a), the latter two earthquakes seem to reflect a rebound at the interface between the Philippine Sea Plate and the continental lithosphere which includes the Wakayama earthquake swarm area, while the Kanto and Boso-Oki earthquakes seem to represent a slip of the Philippine Sea Plate relative to the neighboring plates.

Kanamori (1972b) assumes that the motion of the Philippine Sea Plate is propelled by a rebound at the interface between this plate and the neighboring plates. At the time of the Kanto and Boso-Oki earthquakes, therefore, the stresses within a continental block involving the Wakayama area would become strengthened, stimulating an increase in swarm activity. The coupling between plates being frictional, it would not be unreasonable to suppose a creep-like deformation at depths starting before- hand. In that case, the gradual increase in swarm activity forerunning the great earthquakes could be explained.

One more remarkable aspect of the Wakayama earthquake swarms was a sudden decrease in activity immediately after the 1944 Tonankai earthquake, which was no doubt a rebound at the interface between the Phillippine Sea Plate and the continental lithosphere. It seems reasonable that the compressional stresses became drastically weak, in association with the rebound, resulting in a drop of the swarm activity.

Should the above pattern of swarm activity in relation to the occurrence of a great earthquake in the vicinity of the Kanto district be repeated in the future, then premonitoring an impending great earthquake in the area could be achieved by monitoring the Wakayama swarm activity. The important and interesting point is the fact that the relation between earthquake swarms and great earthquakes, whose locations can be separated by 500 km, can be accounted for to some extent by the current theory of plate tectonics. It may also be pointed out that a great eruption of an active volcano

14 TSUNEJI RIKITAKE

on Oshima Island, about 100 km south of Tokyo, was reported in the interval between the rise of swarm activity and the occurrence of great earthquakes. Since we have only two examples since the beginning of this century, it is not known whether or not the volcanic activity was a mere coincidence.

2.5. ACTIVE FAULTS AND FOLDINGS

Large-scale faults, such as the San Andreas in California, U.S.A., and the Anatolian in Turkey, are the seats of many earthquakes, some of which are extremely large. Some portions of these faults show continual creeping; these faults are therefore active.

No faults which show a now-developing creep have been found in Japan. According to geologists and geographers, however, there are a number of faults which retain evidence that slip movements with speeds of 1 to 5 m/1000 yr occurred in the past. It is feared that these faults may become the site of a strong earthquake some day. Actually, one of them, about 100 km west of Tokyo, was displaced a few meters at the time of a strong earthquake about 40 yr ago. It has also been known that earthquakes of moderately large magnitude sometimes occur in areas where mountain folding is now taking place.

It appears to be a good idea to concentrate various kinds of observation around active geological structures. In other words, active faults and foldings provide targets for earthquake prediction observations.

2.6. CHANGES IN SEISMIC WAVE SPEEDS

It has long been thought that seismic wave speeds in strained rocks were larger than those for non-strained rocks. This idea has been supported by laboratory experiments. An appreciable change in the ratio of the P-wave speed to the S-wave speed has recently been reported before an occurrence of a moderately large earthquake in the Garm district, Middle Asia (Sadovsky et al., 1972).

A program for measuring arrival times of seismic waves originating from man-made explosions at stations scattered over the Kanto district has been underway in Japan. Explosions are made on Oshima Island once a year. Details of the experiments will be given later.

2.7. SEISMOMAGNETIC EFFECT

Much has been reported on geomagnetic changes prior to an earthquake in the li- terature. It was shown by Rikitake (1968b), however, that those earlier reports con- tain large errors because of obsolete techniques of measuring the geomagnetic field. Modern techniques accurate enough for discussing seismomagnetic effect, i.e., change in the geomagnetic field in association with an earthquake occurrence, have become available only recently.

