Possibility of a Great Earthquake in the Tokai District, Central Japan

J. Phys. Earth, 25, Suppl., S 219-S 230, 1977

POSSIBILITY OF A GREAT EARTHQUAKE IN THE TOKAI DISTRICT, CENTRAL JAPAN

Tokuji UTSU Departmentof EarthSciences, Faculty of Science, Nagoya University, Nagoya,Japan (ReceivedMarch 25, 1977)

The Tokai districthas been consideredto be one of the probablesites of great interplateearthquakes in the near future.This idea isbased on the time seriesof historicalgreat earthquakes and the fairlyhigh rateof crustalmovement observed in thisregion for more than 80 years. The lastgreat earthquake was the Ansei earthquakeof 1854 (M=8.4). The westernhalf of the sourceregion of thisearth- quake rupturedagain in the 1944 Tonankai earthquake,but the easternhalf, i.e., theTokai district,remain unrupturedfor 122 years.The recurrencetime intervalof greatearthquakes in thisregion estimated from historicalearthquakes, topograph- icaland geologicalevidences, observed strain rates, the rateof plateconvergence, etc.varies from 120 yearsto more than 600 years. There are severalobservations which may suggestthat thisregion has alreadybeen in the long-termprecursory stage.However, itis questionable that these observations represent real precursors to the futureTokai earthquake. It seems possiblethat the next greatearthquake will occur south of the Izu Peninsula and will release the strain in the Tokai district. In any case, the Tokai district should be watched carefully for every con- ceivable precursors in order to make a successful prediction.

1. Introduction

In the pastseveral years, the possibilityof a greatinterplate earthquake in the Tokai districthas been much discussedby Japanese earth scientists.The Tokai districtis a southern coastalregion of centralJapan (ShizuokaPrefecture and its vicinity)near thenorthern tip of the PhilippineSea plate(Fig. 1, rightbottom). In 1969,the Coordinating Committee forEarthquake Prediction,Japan (CCEP, Prof.T. Hagiwara, chairman) designatedthis region as 'theregion of specifiedobservation' for the follwing reasons. (1)Japan's history shows repeatedoccurrences of great earthquakesin thisregion. The lastone was theAnsei earthquakeof 1854 (M=8.4). (2)The analysisof two triangulationsurveys in 1886-87 and 1954-56 indicatesthat thisregion has been compressedconsiderably to the northwest(HARADA and IZAWA, 1969). This result,together with fairlyrapid subsidence of thewest coastof Suruga Bay observedsince 1889 (GEOGRAPHICAL SURVEY INSTITUTE,1970), seems to indicate the accumulationof considerableamount of strainin thisregion, which must be releasedby the futureTokai earthquake(MOGI, 1970). In 1973,the problem of the Tokai earthquakereceived much public attention through newspaperreports of the researchesby ANDO (1975a),RIKITAKE (1974),and

S 219 S 220 T. UTSU others.In the background,there was a factthat the Nemuro-oki earthquakeof June 17, 1973 (M=7.4) occurredjust within one of the regionsof specifiedobservation designatedby the CCEP in 1970. The reasonsfor the designationwere very similar to the case of the Tokai district.In 1974,the CCEP upgraded the Tokai districtto 'theregion of intensifiedobservation .' In 1976,documents reportingthe effects of theAnsei earthquake along the coast of Suruga Bay were found by chance (EARTHQUAKE RESEARCH INSTITUTE,1977). The documents indicatethat the rupturezone of the Ansei earthquakeextended to the northernend of Suruga Bay (HATORI, 1976;ISHIBASHI, 1976). The importance of thisfact pointed out by Ishibashishocked the local people. The anomalous uplift in the Izu Peninsulasince 1975 (e.g.,TSUMURA, 1977)has alsobeen watched in re- lationto theTokai earthquake.Near theend of 1976,the GeodeticCouncil of Japan (Prof.T. Nagata, chairman) recommended the intensificationof various kinds of observationsand surveysin the Tokai districtto the government of Japan. The recommendation alsoincludes rapid collection of data by a monitoringcenter on the 24-hr basisand organizationof a specialcommittee forevaluating anomalous phenomena which may be short-termprecursors to the Tokai earthquake. This paper reviews the main informationand studiesin connectionwith the predictionof the Tokai earthquakeavailable as of January, 1977. The materials presentedto theCCEP's subcommitteefor the Tokai district(Prof. T. Asada, chairman) were very usefulin preparingthis paper.

