Estimation of Rupture Direction and Frequency-Dependent Rupture Directivity Effect of Northwestern Chiba Prefecture Earthquakes

Abstract

Rupture directions of 17 northwestern Chiba prefecture earthquakes with Mw3.8-5.9 are estimated assuming the unilateral rupture using azimuth-dependency of apparent source spectral ratios. Rupture directions of at least near 1/3 of earthquakes have frequency-dependency. This result can be interpreted that different-sized strong motion generation areas are located at the different directions from the hypocenter. The largest two repeating earthquakes with Mw5.9 are estimated to have the same stress drop, but they ruptured to different direction each other and the directions have frequency-dependency. Ground motions within 100 km of most of earthquakes have rupture directivity effects up to 10Hz.