J. Phys. Earth, 43, 395-405, 1995

Recent Great and Tectonics in

Katsuyuki Abe EarthquakeResearch Institute, The Universityof Tokyo, Bunkyo-ku,Tokyo 113,Japan

In the last 10years (1982-1991), a great earthquakeof MW=7.8occurred in 1983 beneaththe Japan Sea, wherelarge earthquakesand tsunamisseldom occurred. The Akita-Oki(Central Japan Sea) has had a great impacton studiesof and plate tectonicsas wellas studiesof seismicsource mechanisms, and a number of studieshave been made of this event. Mechanismstudies have revealedconsiderable heterogeneityin rupture propagationand slip distributionon the . Considerable attentionhas been focusedon the argumentthat this earthquaketook place on a very youngboundary between the Eurasianand North Americanplates. Japanese researchers have also taken an activeinterest in foreign earthquakes.Reconnaissance teams were frequentlydispatched to manyparts of theworld to makesurveys of the effectsof recent significantearthquakes.

1. Introduction It was very fortunate that Japan had been free from devastating earthquakes for the four decades before 1992 (see "Note added in proof"). It happened that this geologic peace had been produced by the transient lack of occurrence of large earthquakes originating in densely populated cities. In the last ten years (1982-1991), a great earthquake of MW=7.8occurred in 1983beneath the Japan Sea. The Akita-Oki (Central Japan Sea) earthquake has had a great impact on studies of tsunamis and plate tectonics as well as studies of seismic source mechanisms, and a number of studies have been made of this event. Japanese researchers and engineershave also taken an active interest in foreign earthquakes. Reconnaissanceteams were frequently dispatched to many parts of the world for post-earthquake investigations of significant events. Arbitrarily chosen highlights of these works are briefly described below.

2. Damaging Earthquakes Major destructive earthquakes in Japan during the past 100 years are given in Table 1, in which shocks with a loss of more than 100 lives are listed for 1891-1948 and those with more than 10 deaths for 1949-1991. It is to be noted that earthquakes do not need to be of large magnitude to produce severe damage, because the degree of damage depends not only on the physical size of an earthquake but also on various

Received March 18, 1992; Accepted March 5, 1993

395 396 K. Abe

Table 1. Major damaging earthquakes in Japan for the past 100 years, 1891-1991.

Earthquakeswith morethan 100deaths are listedfor 1891-1948and thosewith more than 10 deaths are listedfor 1949-1991.Number of "swepthouses" indicates number of housesswept by .Type indicatesearthquake tectonics: A, interplate;B, outer-risenormal faulting; C, intra-crust;D, unknown mechanism. factors such as where and when an earthquake occurred. Fortunately, no large earth- quakes originating in densely populated cities occurred for the last four decades, and the death toll associated with earthquakes has actually been at a low level since the earthquake of 1948 (e.g., Abe, 1987). The worst death tolls during the past 40 years before 1992 have been those of 139 persons due to the tsunami excited by the great Chilean earthquake of 1960, 104 for the Akita-Oki earthquake of 1983, and 52 for the Tokachi-Oki earthquake of 1968.

3. Recent Major Earthquakes Many earthquakes of magnitudes in the high sevens or eights were concentrated during the 22 years from 1952 to 1973. They are, in chronological order, the Tokachi-Oki earthquake(MW=8.1) of 1952, the Iturup (MW=8.4) of 1958, the Iturup (MW=8.5) of 1963, the Tokachi-Oki (MW=8.2) of 1968, the Hokkaido-Oki (MW=8.2) of 1969, and