2. Tectonic Situation

There is general agreement that the northern part of the Philippine Sea plate moving to the northwest relative to the Eurasia plate underthrusts at the Nankai trough. The great earthquakes along the Nankai trough are caused by the rebound of the compressed crust of the Japanese Islands (FITCH and SCHOLZ, 1971; KANAMORI, 1972; ANDO, 1975a, b). Near its eastern end, the Nankai trough bends to the north and enters into Suruga Bay. This part of the trough is sometimes called the Suruga trough. To the east, the Sagami trough defines the boundary of the two plates. The plate boundary is obscured between the Suruga and Sagami troughs. Collision of the Izu Peninsula with Honshu Island forms a zone of deformation around Mt. Fuji.

The relative motion of the Philippine Sea plate to the Eurasia plate has been discussed by SENO (1976). According to his calculation, the pole of the relative motion is located at 45.3±3.7°N, 150.2±5.5°E near the southern Kurile Islands.

The convergence rate at the Tokai district is about 3cm/yeat. ANDO (1975b) con- structed geodetic models for the fault dislocation of the 1944 Tonankai, 1854 Ansei, and 1707 Hoei earthquakes. The estimated dislocation for the 1944 and 1854 earthquakes is 4m, and for the 1707 earthquake 8m. If the average dislocation is 4m at the Tokai district, the recurrence time will be about 130years.

Since the part of the Philippine Sea plate near the Tokai district is not a simple oceanic plate, the pattern of the plate consumption may be complicated. As explained Possibility of a Great Earthquake in the Tokai District, Central Japan S 221 in the next section,source regions of some great earthquakes near the Tokai district seem to extend beyond the Nankai trough. Tectonic lineshave been assumed south of the Izu Peninsula as indicatedby broken linesin Fig.1 (rightbottom) along which thrust faultingmay take place (ISHIBASHI,1976; AOKI, 1977). However, no direct geophysical or geologicalevidence has been found to support such a hypothesis of multiple plateboundaries.

3. HistoricalEarthquakes

Source regions(rupture zones) of largehistorical earthquakes along the Nankai and Sagami troughsare shown in Fig. 1. Except forthe 1923, 1944,and 1946 earthquakes,the sourceregions have been estimatedfrom the descriptionof macroseismic effects,land deformations,and tsunamisin the historicalrecords. For olderevents,

Fig.1. Historicalearthquakes along the Nankai and Sagami troughs,and volcaniceruptions probably related to them. Map scale,1:15,000,000. S 222 T. UTSU the delineationof the sourceregions is more speculativedue to fewerhistorical materials.The years of greatearthquakes having magnitude 8 or more are indicated by largenumerals. There are 14 greatearthquakes occurring in nine epochslabeled [1],[2],..., [9]. Medium numerals in dashed ellipsesrepresent the approximate positionof earthquakesof magnitude 7 to 7.5. The dashed ellipsein epoch [9]indi- catesthe probable source region of the Tokai earthquakenow generallyassumed (e.g.,ISHIBASHI, 1976). Itis seen that the westernhalf of the sourceregion of the 1854 earthquakerup- turedagain in the 1944 earthquake,but the easternhalf remains unruptured for 122 years. The faultingof the 1707 earthquakealso extended into Suruga Bay, since Cape Omaezaki upheaved by about 1m at the time of the earthquake.However, it isnot certainthat the faultingreached the northernend of Suruga Bay as in thecase of the 1854 earthquake. It seems unlikelythat the source regionsof the 1605 Keicho and 1498 Meio earthquakescovered the Suruga Bay area. The shakingwas not so strongand no evidence of land deformationwas reportedin the Tokai district.However, the tsunamiswere very strongalong the Tokai coast. The tsunamiswere alsovery high in the Sagami Bay area and the Boso Peninsula. Hachijo Islandwas attackedby largetsunamis after the both earthquakes.Hachijo volcanoerupted 9 months after the 1605 earthquake.The othereruptions of Hachijo known in thehistory occurred in 1487, 1518,and 1522-23. These were fairlyclose to the 1498 earthquake. The 1605 earthquakehas been consideredto be two eventsoccurring simultaneously off Nankaido and offthe Boso Peninsula.However, the strongtsunami along the Tokai coastsuggests a possibilitythat the faultslipped slowly off the Tokai district.The sourceregion of the 1498 earthquakeshown in Fig.1 isthe one proposedby HATORI (1976).By studyingthe remainsof ancientboring shells, FUKUTOMI (1934,1938) inferredthat the southern Izu Peninsulaand Niijimaand ShikineIslands uplifted by a few metersseveral hundred years ago. The uplift maycorrespond to the 1498or 1605 earthquake. Itis likely that the faulting of the 1096Eicho earthquakeextended into Suruga Bay,because the tsunami was strongon thebay coast,though no materialsindicating theuplift of the coasthave been found. No directinformation is available indicating that the rupturezones of the 887 and 684 earthquakesreached to the Tokai district.The 887 earthquakewas a very big one, and itwas accompanied by a largeearthquake near Nagano (about 150km north of Mt. Fuji)on the same day. The largeeruptions of Mt. Fuji occurred in 1707, one and half months afterthe greatearthquake, and in 800-01 and 864-66. These eventsare marked in Fig. 1. Only three greatearthquakes (818, 1703, and 1923) are known in the northern part of Sagami trough,though the sourceregion of the 818 earthquakeis fairlyuncertain. By comparing the two series,epochs [1]-[5]and epochs [6]-[9],we can notice some similaritiesbetween them (AOKI, 1977). However, time intervalsare much longerfor the earlierseries. Epochs [1] and [6] aro not very similar.Epochs [2] Possibility of a Great Earthquake in the Tokai District, Central Japan S 223

