Space-time pattern of large earthquakes occurring off the Pacific coast of the Japanese Islands.

Abstract

The regions of intense seismic activity off the Pacific coast of Japan are classified into several types according to the pattern of occurrence of large earthquakes (M≥6) in space, time, and magnitude. In type A regions, great earthquakes occur at fairly regular intervals. In type B regions, large earthquakes of magnitude up to about 7.5 tend to occur in swarms. In type A' regions, large earthquakes of magnitude up to about 7.5 occur sporadically. There are regions labeled as type AB, A'B, A(B), (A)B, etc. Type AB means a mixed type of A and B. Type A(B) represents a type A with slight indication of the character of type B.Investigations of the pattern of occurrence in regions of each type provide useful information for the statistical prediction of large earthquakes. For great earthquakes in type A regions, foreshock activities are usually weak, but the identification of foreshocks, if they are observed, may be easier, because a remarkable seismicity gap is developed before a great earthquake. In regions of types B, (A)B, and AB, the absence of a seismicity gap for large earthquakes does not assure the non-occurrence of a large earthquake in near future. The probability of the successive occurrence of large earthquakes is high after the occurrence of large earthquake in regions of types B and (A)B. A seismic gap for great earthquakes found in regions of types A' A'B, and B is not an indication of a future great earthquake. Swarms in regions of types B, (A)B, and A'B usually have low b-values. Therefore a low b-value is not a criterion for foreshocks in these regions. A remarkable increase in number of destructive earthquakes in the land area of Japan on the inner side of the source regions of great earthquakes along the Nankai trough is recognized during a 50-year period prior to the occurrence of each great earthquake.