Reappearance of Characteristic Seismicity Pattern in the Tokai and Tonankai Regions (Seismic Activity Change in the Tokai Region: Part 6)

Abstract

Changes of the seismicity pattern were examined in the Tokai and the Tonankai seismogenic regions and discussed with respect to possible crustal movements. Temporal transitions of the seismicity pattern were followed by calculating correlation coefficients between two spatial distribu.tions of earthquakes for the period of interest. I sampled M 3.5 and greater earthquakes from the JMA catalog. The catalog contains a problem of non-uniform observations, however the method utilized here is insensitive to this. Following the changes represented in the correlation coefficients, I found that the recent seismicity pattern is approaching the reference, which is precisely the pattern preceding the 1944 Tonankai earthquake. The tendency is common to both the Tokai and Tonankai regions. This implies that a similar crustal movement reappeared after a long hiatus of about sixty years in both regions. One possible cause of common changes in the Tokai and Tonankai regions is the long-term slow slip beneath Lake Hamana that has progressed during the last decade. If the cause of the recent changes in the seismicity pattern is attributed to this slow slip, the same slow slip can be interpreted as having progressed beneath Lake Hamana before the Tonankai event, which created a characteristic seismicity pattern in the past.