and [7] are similar in that both epochs included great earthquakes at the Sagami trough prior to the Tokai-Nankaido earthquakes, and the large eruptions of Mt.

Fuji occurred only in these epochs. Epochs [3] and [8] are very similar, because in each epoch a great Tokai earthquake was followed by a great Nankaido earthquake.

Epochs [5] and [9] resemble each other in that the rupture did not propagate into

Suruga Bay (provided that the Tokai earthquake will not occur in the near future).

It should be mentioned here that earthquakes of magnitude 7 to 7.5 occurred 20 to

50 years after the great earthquakes. An earthquake of this size can be destructive to the Tokai district. Of course it is not proved that the similarities between the two series represent a real physical process, but if we admit the similarities, we can expect a great earthquake corresponding to the 1489 earthquake in a year 2030±28

(AOKI, 1977). The time interval between great earthquakes in the Tokai district is very vari- able. If we consider the earthquakes whose rupture zones extended into Suruga Bay, i.e., the 1096, 1707, and 1854 earthquakes, the intervals between them are 611 and

147 years. The 1498 and 1605 earthquakes probably released the crustal strain in the Suruga Bay area to some extent. The time intervals between the latest four earthquakes in 1498, 1605, 1707, and 1854 are 117, 102, and 147 years, respectively.

The mean interval is 119 years. As 122 years has passed since the last earthquake, considerable amount of strain must be accumulated in the Tokai district during this interval.

4. Recent Crustal Movements

4.1 Horizontal strain Triangulationor trilaterationsurveys around the Suruga Bay area conducted by the GeographicalSurvey Institute(GSI) in 1884, 1931, 1956, 1973,and 1975 re- vealedremarkable compression of the Tokai districtin the NW-SE or WNW-ESE direction.Figure 2 shows the change in distancebetween some representativesta- tionsand the extension,contraction, and maximum shear for three representative trianglesduring the time intervalof 42 yearsfrom 1931 to 1973 (GSI, 1977). The

Fig.2. Horizontalstrain in the Suruga Bay area (GSI, 1977).Large numerals:maximum shearin 10-5.Small numerals:change in distancein cm. S 224 T. UTSU

strain rate for contraction or maximum shear is 3.0 to 3.6×10-7/year. Similar rates

have been obtained for other periods befbre 1973. Such strain rates are fairly high

for a region of Japan where no remarkable earthquakes occurred during the interval

between surveys. The ultimate strain for the crust of the Tokai district is not known

exactly, but if we adopt RIKITAKE'S estimate (1975) for the average ultimate strain, 4.7×10-5, the recurrence time will be 130 to 160 years. RIKITAKE (1977) discusses

the probability of the Tokai earthquake based on new data.

In 1974, the Izu Hanto-oki earthquake (M=6.9) occurred near the southern tip of the Izu Peninsula. This earthquake was caused by a right-lateral strike-slip faulting in the NW-SE direction. A preliminary report from the GSI shows the distances from Iwashina-mura to Sakabe-mura and Ryuzumeyama increased by about 38cm between 1973 and 1975. This fact suggests the release of strain in the central part of

Suruga Bay. Instead, the strain may be increased in the southern part of Suruga Bay.

