2019 Volume 128 Issue 3 Pages 377-389
The Mj 6.4 Eastern Shizuoka earthquake occurred on March 15, 2011, which was 4 days after the 2011 off the Pacific coast of Tohoku Earthquake (MW 9.0). The hypocenter of the earthquake was approximately 6 km southwest of Mount Fuji's peak at a subsurface depth of about 15 km. There had not been an earthquake with a magnitude of 6 or higher in the region for the last 80 years, and many experts were concerned that the Eastern Shizuoka earthquake increased the probability of a Mount Fuji eruption. Estimating spatial variations of the b-value of the Gutenberg–Richter relation is important in examining time and spatial changes of volcanic and seismic activity after large earthquakes. Changes in seismic activity after the 2011 Eastern Shizuoka earthquake are evaluated using spatial variations of the b-value. Hypocentral data from July 2001 to December 2017 were obtained from the JMA catalog. Only data for earthquakes with focal depths of 25 km or less were selected for the investigation. As a result, the b-values for earthquakes around Mount Fuji were significantly smaller during the period after the 2011 Eastern Shizuoka earthquake than during the period from July 2001 through February 2011, and continued to gradually recover after the Eastern Shizuoka earthquake. Furthermore, in the source region of the northeastern part of Mount Fuji, the b-value increases from the southwest toward the northeast, and the spatial variations of the b-values may coincide with the density distribution of volcanic fluid.