J. Phys. Earth Recent Great Earthquakes and Tectonics in Japan 397 the Nemuro-Oki(M=7.8) of 1973. These six earthquakes occurred along the Kurile and Japan trenches, filling the entire seismic belt (e.g., Mogi, 1985a). The Nemuro-Oki event took place in an as yet unfilled seismic gap identified as such more than 5 years earlier. All of these earthquakes were tsunamigenic (e.g., Abe, 1988). The activity of these earthquakes is a manifestation of the mechanical interaction between the sub- ducting and the overriding plates along the zone in a plate tectonic frame- work (Lay et al., 1982; Sato et al., 1986). The seismic slip rate inferred from these interplate thrust earthquakes is known to be smaller than the rate of plate motion. To resolve this discrepancy it has been proposed that aseismic coupling of plates accounts for a large fraction of interplate slip (Kanamori, 1977; Peterson and Seno, 1984; Kawasaki et al., 1991). The last large interplate event along the Pacific is the Miyagi-Oki earthquake (MW=7.6) of 1978 in northeastern Honshu (e.g., Seno et al., 1980). Large earthquakes of the past are often subjected to repeated analyses, because new methods of analysis are continually being developed. The 1968 Tokachi-Oki earthquake is such event (Iida and Hakuno, 1984; Mori and Shimazaki, 1985; Kikuchi and Fukao, 1985; Satake, 1989). The seismic activity in the last 10 years is shown in Fig. 1. The earthquake catalogs of the Japan Meteorological Agency for the area of Japan list 975 shocks of MS and over, and 147 of M6 and over for 1981-1990. In the last 10 years, no great earthquakes occurred along the Pacific coast. Instead of this region, a great event of MW=7.8 occurred in the Japan Sea on May 26, 1983.

Fig. 1. Seismicity of Japan for the past 10 years, 1981-1990. Open circles, shallow

(focal depth <100km); Crosses, deep (focal depth•†100km). Size of symbols denotes difference of magnitude. Broken line denotes proposed boundary between the Eurasian and North American plates. Plates are identified as: P, Pacific; Ph, Philippine Sea; EU, Eurasian; and NA, North American. data are from Japan Meteorological Agency.

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The earthquake-generated tsunami caused severe damage in coastal regions. This earthquake is unique because it took place in the Japan Sea where large earthquakes and tsunamis seldom occur. The last largest event in the Japan Sea was the Niigata earthquake(MW=7.6) of 1964. This earthquake has been the subject of repeated anal- yses owing to long-standing questions regarding the fault orientation (Satake and Abe, 1983; Hamada, 1983; Fujiwara and Seno, 1985; Mori and Boyd, 1985; Matuhashi et al., 1987). A shallow earthquake (M=7.5) occurred on April 5, 1990 just beneath the Mariana trench. This is the greatest shallow event ever recorded in this region. The tsunami washed against the Pacific coast of Japan (Satake et al., 1992). The earthquake is an outer-rise normal faulting event in a weakly coupled subduction zone (Yoshida et al., 1992 a).

4. The Akita-Oki Earthquake of 1983

This earthquake is officially called the "Nihonkai-Chubu (Central Japan Sea) Earthquake" (Japan Meteorological Agency, 1984), but, in view of its proximity to the city of Akita Prefecture, here we call it the "Akita-Oki" earthquake. The Japanese word "oki" means "off the coast ." The excellent seismic data sets recorded for this earthquake have been the subject of numerous studies. These studies have been focused on the seismological, geodetic, geophysical, geological, and engineering aspects of the earth- quake. The earthquake had a predominant double event source like the 1985 Mexican earthquake (e.g., Satake, 1985; Kanamori and Astiz, 1985; Kosuga et al., 1986). The focal mechanisms and source parameters have been obtained from digital waveform data recorded by very-broad-band seismographs. The average feature of the source mechanism is thrust faulting along southern and northern planes dipping 110 NE and 70 NE with a dip angle of 20 to 30 degrees. The estimate of the total seismic moment ranges from 4 to 8•~1020 Nm and the average is 6•~1020 Nm (M=7.8). The estimate of the total fault length varies from 90 to 150km, and the average is 110km. The detailed rupture process has been studied using analyses of body wave and strong motion records (Sato, 1985; Fukuyama and Irikura, 1986; Koyama, 1987; Iwata and Irikura, 1988). The main rupture consists of a large subevent at the beginning of the rupture, followed by other large subevents about 50km northeast and 25s later. Izutani and Hirasawa (1987) analyzed the directivity of the strong-motion duration for a rapid estimate of the fault length. Kuroiso et al. (1986) discussed the correlation between spectral characteristics of the and the rupture process of the main shock. A number of aftershocks were located using the existing regional array or temporary arrays of high-gain seismographs (Umino et al., 1985; Sato et al., 1986; Nosaka et al., 1987) and ocean bottom seismographs (Urabe et al., 1985). activity was reported (Umino et al., 1985). The activity started 12 days before the main shock within a concentrated area in the vicinity of the main shock and the largest magnitude is 5.0. The exhibited a remarkable similarity of waveforms