The Japanese program on earthquake prediction places much emphasis on obtaining seismomagnetic data through a nationwide array of proton precession magnetometers; the working principle relies on the magnetic moment of a proton. The magne-


tometer is affected very little by environmental conditions, such as temperature. It is thus possible to determine the absolute field intensity with an accuracy of + 1 x 10- 9 T (1 tesla corresponds to 104 gauss = 109 gamma). It turns out, however, from observations made by an array of about 10 magnetometers spread over the Japanese Islands, that the values observed simultaneously at two observatories involve an error several times that of the instrumental accuracy. Magnetic fields produced by electric currents excited in the Earth by time-variations of the external magnetic field are controlled largely by the non-uniform electrical conductivity in the Earth, and so they differ from place to place. It has been concluded tentatively that the geomagnetic field intensity at a station can be compared to that at a reference station, a few hundred km distant, with a standard error of about 2 x 10 .9 T (Mori and Yoshino, 1970).

A seismomagnetic effect of about 10 .8 T was observed during the most violent stage of the 1965-7 Matsushiro seismic activity (Rikitake, 1968b). However, no geomagnetic changes exceeding the overall accuracy was reported in association with an earthquake (M = 6.2) which occurred in northern Honshu (the main island of Japan) on October 16, 1970 (Geomagnetic Group, 1971). One of the stations where measure- ments were made with a portable proton precession magnetometer is within about 10 km from the seismometrically-determined epicenter. The time-interval between the two surveys, during which the shock occurred, was 40 days.

Judging from the magnetic results since the start of the prediction program, the magnetic method does not seem very promising for earthquake prediction, although a much larger seismomagnetic effect may possibly be expected for an earthquake with M > 7 .

2.8. LABORATORY EXPERIMENTS AND THEIR APPLICATION TO FIELD WORK

It has been made clear by rock-breaking experiments (Mogi, 1962) that, when a specimen is moderately non-uniform, numerous shocks associated with microfracture production take place prior to the main shock, or the rupture. When the specimen is extremely non-uniform, no main shock occurs. On the other hand, no microshocks precede the main shock for a highly uniform specimen. This kind of study provides a physical basis for the empirical relation between foreshocks and main shock. A program for making similar experiments under conditions prevailing within the Earth's crust is now in progress.

Changes in physical properties of rocks subjected to large strains provide key points of possible ways to predict earthquakes. Experiments on the piezomagnetic effect have been advanced considerably in recent years. A decrease in magnetization of basaltic rocks in the direction of compression amounting to one part in 103 per MPa (1 Megapascal= I0 bars) has been ascertained (Nagata, 1970). This provides a basis for the seismomagnetic effect.

Yamazaki (1965, 1966, 1967, 1968) reported that the electric resistivity of some porous rocks is extremely sensitive to stress. For a particular sedimentary rock, the rate of resistivity change is larger than the mechanical strain by a factor of several hundred or more.

16 TSUNEJI R I K I T A K E

Quite striking changes in resistivity on the occasion of large earthquakes have been found with a resistivity variometer constructed by Yamazaki (1967, 1968). Figure 5 (Rikitake and Yamazaki, 1969, 1970) shows an example of a resitivity step at the time of the 1968 Tokachi-Oki earthquake ( M = 7.9), which occurred at a location about 700km distant from the observation point on Miura Peninsula (see Figures 5 and 7). The gradual change in resistivity is caused by the change in crustal strain associated with tidal loading. The jump at the time of earthquake is not quite instan- taneous. The strain step which produces the jump is the order of 1 x 10 -7. It is in harmony with the value calculated from the dislocation theory. About 10 resistivity steps associated with an earthquake have been observed during a 4-yr interval of

J

decr~ose May 16. 1968 a,

o

- - I h o u r -

I

- - o

Fig. 5. Resistivity step at the time of the Tokachi-Oki earthquake on May 16, 1968, as recorded by the high (H) and low (L) sensitivity channels of the resistivity variometer. The observation point

(A) and epicenter are indicated by a solid circle and a cross, respectively (after Rikitake and Yamazaki, 1970).

observation. As can be seen in Figure 5, some of the steps observed are preceded by an anomalous premonitoring change, starting a few hours prior to the main step. It is not clear whether or not such changes are precursory effects at the present stage of investigation.