4.2 Vertical movement Preciseleveling surveys were conductedseveral times since about 1885 by the GSI. These surveysshow a considerablesubsidence of the west coastof Suruga Bay at almost a constantrate for about 90 years. Figure 3 shows the movements of Shizuoka and Okitsurelative to Iwata and Numazu. Sincethe lattertwo reference stationsare believedto be stable,the subsidencerate is about 5 to 6mm/year. The tidegauge recordsalso show the subsidenceof Shimizu relativeto Uchiura at a rate of about 5mm/year. On the other hand, the Akaishi Mountains standingbehind the subsidencezone upheaved by more than 30cm during the last90 years. Ifthe amounts of co-seismicuplift and post-seismicrapid subsidence due to a

Fig. 3. Vertical movements of Okitsu and Shizuoka relativeto Numazu and Iwata (SATO, 1977a). Possibility of a Great Earthquake in the Tokai District, Central Japan S 225

great interplate thrust-type earthquake are denoted by D and δ, respectively, the recurrence time interval R is given by

where υ is the rate of secular upheaval (averaged for thousands of years) and r is the rate of steady subsidence between earthquakes. In the case of the 1854 earthquake,

D has been estimated to be 1 to 2m on the west coast of Suruga Bay, but δ is not known. The secular vertical movement has not been determined accurately, since the marine terraces are not so easily identified. Geologists (e.g., MATSUDA, 1975) describe that the secular rate at Omaezaki area is small as compared with those at

Cape Muroto in Shikoku (1.5mm/year) and at the southern Boso Peninsula (3.3 mm/year). If we assume that D-δ=1m, υ=0.5mm/year, and r=5mm/year, we obtain R=180 years.

5. RecentSeismicity

Figure4 representsthe distributionof shallowearthquakes (h<60km) located by the Japan MeteorologicalAgency (JMA) in the years from 1926 to June, 1976. The earthquakesoff the Kii Peninsulaare the aftershocksof the 1944 Tonankai earthquake. Before that earthquake,the region was very calm, and at presentit becomes calm again. If we neglectearthquakes of magnitude lessthan 5, we can

Fig. 4. Shallow earthquakes located by JMA, 1926-June, 1976. S 226 T. UTSU observea seismicitygap in Suruga Bay and Omaezaki area. SEKIYA (1976) has reported on the anomalous seismicactivities preceding several earthquakes in Japan. The natureof theseactivities isdifferent from ordinaryforeshocks. The time interval betweenthe anomalous activityand the main shock isfairly long, depending on the magnitude of the main shock. Sekiyaalso pointed out thatthe activityin the pre- sumed region of the Tokai earthquakeincreased during 1961-66. It isnot clear, however, thatthis activity represents the anomalous activityreported by Sekiya. Figure 5 shows the distributionof microearthquakesfor the last2 yearslocated by Nagoya University.Some activitiesare found in Suruga Bay, but no earthquakes with magnitude above 3 occurredin thisperiod.

Fig. 5. Microearthquakes located by Nagoya University, June 1974-Oct. 1976. Possibility of a Great Earthquake in the Tokai District, Central Japan S 227

MOGI (1969)reported that the seismicactivity increased remarkably in areas surroundingthe focalregions of the 1944 and 1946 earthquakesduring about 20 yearsprior to thesegreat earthquakes. According to UTSU (1974)the rateof occur- renceof destructiveearthquakes in westernJapan duringthe 50 year intervalsprior to and 10 year intervalsafter the greatearthquakes along the Nankai trough was about fourtimes as high as therate for the otherperiods. It isnot certainthat large earthquakesoccurring near or behind the Tokai districtin recentyears, e.g., the Izu Hanto-oki earthquake of 1974 (M=6.9), the Atsumi Hanto-okiearthquake of 1971 (M=6.1), the CentralGifu earthquakeof 1969 (M=6.6), the Shizuoka earthquake of 1965 (M=6.1), the Kitamino earthquakeof 1961 (M=7.0), etc.represent such a kind of activitypreceding the Tokai earthquake.

6. PossiblePrecursors and Probability Multi-Elementof Prediction

It isnot known thatthe focalregion of a great earthquakealong the Nankai troughbecomes dilatantbefore its occurrence. Thus itis questionable that some pre- cursorssometimes interpreted as due to the dilatancyof the crustalrock, i.e., VP/VS anomalies,foreshocks, changes in radon contentin ground water,etc., can be ob- servedbefore the Tokai earthquake. However, nearly aseismicfault slip is more likelyto occur before such a great earthquake. In this case,precursory crustal movement and associatedphenomena may be expected.

6.1 Change inthe rate of crustalmovements There isno clearevidence to indicatethat the rateof horizontalstrain or vertical movement in the Tokai districthas been increasedor decreasedin recentyears. The tidegauge recordat Omaezaki and the levelingsurveys to Omaezaki givesome indi- cationof the acceleratedsubsidence in the lastseveral years, but some more years are necessaryto evaluatethis phenomenon.

