Abstract
The anomaly in focus is relative quiescence which is defined as a significant decrease of earthquake activity compared with the predicted occurrence rate by the estimated statistical model for the standard seismicity of the region: we use a point-process model called Epidemic Type Aftershock-Sequences (ETAS) model. Regardless of the seismicity level, the relative quiescence can take place. Size of such quiescence is seen in cumulative and M-T diagrams of transformed occurrence times by using the estimated ETAS model. By this method, we recognized significant, relatively quiet stages in shallow seismicity over M 5 class in a very wide area preceding all studied great earthquakes of M 8 class in and around Japan and also those of M 9 class in the world. However, we saw no relative quiescence for about 20 years up to 1990 in the wide areas which include the Tokai and Boso gaps [OGATA (1992)]. On the other hand, a few authors recently showed a very significant quiescence for the last 20 years since early or mid 1970's in the seismicity of certain areas in or around Tokai region. However, it turns out that these quiescence are seeming ones owing to magnitude shifts which took place during 1975?76. The magnitudes below MJ5.0 are substantially underestimated after the period. This shift is found and estimated by a statistical comparison of magnitudes between the JMA and USGS catalogs. Nevertheless, the recent seismicity with level of MJ≥5.5 in a very wide region including central and western Japan shows a significant relative quiescence since 1992, which may be related to the 1995 Kobe Earthquake of MJ7.2 but seems to be further extended to late 1996 when many M 6 class events successively occurred about the boundaries of the quiet region within a half year span.