It has also been planned in Japan to study the reaction of the crust to man-made perturbations. Unlike the fluid injection experiments near Denver, Colorado, U.S.A., no large scale experiment could be undertaken in Japan because almost all parts of the country are populated, and it is feared that a destructive earthquake might be caused by such an experiment. A water injection experiment was made at the central portion of the Matsushiro earthquake area in 1969-70. However, no clear-cut relation between seismic activity and the amount of water injected was found; probably because almost all the strain energy had already been released and the volume of water was too small.


2.9. PREDICTIONS BASED ON EARTHQUAKE STATISTICS

Kawasumi (1970) pointed out that large earthquakes had occurred in the South Kanto district with a period of 69 yr. About 50 yr have passed since the 1923 Kanto earthquake, and some persons believe that a catastrophe of the world's largest city is ap- proaching. Journalistic interest has been focused on the reality of such warning. The local government of Tokyo has started a plan for minimizing earthquake hazards in case a large earthquake strikes Tokyo.

Usami and Hisamoto (1970, 1971) undertook studies to estimate the probability of a future earthquake with intensity V or larger (in the Japan Meteorological Agency scale) in the Tokyo and Kyoto areas. The intensity scale (which is not a magnitude scale) is different from the modified Mercalli scale used widely in western countries. Intensity V in the Japanese scale corresponds to about VII or VIII in the Mercalli scale. Although 36 and 38-yr periods were obtained for the respective cities, they show that such periodicities may be apparently obtained even though earthquake occurrence is assumed to be random.

Shimazaki (197 la) argued that the method of discussing the significance of detected periodicity may be in error, emphasizing that no plausible periodicity for the occurrence of large earthquakes is found in the South Kanto area.

It does not appear that we can place much emphasis on statistical prediction. Earth- quake prediction must be based on physically observed elements of geophysical significance.

3. Organization and Strategy of Earthquake Prediction

3.1. ORGANIZATION OF EARTHQUAKE PREDICTION WORK

To accelerate the processing of earthquake prediction data 3 centers have been set up: in the Geographical Survey Institute, the Japan Meteorological Agency, and the Earth- quake Research Institute (University of Tokyo). These are, respectively, the Crustal Activity Monitoring Center (geodetic, tide-gauge data, etc.), the Seismicity Mon- itoring Center (for earthquakes of M > 3), and the Earthquake Prediction Observation Center (microearthquakes, crustal deformation, magnetic, and other data from university sources).

Processed data from these centers are sent to the Coordinating Committee for Earthquake Prediction, located in the Geographical Survey Institute, which is the headquarters for earthquake prediction in Japan. The Committee, which consists of about 30 specialists from universities and governmental organizations, is responsible for analyzing the data sent through the 3 channels. The Committee issues information for a particular area on the occurrence of anomalous phenomena, such as land deformation and seismic activity, which may be related to the danger of a large earthquake occurring. However, it is not possible to issue a definite earthquake alarm in the present stage of earthquake prediction research.

18 TSUNEJ[ RIKITAKE

3.2. STRATEGY OF EARTHQUAKE PREDICTION

The strategy of earthquake prediction as proposed originally by the Geodetic Council and supported almost unanimously by Japanese seismologists will be briefly described. First of all, levelling, geodimeter, gravity, and magnetic surveys covering the whole country are to be speeded up. Anomalous effects, if any exist, could be detected by repetitions of such surveys. Also, routine work, such as tide-gauge observation and seismographic observation, is to be increased. In addition, intensified observations are to be carried out over the 'areas of special observation', which: (1) have experienced destructive earthquakes, (2) include active faults, (3) are characterized by the frequent occurrences now of earthquakes, or (4) are sociologically and economically important, such as the Tokyo area. Every effort is made in these areas to detect anomalous effects by establishing observatories for crustal deformation and microearthquakes, along with frequent repetitions of geodetic surveys.