6.2 Anomalous crustalmovements and change inground water Various observationssuggest that anomalous crustalmovements preceded the 1854 Nankaido, 1944 Tonankai, and 1946 Nankaido earthquakes. SATO (1970, 1977b)reported the disturbanceto the levelingsurvey near Omaezaki on the day and theprevious day of the 1944 earthquake.Water levelsof many wellsin the Kii Peninsula andShikoku went down a few days beforethe 1946 earthquake.Some wells became muddy (HYDROGRAPHIC DEPARTMENT, 1948; SAWAMURA, 1967). Borehole strainmetersof Sacks-Evertsontype have been in operationsince April, 1976. The instrumentsat Omaezaki and Mikkabi (by Lake Hamana) indicateremarkable contraction,approximately 10-6/month (JMA, 1977). TWO additionalinstruments of the same designwill be installedin February,1977 to check thisanomalous strainrate.

6.3 Seismicitypattern The problems of seismicityare describedin the previoussection. The seismicity S 228 T. UTSU

of the Tokai district has been monitored mainly by the networks of JMA, Nagoya University, and Tokyo University. The JMA will install ocean bottom seismometers connected with a 200km long submarine cable south of Omaezaki in 1978.

6.4 Seismic wave velocities Severalindependent observations indicate that the crustal P-wave velocityin the Tokai districtis fairly low. These observationsinclude a seismicrefraction survey in the Sea of Enshu (offOmaezaki) (ASADA, 1975),observations of the explosionsat Oshima and Inabu (IKAMI, 1976, 1977),travel-time delays of naturalearthquakes (RESEARCH GROUP FOR AFTERSHOCKS, 1975;UTSU, 1975a,b). However, itis ques- tionablethat these observations represent a precursorydecrease in P-velocityto the Tokai earthquake.The P-traveltime from Oshima to Shizuoka did not change by more than 0.02 sec between threeexplosions in the fallsof 1974, 1975, and 1976. This path crossesthe upheaval region in the Izu Peninsulaand the centralpart of Suruga Bay.

6.5 Otherprecursors Observationsof geomagnetism,gravity, radon contentin ground water, etc. have been made and willbe made more intensivelyin the Tokai districtmainly by the members of the Universityof Tokyo.

6.6 Multi-elementprediction One of the difficultproblems in the earthquakeprediction is how to evaluatean anomalous phenomenon which willbe found in the courseof observation.Since the signal-to-noiseratio for an earthquakeprecursor is generally low, the probability thatan anomalous phenomenon isa realprecursor is fairly small. However, if we findtwo or more precursor-likephenomena of differentkinds at the same time,the probabilityof successfulearthquake prediction will be increasedconsiderably. UTSU (1977)proposed the following formula forthe probabilityp that the earthquakepre- dictionwill be successfulwhen N independentanomalous phenomena are observed at the same time.

where pi is the probability for the successful prediction based on the i-thanomalous phenomenon alone, and p0 is the probability of success when a prediction is issued at random without making any observation. P0 is estimated from the secular seismicity of the region concerned. Of course, the criteria for 'anomalous phenomenon,' 'earthquake ,'and 'the same time' must be set up definitely. For example, if p0= 0.05 and p1=p2=0.2, the above equation for N=2 gives p=0.54. If we add the third element with p3=0.2, we get p=0.85. In this case we can issue a warning with considerable confidence. Possibility of a Great Earthquake in the Tokai District, Central Japan S 229

7. Conclusion

Althoughseveral pieces of evidencesuggest the possibility of the Tokai earthquake in not very distantfuture, it is stilluncertain that the region has been in the long-term precursory stage. Geodetic surveys repeated during the last80 years show considerable strain accumulation in this region. The recurrence time intervalof great earthquakes in the Tokai districtestimated from various kinds of evidence varies from 120 years to more than 600 years. If a great earthquake occurs in the region indicatedby a dashed ellipsein Fig. 1 [9],the damage will be tremendous in the worst case. It seems possible,however, that a great earthquake will take place south of the Izu Peninsula likethe 1498 earthquake and will releasethe strainin the Tokai districtat leastpartially. Various kinds of observations are going to be in- tensifiedby several institutionsaiming at the detection of medium-to short-term precursors. It isalso important to carry out geophysicaland geologicalinvestigations on land and at sea to clarifythe complex tectonicsin and near the Tokai district, and to collectmore information on historicalearthquakes.

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Note added inproof During severalmonths afterthe submissionof thispaper, a considerableamount of new informationon the Tokai problem has been provided. Some of the new resultsare found in Rep. Subcomm. TokaiDistr., Coord. Comm. Earthq.Predict., 81 pp.,Nov., 1977 (inJapanese).