地震発生前の地震活動と地震予知

Abstract

The Izuhanto-oki earthquake of May 9, 1974 (M6.9) was preceded by an anomalous seismicity as shown Fig. 1. The duration from the anomalous Seismicity to the mainshock was about 10 years and 4 months. Similar phenomena were also found for 10 events which occurred at shallow depths in Japan.Considering that these data show a linear relation between the magnitude of earthquakes M and duration log T (day), the author calculated the coefficient of logT=bM+a by using the method of least squares as follows: logT=bM+a=0.77M-1.65The relation between the above formula and that of TSUBOKAWA (1969), RIKITAKE (1975) and SCHOLZ et al. (1973), which indicate with a certain accuracy the magnitude and time of an expected earthquake was given by Fig. 7.The author believes that the anomalous seismicity preceding earthquakes is one of the potential parameters which could be employed for earthquake prediction. On the other hand, in some certain fracture zones were found a premonitory change before some events. These phenomena generally are called foreshocks. Anomalous seismicity preceding smaller events cannot clearly be found by data of the Japan Meteorological Agency as shown Fig. 8. It is, however, not certain yet whether such earthquakes were not really preceded by anomalous seismicity.Our study requires further accumulation of examples from the viewpoint of ascertaining the existence of anomalous seismicity preceding earthquakes by observation network for small earthquakes.