Whenever the Coordinating Committee confirms that something anomalous is taking place in a certain area it is designated an 'area of intensified observation'. The area will then be monitored by observation teams of various disciplines. In case these intensified observations lead to the conclusion that the anomaly seems likely to be correlated to an earthquake occurrence, then the Committee will designate the area as an 'area of concentrated observation'. Every effort will then be made toward possible earthquake prediction by concentrating all kinds of observations there. At the moment (t972) 8 areas of special observation have been designated and one of intensified observation.

The above strategy may sound splendid. However, there are difficulties in actual performance. The Committee handles only 'coordination' ; it is not authorized to issue an 'order' , so that it is not always possible to conduct timely observations. If actual prediction is to be made in the future, it will be necessary to set up an institution which can conduct earthquake prediction work in a more direct way.

3.3. QUANTIFICATION OF EARTHQUAKE WARNING

The designation of the degree of an earthquake threat as defined above could be biased by subjective judgement. To make the rating more objective Rikitake (1969) presented an attempt to utilize the probability of magnitude and the occurrence-time of an earthquake prediction.

If a seismic observation has been carried out over an area for a fairly long period, it is possible to determine the coefficients of the Gutenberg-Richter formula, which is written as

log n = a - b M (5)

where n is the frequency of earthquakes as a function of M. We also have a relation between the energy E and magnitude M of an earthquake,

log E = o~ + t i M , (6)

where ~ and fl are constants determined empirically.


F r o m (5) and (6), a long with addi t ional considerations, it is possible to calculate the probabi l i ty for an ear thquake falling within a magni tude range. Riki take (1969) calculated such probabil i t ies for the K a n t o district, approximate ly a 200 km x 200 k m area surrounding Tokyo . These probabili t ies, which are deduced entirely f rom the seismic activity in the past , are called preliminary probabilities. Such probabili t ies, which usually indicate a low value for a large magni tude, can be modified by adding a new observat ion of a different kind.

Let us assume tha t an anomalous land-deformat ion is found in the area considered. I f the effective radius of tha t area is determined, then Mo, the magni tude of an earthquake which is mos t likely to occur in association with the deformat ion, is obta ined f rom Equat ion (1). I f we fur ther assume that the difference between the actual magnitude, M, and Mo follows a Gauss ian distribution, then the probabi l i ty that the magnitude falls in a certain range can be calculated, provided tha t the s tandard deviat ion of M obta ined f rom Equat ion (1) is known. The probabil i t ies thus obtained are denoted for respective magni tude ranges as W1(1), W1(2),... Wl(n), whereas the prel iminary ones are Wo(1), Wo(2) .... Wo(n). Hence, the theory of probabi l i ty leads to a synthesized probabi l i ty which is given by

W(s) = Wo(s ) Wl(s)/[Wo(1) W1(1) + Wo(2) W, ( 2 ) - . . + Wo(n) Wl (n) ] . (7)

We see that the probabi l i ty tha t an ear thquake falls in a magni tude range becomes the one calculated f rom Equat ion (7) by adding an ear thquake predict ion element.

8.0

M 7.0

6.0

r = 5 o k m

r =lOkm . . . . . . . . . . . . . .

( ~

5.O , , , I . . . . I , , , I , , , , I , , , I . . . . I

0.1 0.5 I 5 10 50 IOOyeers t - - - - - - - ~

Fig. 6. Distribution of the synthesized probabilities for an earthquake to fall in a magnitude range of 0.5 and a time range from t = 0 to 5, when an anomalous crustal deformation is taken into account in the Kanto district; r is the effective radius of the deformed area (after Rikitake, 1969). The probabilities 0.789 and 0.822 are the highest attained for the respective radii. The points indicate the

time when first attained.

20 TSUNEJI RIKITAKE

Similar calculations can readily be performed by adding other elements, such as anomalous geomagnetic change, foreshock activity, etc., although empirical relations like Equation (1) for these elements have not yet been well established. The situation is much the same as testimony by a witness. For the purpose of having a high value of synthesized probability, the witness should be highly reliable. If there are a number of reliable witnesses then the synthesized probability becomes high.

