総合報告: 古地震研究によるプレート境界巨大地震の長期予測の問題点 -日本付近のプレート沈み込み帯を中心として-

特集: 大地震の長期予測はどこまで可能か? I. 古地震調査と長期予測

抄録

We review long-term forecasts of great earthquakes along subduction zones around Japan and discuss the related problems from a paleoseismological point of view. Rich historical data in Japan show the recurrence of great earthquakes along subduction zones, particularly at the Nankai trough, for more than 1, 000 years. On the basis of such historical data and interseismic/coseismic vertical crustal movements, Imamura made a rather vague forecast of great earthquakes along the Nankai trough as early as 1933, which turned out to be successful by the occurrence of the 1944 Tonankai (M_w 8.1) and 1946 Nankai (M_w 8.1) earthquakes. After plate tectonics theory was established, the concept of seismic gap in subduction zones has been thought as a powerful tool for long-term earthquake forecasts. Great interplate earthquakes have been predicted from examinations of not only seismic gaps but also other observations such as seismic quiescence, earthquake recurrence history, current crustal deformation in coastal areas, or seismic crustal movements in geologic records. The 1973 Nemuro-oki earthquake (M_w 7.8) along the Kurile trench was predicted in 1972, although it was slightly smaller than the predicted size. The 1978 Miyagi-oki earthquake (M_w 7.6) along the Japan trench was also predicted in 1977, although the size and place were somewhat different from the prediction. The Tokai earthquake was predicted in 1976, and its occurrence has been considered imminent. Despite that short-term surveillance system has been in operation in the last 20 years, this earthquake has not occurred yet. In 1994, two great/large earthquakes occurred in subduction zones off northeast Japan, but no forecasts had been made on these. The off-Hokkaido event (M_w 8.2), whose aftershock area apparently coincides with that of the 1969 interplate earthquake (M_w 8.2), has been interpreted as an intraplate event within the subducted Pacific slab. The Sanriku-oki earthquake (M_w 7.7) was aninterplate event, but its rupture zone overlapped with a previous interplate event, the 1968 Tokachi-oki earthquake (M_w 8.2). The recurrence history of interplate earthquakes along the Nankai trough has been updated by seismo-archaeologlcal data such as liquefaction evidence at archaeoiogical sites. and it now seems more regular and can be explained by time-predictable model. In the mean time. historical data indicate that the 1605 earthquake was an unusual “tsunami earthquake” and the source process is very different from the other repeated events. This event may have been affected by a preceding large inland earthquake in 1596. Paleoseismological investigation is still very important for long-term forecast of earthquakes. Future research should emphasize to complement recurrence history of interplate earthquakes from historical as well as seismo-archaeological data, to distinguish intraplate (slab) earthquakes from interplate earthquakes in historical catalog, to investigate the co-relation between interplate and inland earthquakes, and to combine historical, geologic and other kinds of data to study earthquake recurrence as demonstrated in the Cascadia subduction zone.