抄録
Historical seismicity data since the year 1850in Tohoku and Hokkaido, northern Japan, show a spatial and temporal correlation between intraplate seismicity in the land area and occurrence of large interplate earthquakes along the Japan and Kuril trenches. In the continental lithosphere adjacent landward to the rupture zones of recent large interplate earthquakes, almost all the earthquakes of magnitude 6.0 and above occurred during the periods from the 50 years before to the 10 years after the occurrence of the large interplate events. A simple statistical test shows that this correlation is significant at a 99.5% confidence level. During the period for the last 20 years, a group of intraplate earthquakes occurred in middle Tohoku with no large interplate earthquake seaward of their epicentral locations; this suggests a possibility of a large interplate event off the southern Sanriku coast in the near future. We examine other geophysical data to rate the possibility. All the examined data, i.e., historic and recent seismicity off the Sanriku, and crustal deformation revealed by the geodetic work, support the possibility of occurrence of a large event off the southern Sanriku in the near future. Source parameters are estimated for this expected event; they are conformable to those of recent large interplate events along the Japan and Kuril trenches. We can use the hypothesis on the correlation between intraplate seismicity and large interplate earthquakes as a criterion for forecasting the land area of high seismic risk for intraplate events as well as a criterion for forecasting a rupture zone of an impending large interplate earthquake. This may be of significance for instrumentation to detect various kind of phenomena associated with imminent intraplate and/or interplate earthquakes.