(Hasegawa et al., 1985). This suggests the possibility of identifying a foreshock sequence

J. Phys. Earth Recent Great Earthquakes and Tectonics in Japan 399 by monitoring waveform similarity in the time domain on routine basis, as was proposed by Motoya and Abe (1985). Mogi (1985b) has suggested, from the post-earthquake investigation of space-time distribution of shallow earthquakes, that the formation of seismic quiescence over the focal region preceded the main event. Mogi (1985c) has also pointed out that the Akita-Oki earthquake took place at the end of proposed tectonic line crossing northeastern Honshu. A small island, Kyuroku-shima, near the main shock epicenter was found to have subsided by 32cm from photographic analyses of snapshots (Yamashina et al., 1985). A slight subsidence of the order of centimeters was measured along the coast by precise leveling (Geographical Survey Institute, 1984). This earthquake was very important in tsunami studies because the greatest tsunami in the last two decades was generated (Fig. 2). The maximum run-up height is 13.75m at Hachimori, north of Akita Prefecture (Shuto, 1984). Hatori (1983) estimated tsunami source area from the travel time data. Aida (1984) and Watanabe (1987) interpreted the observed heights in terms of fault dislocation models. Satake (1989) applied inversion technique to tsunami waveforms in a way similar to seismic waveform analysis, and estimated the heterogeneity of fault slip at the source. The source models derived from these studies are in close accord with the solutions from studies. Abe (1985) noted that tsunami heights from earthquakes in the Japan Sea are systematically larger than those from earthquakes with a comparable MWin the Pacific, and explained this discrepancy in terms of the effective difference both in source depth and in fault geometry. This earthquake had a great impact on studies of plate tectonics because it took

Fig. 2. Geographic distribution of observed and estimated heights for the Akita-Oki tsunami (Mt=8.1) of May 26, 1983. Solid circles represent the local-mean run-up heights in the segment interval of about 40 km along the coast of the Japan Sea. Error bars represent the standard deviation of the average. Cross symbols represent heights estimated from the simple relation proposed for a near-field tsunami warning purpose (Abe, 1989).

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place in the Japan Sea, where large earthquakes seldom occur. The location and the fault motion of the Akita-Oki earthquake roused strong argument that the eastern margin of the Japan Sea is a young between the Eurasian and North American plates (Seno, 1983; Seno and Eguchi, 1983; Kobayashi and Nakamura, 1983; Nakamura, 1983). Nakamura (1983) suggested that the convergence started 1-2Ma. Global plate motion implies convergence at the rate of 1.1cm/year at the main shock epicenter (Kanamori and Astiz, 1985). According to this model, northern Honshu, Hokkaido and the Kuriles are on the North American plate (Fig. 1). In this case the earthquake can be considered as a subduction event that occurred at an essentially aseismic plate boundary. On the other hand, Seno (1985a) analyzed the direction of slip vectors of large earthquakes in northern and central Honshu, and suggested the possibility that an incipient jump of the North American-Eurasian plate boundary from Sakhalin-central Hokkaido to the eastern margin of the Japan Sea has not yet been accomplished. In this case northern Honshu would be a microplate without a driving force. Seno (1985b) later rejected this inference and suggested that the jump was completed by 0.5Ma. In view of the currently proposed modes of seismotectonics, there is concern that a great earthquake could rupture a historically quiet segment along the eastern margin of the Japan Sea. In this meaning, the Juan de Fuca subduction zone of young convergence shares many features with the eastern margin of the Japan Sea, and there is concern that it could be the source of a great earthquake (Heaton and Kanamori, 1984; Kanamori and Astiz, 1985).