Preliminary probabilities for the occurrence-time of an earthquake can also be estimated from the record of seismic activity, provided that earthquake occurrence is assumed to be stationary and random with respect to time. When an anomalous land-deformation, which can be regarded as a premonitoring effect is observed, then synthesized probabilities can be calculated from Equation (4). Similarly, synthesized probabilities can be calculated when other prediction elements are introduced. In contrast to the estimate of magnitude probability, it is believed that the estimate of occurrence-time can be made only with low accuracy because of scanty data.

If it is assumed that the magnutide and time distribution of earthquakes are inde- pendent then the probability that an earthquake falls in a magnitude and occurrence- time range is obtained from the product of the probabilities for magnitude and for occurrence-time, separately obtained. Figure 6 shows the synthesized probabilities for an earthquake to fall in the magnitude interval between ( M - 0.25) and (M+0.25) and to occur between 0 to t in time, under the condition that an anomalous crustal deformation having an effective radius of either 10 km or 50 km is detected at t = 0 in the Kanto district.

On specifying probabilities corresponding to various ratings, as discussed in Section 3.2, the designations of 'intensified' and 'concentrated' areas of observation can be made on the basis of the synthesized probability thus obtained.

Applications of the present theory are in a testing stage. The most difficult point is the fact that we do not know whether a crustal deformation will lead to an earthquake occurrence even if we find an anomaly; land-deformations appear to exist which are not accompanied by an earthquake. Since the above theory is formed on the assump- tion that an anomaly in a prediction element always leads to an earthquake occurrence, the probability may sometimes be over-estimated. The theory should be improved by accumulating additional data related to earthquake prediction.

4. Operation South Kanto

A levelling survey conducted in 1969 over the Boso Peninsula in the South Kanto district (Figure 7) revealed a land upheaval. During the 1923 Kanto earthquake the tips of the Boso and Miura Peninsulas (Figure 7) were raised 1 to 2 m. Since then, the peninsulas have been subsiding steadily about 1 cm/yr. It is generally believed by Japanese seismologists that sudden upheavals, related to large-scale earthquakes, followed by gradual subsidences afterward are characteristic of crustal deformations of peninsulas of the Japan Islands facing the Pacific, and that no catastrophe occurs while the peninsulas are subsiding steadily.


F ig . 7.

N

§ o

Yokohama ~ ,2-'-~.jJl "-%/

MIURA .."

I ~ SO PEN.

30 km I I I I

Changes in height, in cm, from Feb., 1969 to Feb., 197t (after Geographical Survey Institute, 1971).

The Geographical Survey Institute, which was surprised by the unexpected 1969 sm'vey, hurriedly carried out another survey along a route over the Miura Peninsula in 1970 and a similar land upheaval was revealed there also. The Coordinating Com- mittee for Earthquake Prediction had designated the district as an area of 'intensified' observation toward the end of 1969, and a major effort to trace the anomaly, denoted 'Operation South Kanto' , was begun.

This anomalous upheaval seems to be continuing even now. Figure 7 (Geographical Survey Institute, 1971) shows the changes in height from February 1969 to February 1971. The tide-gauge record at a station near the tip of the Miura Peninsula seems to confirm the upheaval, although the record is affected by other effects, of oceanographic origin.

Crustal deformation observatories, one each on the two peninsulas, have been in operation. Although nothing definite can be said about the results at the observatory on the Boso Peninsula, because the observation interval is too short, a marked tendency since 1961 of eastward tilting amounting to 0."2 per year has been detected at another observatory, on the Miura Peninsula, which has been in operation more than 20 yr. The tilting seems to have slowed down about 1966, and then reversed its direction of motion. Generally speaking, observations of crustal deformation seem to be compat- ible with anomalous land-deformation as determined by levelling.

The similarity between the changes in height at the tip of the Miura Peninsula, as revealed by levelling, and changes in the east-west tilting at the observatory has been noticed by T. Hagiwara (personal communication, 1970). K. Kasahara (personal communication, 1970) stressed the importance of ground tilting as a possible indicator of the general crustal movement of the peninsula.

