ANNALS OF GEOPHYSICS, VOL. 47, N. 2/3, April/June 2004 Status of historical seismology in Japan Katsuhiko Ishibashi Department of Earth and Planetary Sciences at Faculty of Science, Kobe University, Kobe, Japan Abstract Japan’s combination of high seismicity and a long history has produced copious written records of histori- cal earthquakes. Systematic collection and investigation of such historical documents began late in the 19th century. Now, almost all of Japan’s known historical materials on earthquakes have been transcribed into 25 printed volumes. The collections include records of about 400 destructive earthquakes from A.D. 599 to 1872. Epicentral coordinates and magnitudes have been estimated for about half these events and details of earthquake and tsunami disasters have been summarized in catalogues. The space-time pattern of great Tokai and Nankai earthquakes is a good example of revealed earthquake history. The existing collections of historical sources, however, contain low-quality records that produce errors and fictitious (fake) earth- quakes, and are difficult of full utilization because of volumes. Moreover, there are peculiar problems to Japan’s historical times such as calendar and time of day. Systematic ways of estimating seismic intensities, epicenters, focal depths and magnitudes have not yet been established. Therefore, historical earthquake cat- alogues are yet incomplete. Constructing a reliable database of the whole historical documents in collabo- ration with historians to give wide-ranging researchers easy and full utilization of old earthquake records is urgent task. Revision of earthquake catalogues and construction of a seismic intensity database with inter- national standard are also necessary. Key words historical seismology – Japan – histor- been intensively conducted in Japan for more ical documents – earthquake catalogue – database than a century. Purposes or contents of historical earth- quake study can be categorized into three. The 1. Introduction first is scientific observation and analysis of historical earthquakes such as underground The seismic activity in and around Japan natural phenomena, that is, historical seismol- is very high because it is located in an active ogy, an indispensable branch of earthquake plate boundary zone where four lithospheric science. The second is investigation of histori- plates converge on one another. The Japanese cal earthquake disasters including physical people have suffered from severe earthquake and mental aspects from the viewpoint of pres- and tsunami disasters frequently throughout ent and future earthquake countermeasures their history, and have left a huge amount of (e.g., Miki, 1979). The third is analysis of his- written documents on these events. Based on torical societies and people related to earth- them, study of historical earthquakes has quake disasters from the viewpoint of social and human sciences (e.g., Kitahara, 1983). Though these three fields are interdisciplinari- ly related to one another and should collabo- rate closely, this paper focuses on the first Mailing address: Prof. Katsuhiko Ishibashi, Depart- field and describes the past progress, present ment of Earth and Planetary Sciences at Faculty of Science, Kobe University, Kobe, 657-8501 Japan; e-mail: situation and future problem of historical seis- [email protected] mology in Japan. 339 Katsuhiko Ishibashi Both rekishi-jishin (historical earthquakes) gins from 1885. The method of historical seis- and ko-jishin (old earthquakes, paleo-earth- mology, however, is useful even after these quakes) are used in Japan to denote earthquakes years, especially until the 1923 great Kanto in historical times. Concerning these terms, earthquake (Mw 7.9) or until around the end of Ishibashi (1987) proposed definitions as fol- World War II. Takemura (1999, 2001), for exam- lows; «paleo-earthquakes» is the general term ple, investigated strong ground motions due to for earthquakes during the pre-instrumental pe- the 1923 Kanto earthquake and very large after- riod («pre-instrumental earthquakes») includ- shocks immediately afterwards based mainly on ing «historical earthquakes» and «prehistoric many descriptions of personal experiences. earthquakes», whereas «historical earthquakes» Archaeoseismology and paleoseismology means earthquakes in pre-instrumental histori- have recently begun to complement historical cal times when written records exist, and «pre- earthquake research in Japan (e.g., Sangawa, historic earthquakes», ones during older times 1992; Ota and Shimazaki, 1995; Matsuda, without written records including the Holocene 1999). Field records of historical earthquakes and Pleistocene. We should note, however, that include surface fault displacements, surface pre-WWSSN (World-Wide Standardized Seis- fractures, landslides, liquefaction features, up- mograph Network) (pre-1963) earthquake lifted marine terraces, subsided marshes, dam- records and earthquakes are sometimes called aged archaeological sites, tsunami deposits, tur- as «historical seismograms and earthquakes» bidites, dendrochronological