5. Reconnaissance Dispatch to the World

The Japanese group for the Study of Natural Disaster Science, with the financial support of grant in aid from the Ministry of Education , Science and Culture of Japan, has frequently dispatched scientific reconnaissance teams to foreign countries for post-earthquake investigations of significant events. The members of the teams consisted of academic researchers in the field of , architecture, civil engineering and social science. The main objectives of the delegations were to document perishable information and to establish contact with appropriate research counterparts for further surveys and research. Some of the reports have been published in Japanese and some are in English. The recent earthquakes which have been investigated were, in chrono- logical order, the Valparaiso, Chile earthquake (M=7 .8) of March 3, 1985 (Hakuno, 1986), the Michoacan, Mexico earthquake (M=8 .1) of September 19, 1985 (Izumi, 1986), the Colombia-Ecuador border earthquake (M=6 .9) of March 6, 1987 (Hakuno et al., 1988), the Whittier Narrows, Los Angeles earthquake (M=5 .8) of October 1, 1987 (Minami, 1988), the Nepal- border earthquake (M=6 .6) of August 20, 1988 (Fujiwara et al., 1989), the Loma Prieta, earthquake (M=7.1) of October 18, 1989 (Kameda, 1990), the Rudbar, earthquake (M=7 .7) of June 20, 1990 (Tsukuda et al., 1991 b), and the Luzon, Philippines earthquake (M=7.8) of July 16, 1990 (Abe, 1990; Yoshida et al., 1992b). Nakata et al. (1990) measured the surface offset at more than 50 sites along the 120-km-long surface fault of the Philippines earthquake. Tsukuda et al. (1991a) established a temporary array of seismographs to

J. Phys. Earth Recent Great Earthquakes and Tectonics in Japan 401 locate the aftershocks of the Iranian earthquake. These works are particularly notable among the most recent surveys.