22 T S U N E J I R I K 1 T A K E

An extensive program of geodimeter surveys covering the South Kanto district has been undertaken by the Geographical Survey Institute in the last few years. Figure 8 (Geographical Survey Institute, 1971) shows the changes in horizontal distance between triangulation stations from 1925 to 1971. The changes were obtained by comparing recent geodimeter results with those of a triangulation survey made after the great 1923 Kanto earthquake. I t is striking that the Miura and Boso Peninsulas seem to be under contraction of the order of 1 x 10-5 in linear strain in an approximately north-

~TOKYO BAY

/~ PEN.

3 0 k m ~ b i i i

Fig. 8. Changes in horizonta] distance, in cm, from 1925 to 1971. Those for the interval from Feb- ruary to October 1971 are indicated by numeral in parentheses (after Geographical

Survey Institute, 1971).

south direction and the neighboring sea, i.e., Sagami Bay, in a northwest-southeast direction. A considerable amount of shear is also found there.

The land-deformation, which has given rise to such remarkable horizontal displacements, seems to be in progress even now. The differences in distance between two geodimeter surveys made in February and October 1971 are indicated by numerals in parentheses in Figure 8. The displacements even for such a short interval seem to exceed the range of error of measurement, although the physical significance of these deformations is not clear.

Shimazaki (1971b), with the aid of a computer, plotted 678 epicenters of shallow earthquakes which had occurred in the South Kanto district f rom 1926 to t967. As may be seen in Figure 9, very few earthquakes took place in an area elongated in a northwest-southeast direction which covers the southern parts of the Miura and Boso Peninsulas and eastern Sagami Bay. The area coincides with the trend of the Sagami trough, a deep submarine valley running down to the Japan trench, which may be

Fig. 9.

E A R T H Q U A K E P R E D I C T I O N S T U D I E S I N J A P A N 23

36~N �9 4 - " - - - ~ , �9 . . .

0* �9 �9 �9 �9 �9 �9 �9 ~ oe �9 �9 - " - �9 -II II I f * ~

�9 " �9 �9 - a " a -" a : ~ - " Z

�9

- . . . . . . ~ . : y . ' - - - / : . : . . . . . .

�9 " l " Y " " " : % : " " " ' " . ~ ~ - . . . , ~,........

~ �9 �9 U o �9 �9

o j � 9 l e �9

, ' ' ; ' . . " " : �9 . �9

347 " �9 ~01 "81~

139 ~ 141~E

] E p i c e n t e r s o f m a j o r e a r t h q u a k e s , t h e l o c i of" w h i c h a r e s h a ] ] o w e r t h a n 6 0 k i n , f r o m ] 9 2 6 t o

1 9 6 7 ( a f t e r S h i m a z a E i , 1 9 7 1 b ) .

identified as the surface trace of a fault system associated with historical great earthquakes, such as that of 1923 (Kanamori, 1971 ; And�9 1971).

Figure 10 (Shimazaki, 197ib) shows the epicenters of large earthquakes (M>~7.0) from A.D. 416 to 1964, as listed in a table of major earthquakes in Japan by Usami (1966). Although the locations of historical earthquakes may not have been determined accurately, it is surprising that many large earthquakes seem to have originated in areas now quiescent. It may therefore be surmised that the area concerned is capable of storing a considerable amount of strain energy which may be released as a large earthquake from time to time. Judging from the geodimeter results, the area seems to be strained to some extent, although the strain has not as yet reached a critical amount.

A recent microearthquake observation over the South Kan t �9 district (Ishibashi and Tsumura, 1971) also indicated a low seismicity over the area in question. It would

24 TSUNEJI RIKITAKE

139~ 141~

189411, 1111921 1895

19230_1241 ~ ~) (, ~09 1633-- --1"1~648 %] 3 r 1912" T

1930 m~ i~433 @ f 1923

34"N

Jl �9 ~1923 �9 1906 905 703

0 1605

0

Fig. 10.

