CUTOFF DEPTH OF SEISMICITY AND LARGE EARTHQUAKES NEAR ACTIVE VOLCANOS IN JAPAN

Cutoff depths of seismicity in the earth's crust change from place to place over a range of 5-25 km. These variations are closely related to the values of the surface heat flow. In order to examine regional variations in greater detail, available cross sections of well-determined focal depths have been collected for five areas located near active volcanoes, where the thermal structure is considered to change abruptly. The seismic-aseismic boundary is located at depths of about 5-8 km immediately below the craters, with increasing depth towards the flanks of the volcanoes. The boundary is found at depths of 10-15 km at distances of about 10 km from the center of the volcanoes. Further, an examination was performed of the magnitudes of earthquakes versus the distances from the craters for seven volcanoes, near which large earthquakes (M greater-than-or-equal-to 6) occurred during the past 30 years. All of the large earthquakes were found to have occurred at distances of approximately 10 km or greater from the center of the volcanoes. Moreover, the distances between active volcanoes and earthquakes were calculated for all large intra-plate earthquakes in Japan that have occurred during the period 1885-1990. It was found that all the large earthquakes occurred at distances of more than 10 km from active volcanoes. These facts suggest that the seismogenic layer near the center of an active volcano is thin and not strong enough to accumulate sufficient stress for the occurrence of large earthquakes.