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Mori, J. and T. Boyd, Seismological evidence indicating rupture along an eastward dipping fault plane for the 1964 Niigata, Japan earthquake,J. Phys. Earth, 33, 227-240, 1985. Mori, J. and K. Shimazaki, Inversion of intermediate-period Rayleigh waves for source characteristics of the 1968 Tokachi-Oki earthquake, J. Geophys. Res., 90, 11374-11382, 1985. Motoya, Y. and K. Abe, Waveform similarity among foreshocks and aftershocks of the October 18, 1981, Eniwa, Hokkaido earthquake, Earthq. Predict. Res., 3, 627-636, 1985. Nakamura, K., Possible nascent trench along the eastern Japan Sea as the convergent boundary between Eurasian and North American plates, Bull. Earthq. Res. Inst., Univ. Tokyo, 58, 711-722, 1983 (in Japanese). Nakanishi, I., S. Kodaira, R. Kobayashi, and M. Kasahara, The 1993 Japan Sea earthquake, EOS, Trans. Am. Geophys. Union, 74, 377-380, 1993. Nakata, T., H. Tsutsumi, R. S. Punongbayan, R. E. Rimando, J. Daligdig, and A. Daag, Surface faulting associated with the Philippine earthquake of 1990, J. Geogr. Jpn., 99, 515-532, 1990 (in Japanese). Nosaka, M., K. Suyehiro, and T. Urabe, Aftershock distribution of the 1983 Japan Sea earthquake revealed by ocean-bottom and land-based stations, J. Phys. Earth, 35, 209-224, 1987. Peterson, E. T. and T. Seno, Factors affecting seismic moment release rates in subduction zones, J. Geophys. Res., 89, 10233-10248, 1984. Satake, K., The mechanism of the 1983 Japan Sea earthquake as inferred from long-period surface waves and tsunamis, Phys. Earth Planet. Inter., 37, 249-260, 1985. Satake, K., Inversion of tsunami waveforms for the estimation of heterogeneous fault motion of large submarine earthquakes: the 1968 Tokachi-Oki and 1983 Japan Sea earthquakes, J. Geophys. Res., 94, 5627-5636, 1989. Satake, K. and K. Abe, A fault model for the Niigata, Japan, earthquake of June 16, 1964, J. Phys. Earth, 31, 217-223, 1983. Satake, K., Y. Yoshida, and K. Abe, Tsunami from the Mariana earthquake of April 5, 1990: its abnormal propagation and implications for tsunami potential from outer-rise earthquakes, Geophys. Res. Lett., 19, 301-304, 1992. Sato, R., K. Abe, Y. Okada, K. Shimazaki, and Y. Suzuki, Handbook of Fault Parameters of Earthquakes in Japan, Kajima Publ. Co., Tokyo, 390 pp., 1989 (in Japanese). Sato, T., Rupture characteristics of the 1983 Nihonkai-Chubu (Japan Sea) earthquake as inferred from strong motion accelerograms, J. Phys. Earth, 33, 525-557, 1985. Sato, T., M. Kosuga, K. Tanaka, and H. Sato, Aftershock distribution of the 1983 Nihonkai- Chubu (Japan Sea) earthquake determined from relocated , J. Phys. Earth, 34, 203-223, 1986. Seno, T., A consideration on the "Japan Sea subduction hypothesis," J. Seismol. Soc. Jpn., Ser. 2, 36, 270-273, 1983 (in Japanese). Seno, T., Is northern Honshu a microplate?, Tectonophysics, 115, 177-196, 1985a. Seno, T., "Northern Honshu microplate" hypothesis and tectonics in the surrounding region, J. Geod. Soc. Jpn., 31, 106-123, 1985 b. Seno, T. and T. Eguchi, Seismotectonics of the western Pacific region, in Geodynamics of the Western Pacific-Indonesian Region, ed. T. W. C. Hilde and S. Uyeda, Geodynamics Ser., Vol. 11, pp. 5-40, Am. Geophys. Union, Washington, D.C., 1983. Seno, T., K. Shimazaki, P. Somerville, K. Sudo, and T. Eguchi, Rupture process of the Miyagi-Oki, Japan, earthquake of June 12, 1978, Phys. Earth Planet. Inter., 23, 39-61, 1980. Shuto, N. (ed.), Heights of the 1983 Nihonkai-Chubu earthquake tsunamis, Tsunami Eng. Rep., Tohoku Univ., 1, 88-267, 1984 (in Japanese).

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Takeo, M., S. Ide, and Y. Yoshida, The 1993Kushiro-Oki, Japan earthquake: a high stress-drop event in a subducting slab,Geophys. Res. Lett., 20, 2607-2610, 1993. Tsukuda, T., K. Sakai, S.Hashimoto, M. R. Gheitanchi,H. Suzuki,S. Soltanian, and P. Mozaffari, A field study on various phenomena associated with the 1990 Rudbar, northwest Iran, earthquake of M7.3, Bull. Earthq. Res. Inst., Univ.Tokyo, 66, 419-454, 1991b (in Japanese). Tsukuda, T., K. Sakai, S. Hashimoto,M. R. Gheitanchi,S. Soltanian,P. Mozaffari,N. Mozaffari, B. Akasheh, and A. Javaherian,Aftershock distribution of the 1990Rudbar, northwest Iran, earthquake of M7.3 and its tectonic implications,Bull. Earthq. Res. Inst., Univ. Tokyo,66, 351-381, 1991a. Umino, N., A. Hasegawa, K. Obara, T. Matsuzawa, H. Shimizu, and A. Takagi, Hypocenter distribution of foreshocks and aftershocks of the 1983 Japan Sea earthquake, J. Seismol. Soc. Jpn., Ser. 2, 38, 399-410, 1985 (in Japanese). Urabe, T., K. Suyehiro,T. Iwasaki, N. Hirata, T. Kanazawa,A. Nishizawa, and H. Shimamura, Aftershock distribution of the 1983 Japan Sea earthquake as determined from helicopter- dispatched OBS observation,J. Phys. Earth, 33, 133-147, 1985. Watanabe,H., Features of tsunamioccurrence by the 1983Nihonkai-Chubu earthquake, especially the relationship between tsunami occurrence and earthquake mechanism, J. Seismol.Soc. Jpn., Ser. 2, 40, 425-433, 1987 (in Japanese). Yamashina, K., K. Nakamura, T. Fukudome, T. Sato, and K. Tanaka, Subsidence of Kyuroku-shima island associatedwith the 1983Japan Sea earthquake, J. Seismol.Soc. Jpn., Ser. 2, 38, 81-91, 1985 (in Japanese). Yoshida, Y., K. Satake, and K. Abe, The large normal-faulting Mariana earthquake of April 5, 1990 in uncoupled subduction zone, Geophys.Res. Lett., 19, 297-300, 1992a. Yoshida, Y. and K. Abe, Source mechanism of the Luzon, Philippinesearthquake of July 16, 1990,Geophys. Res. Lett., 19, 545-548, 1992b.