8 .0<M �9 7.5-< M < 8.0 �9 7.0<_ M < 7.5

Epicenters of historical, large earthquakes (M i> 7.0) (after Shimazaki, 1971b).

not be unreasonable, therefore, to suppose that a fault which probably underlies the low-seismicity area is at present in a locked state, and that strain energy is steadily accumulating. However, nothing certain can be said about possible relations between anomalous land upheaval, horizontal displacements, and unusually low seismicity in the vicinity of the Miura and Boso Peninsulas.

In order to monitor the growth and decay of seismicity, a microearthquake observation network will be set up over the South Kanto area, adding a few temporary observatories to the permanent ones. Sea-bottom seismographs will be operated occasion- ally in Sagami Bay. It has been pointed out that the seismicity was high around


the Tokyo area before the 1923 Kanto earthquake; it was unusually high in 1922 (Suyehiro and Sekiya, 1972). Should a large earthquake be impending there, we hope to premonitor it through microseismic observations. In addition to the observations to the south of Tokyo, a deep-hole seismic observation station has been planned by the National Research Center for Disaster Prevention at a point a few tens of kilometers north of Tokyo. A set of seismographs, along with tiltmeters, will be installed at a depth of 3000 m in the hope of obtaining high sensitivity observations; ground noises would be very low at that depth.

Detonations of about 500 kg of explosives have been made at the northern-most part of Oshima Island (Figure 8), roughly at one-year intervals since 1968. Seismic waves from the origin are observed at a number of stations scattered on the mainland. Although the P-wave speed crossing Sagami Bay has increased by about 1 part in 103, or a little less for the 1968-9 interval, marked changes in speed have not been noted during the entire interval of observation from 1968 to 1971 (Iizuka, 1971). These observations will be continued. Since 1971 another explosion point has been provided, near the extremity of the Boso Peninsula. Magnetic, gravimetric, and geological surveys have also been carried out over the South Kanto district.

It seems premature to say anything definite on the ability to predict earthquakes based upon the above survey results. However, the theory described in Section 3.3 does indicate that the probability of having an earthquake of magnitude 7 in this area may become as large as 0.5 within 10 yr, providing that the anomalous land upheavel observed is related to the occurrence of an earthquake.

5. Summary

Earthquake prediction research has become one of the important scientific projects in Japan since 1960. As a result of intensive studies in various disciplines, basic data necessary for earthquake prediction studies have been accumulated. However, many additional experiments and analyses are required if useful prediction techniques are to be developed.

Modern techniques have been introduced for obtaining seismic, magnetic and other observations, such as use of the geodimeter for detecting land-deformation. Progress has been made in interpreting the observed data. Thus, relations between precursory land-deformation and the magnitude and occurrence-time of an accompanying earthquake have been developed, although these relations will be improved by future investi- gations.

A Japanese committee which serves as the central agency for earthquake prediction has been formed. However, it is responsible only for coordination. To achieve actual predictions it will be necessary to establish a more powerful agency.

A tentative plan, or guideline, leading to predictions has been proposed. An attempt to estimate ratings of earthquake danger is postulated on the basis of probability theory; such an objective method of rating will be necessary for prediction work in the future. Studies on the optimum distribution of observatories, as well as the selec-

26 TS~rN~ RIKITAKE

t ion of survey areas, have been made. Such studies, based on opera t ions research,

should be stressed in the future.

A n ea r thquake pred ic t ion opera t ion has been under way in an area south of T o k y o

since 1969. Var ious observa t iona l results suggest tha t a large ea r thquake might occur.

I f an anoma lous land upheavel recently found there should be cor re la ted with ear th-

quake occurrence, then present pred ic t ion theory, a l though no t yet ent irely rel iable,

indicates tha t the p robab i l i t y of having an ea r thquake of a b o u t magni tude 7 within

10 yr is no t low.

