J. Phys. Earth, 43, 395-405, 1995 Recent Great Earthquakes and Tectonics in Japan 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) earthquake has had a great impacton studiesof tsunamis 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 fault. 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 tsunami.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 Fukui 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 subduction 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. Epicenter data are from Japan Meteorological Agency. Vol. 43, No. 4, 1995 398 K. Abe 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 aftershocks 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). Foreshock 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 hypocenter and the largest magnitude is 5.0. The foreshocks 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).