NOTEADDED IN PROOFAS OF MAY1995 As this manuscript was last revised on March 1992, large earthquakes after that date are not covered in this paper. For only a few years after the last revision, Japan successively suffered large earthquakes. Following are short notes concerning the recent shocks. On January 15, 1993, a large earthquake (11 : 06 UT, 42.85N, 144.38E, MW= 7.6) took place off the coast of Kushiro, east Hokkaido. The Kushiro-Oki earthquake originating at a depth of 107km is very unique because large shocks around this intermediate depth seldom occur in Japan. The mechanism is a rupture within the subducting Pacific plate (Takeo et al., 1993). On July 12, 1993, a tsunamigenic earth- quake (13 : 17UT, 42.78N, 139.20E, M=7.7, Mt=8.1) occurred beneath the Japan Sea off the coast of southwest Hokkaido. The Hokkaido-Nansei-Oki earthquake caused severe damage with a loss of 230 lives and 601 houses, many of which were due to the tsunami waves. The tsunami run-up reached 31m at Okushiri Island (Hokkaido Tsunami Survey Group, 1993). This is the highest record since the 1896 Sanriku tsunami, in which the run-up reached 38.2 m. The earthquake is a thrust event originating in the gap between the locations of the 1940 Shakotan and 1983 Akita-Oki earthquakes in the Japan Sea (Nakanishi et al., 1993). A great earthquake which occurred on October 4, 1994 (13 : 22 UT, 43.37 N, 147.67E, MW=8.3, Mt=8.2) in the Kurile subduction zone

J. Phys. Earth Recent Great Earthquakes and Tectonics in Japan 405 is not a typical interplate earthquake but is an intraplate thrust earthquake, though the aftershock area of the Hokkaido-Toho-Oki earthquake overlaps the area ruptured by the interplate thrust earthquake of 1969 (MW=8.2). The earthquake-generated tsunami rose as high as 10 m at Shikotan Island. On December 28, 1994 (12 : 19UT, 40.45N, 143.72E, MW=7.7, Mt=7.7), a major earthquake occurred off the coast of Sanriku. The mechanism is a typical interplate thrust. The aftershock area of the Sanriku-Oki earthquake overlaps the ruptured area of the Tokachi-Oki thrust earthquake of 1968 (MW=8.2). It is suggested that the 1994 event ruptured the slip deficit region of the southern part of the 1968 break. On January 16, 1995, a tragic shock (20 : 46UT, 34.61N, 135.04E, MW=6.9) struck central-western Japan in the early morning, at 5:46 a.m. local time, and caused widespread damage in a major urban area. The epicenter is close to Kobe city with a population of 1.5 million. There were 5,502 deaths, 36,938 injuries, and 186,902 houses and buildings destroyed. The mechanism is intra-crust strike-slip faulting (Kanamori, 1995). Surface faulting of length 10km occurred in Awajishima island. Strike-slip faulting in this area is no geological surprise. The Hyogoken-Nanbu earthquake is one of the most disastrous events in Japan since the 1948 Fukui earthquake, and is of great importance for any study of seismic hazards in a modern urban environment.

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