References

Ando, M.: 1971, Bull. Earthquake Res. Inst., Univ. Tokyo 49, 19. Dambara, T.: 1966, Y. Geod. Soc. Japan 12, 18 (in Japanese). Geographical Survey Institute: 1971, Rep. Coordinating Com. Earthquake Prediction, Geogr. Sur.

Inst. 6, 25 (in Japanese). Geomagnetic Group: 1971, Rep. Coordinating Com. Earthquake Prediction, Geogr. Sur. Inst. 5, 22

(in Japanese). Hagiwara, T. and Rikitake, T.: 1967, Science 157, 761. Hagiwara, Y.: 1971, or. Geod. Soe. Japan 17, 38 (in Japanese). Iizuka, S. : 1971, Rep. Coordinating Com. Earthquake Prediction, Geogr. Sur. Inst. 6, 15 (in Japanese). Ishibashi, K. and Tsumaura, K. : 1971, Bull. Earthquake Res. Inst., Univ. Tokyo 49, 97 (in Japanese). Kanamori, H.: 1970, Tectonophys. 9, 291. Kanamori, H.: 1971, Bull. Earthquake Res. Inst., Univ. Tokyo 49, 13. Kanamori, H. : 1972a, Phys. Earth Planetary Int. 5, 129. Kanamori, H. : 1972b, Teetonophys. 14, 1. Kawasumi, H.: 1970, Chigaku-zasshi 79, 115 (ih Japanese). Mogi, K.: 1962, Bull. Earthquake Res. Inst. Univ. Tokyo 40, 125. Mogi, K.: 1963. Bull. Earthquake Res. Inst., Univ. Tokyo 41,615. Mori. T. and Yoshino, T.: 1970, Bull. Earthquake Res. Inst., Univ. Tokyo 48, 893. Nagata, T. : 1970, IAGA Bull. 27, 12. Rikitake, T.: 1966, Tectonophys. 3, 1. Rikitake, T.: 1968a, Earth-Sci. Rev. 4, 245. Rikitake, T.: 1968b, Teetonophys. 6 (1), 59. Rikitake, T.: 1969, Teetonophys. 8, (2) 81. Rikitake, T. and Yamazaki, Y.: 1969, Bull. Earthquake Res. Inst., Univ. Tokyo 47, 99. Rikitake, T. and Yamazaki, Y. : 1970, Tectonophys. 9, 197. Sadovsky, M. A., Nersesov, I. L., Nigmatuelaev, S. Kh., Latynina, L. A., Lukk, A. A., Semenov,

A. N., Simbireva, I. G., and Ulomov, V. I., 1972, Tectonophys., in press. Shimazaki, K. : 1971a, Kagaku 41, 688 (in Japanese). Shimazaki, K. : 1971b, Tectonophys. 11,305. Suyehiro, S. and Sekiya, H.: 1972, Tectonophys., in press. Tsuboi, T., Wadati, K., and Hagiwara, T.: 1962, Prediction of Earthquakes-Progress to Date andPlans

for further Development, Earthquake Res. Inst., Univ. Tokyo, Tokyo, 21 pp. Tsubokawa, I., Ogawa, Y., and Hayashi, T.: 1964, J. Geod. Soc. Japan 10, 165. Usami, T.: 1966, Bull. Earthquake Res. Inst., Univ. Tokyo 44, 1571 (in Japanese). Usami, T. and Hisamoto, S.: 1970, Bull. Earthquake Res. Inst., Univ. Tokyo 48, 331 (in Japanese). Usami, T. and Hisamoto, S.: 1971, Bull. Earthquake Res. Inst., Univ. Tokyo 49, 115 (in Japanese). Yamazaki, Y. : 1965, Bull. Earthquake Res. Inst., Univ. Tokyo 43, 783. Yamazaki, Y.: 1966, Bull. Earthquake Res. Inst., Univ. Tokyo 44, 1553. Yamazaki, Y.: 1967, Bull. Earthquake Res. Inst., Univ. Tokyo 45, 849. Yamazaki, Y. : 1968, Bull. Earthquake Res. Inst., Univ. Tokyo 46, 957.

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