materials, and (e.g., Lee et al., 1988). remnant magnetization. For many unknown The time window of historical earthquakes earthquakes during historical times, whose as defined above differs from place to place in written records do not exist, archaeoseismology the world. In Japan, the beginning of the age of and paleoseismology are the only ways to in- historical earthquakes is about the 6th century. vestigate. In this review, however, I will con- The oldest known earthquake dates back to centrate on historical seismology dealing with A.D. 599, which is recorded in Nihon Shoki written historical records of old earthquakes. (e.g., Kuroita, 1951, 1952), the Japan’s oldest Japan is a volcanic country as well, having chronicle completed in 720. Although an older experienced violent volcanic eruptions many earthquake is also described in Nihon Shoki and times, and study of historical eruptions has it is often said of A.D. 416 (e.g., Usami, 2002b, been conducted in parallel with that of histori- 2003), historical events before the early 6th cal earthquakes. Therefore, what will be dis- century A.D. belong to a mythical age for cussed in this paper can be applied to historical which precise dating is almost impossible; the volcanology to a considerable extent. As for the year of 416 is a holdover of the ultranationalis- investigation of historical natural disasters oth- tic view of history during the pre-World War II er than earthquakes, tsunamis and volcanic period claiming that Japan had been founded by eruptions, collaboration with historical seismol- Emperor Jin’mu in 660 B.C., which had a ogy has not been active in Japan so far. strong influence on the pre-1945 historical seis- mology as well. The lower limit of the age of historical earth- 2. Tectonic setting and seismicity of Japan quakes in Japan is usually considered to be 1872 or 1884. The year 1872 is taken because the Gre- The Japanese Islands are located in the gorian solar calendar was adopted and instru- midst of the plate convergence zone at the mental observation of earthquakes, though very northwestern margin of the Pacific Ocean primitive, was started in Japan in 1873 (e.g.,Us- where two oceanic plates are being subducted ami, 1979, 1982, 1988). The year 1884 is taken beneath two continental plates (fig. 1). North- because Utsu’s (1979, 1982, 1988) parametric east Japan is considered to be on the North catalogue of large earthquakes (M ≥ 6) and dam- American plate or the Okhotsk (Sea of Okhot- aging earthquakes (M < 6) in Japan based on sk) microplate, and southwest Japan, on the careful re-interpretation of instrumental data be- Eurasian plate or the Amurian microplate. Al- 340 Status of historical seismology in Japan Fig. 1. Plate-tectonic setting of the Japanese Islands. Hokkaido, Honshu, Shikoku and Kyushu are four main islands of Japan. Arrows indicate the motions of the Pacific and the Philippine Sea plates relative to north- east Japan roughly. Subduction boundaries of these two plates are shown by solid lines. Sgt – Sagami trough; Srt – Suruga trough. As for the continental plates and the boundary between them (broken line), see text. K and E denote two old capitals, Kyoto and Edo (now Tokyo), respectively. though the plate geometry and kinematics of The Philippine Sea plate is being subducted these two blocks remain controversial, there is beneath the southern part of northeast Japan general agreement that these two parts of Japan (Tokyo metropolitan area and its vicinities) and have been converging with each other in an beneath southwest Japan. The convergence di- east-west direction at the rate of 1 to 2 cm/yr, rections and rates are north-northwestward and roughly, since the late Pliocene or the early about 3 cm/yr at the Sagami trough, and west- Pleistocene, and that the juvenile convergence northwestward and 4 to 6 cm/yr at the Suruga- boundary between the two plates runs along the Nankai trough and the Ryukyu trench. The Pa- west coast of northeast Japan and crosses the cific plate is being subducted beneath northeast middle of Honshu toward the Suruga trough. Japan west-northwestward at the Kuril and 341 Katsuhiko Ishibashi Japan trenches at the rate of about 8 cm/yr and large or great earthquakes in terms of plate tec- beneath the Philippine Sea plate west-north- tonics in subduction zones take place in Japan westward at the Izu-Bonin trench at the rate of (fig. 3). They are interplate events along the sub- about 6 cm/yr. duction boundaries, shallow intraplate events in Because of these plate interactions the seismic the crust of overriding plates, offshore shallow in- activity in and around Japan is very high (fig. 2). traplate events in the oceanic plates, and deeper Figure 2 shows only the shallow seismicity, but slab earthquakes in slabs (subducted oceanic deep seismic zones reach as deep as about 600 km plates). Major historical disastrous earthquakes in in the subducted Pacific plate and as deep as about Japan include great interplate events along the 300 km in the subducted Philippine Sea plate Kuril and Japan trenches, Sagami trough and Su- along the Ryukyu trench.