Deep Subsurface S-wave Structure of Okinawa Islands, Japan

Abstract

Okinawa Islands, southwestern Japan, have often been damaged by strong ground motion during some M7 class earthquakes, and there are high risks of the disaster in future earthquakes around these islands. We conducted array observation of microtremor survey at 23 sites on the islands and modelled one-dimensional S-wave velocity structure for each site with the inversion of estimated phase velocity. The S-wave velocity structures are modeled with one to three layers overlying a seismic bedrock, velocity layer over 3 km/s. Depths of the bedrock are from 0.2 to 0.4 km at the sites of the northern part of Okinawa island and Kume island, whereas these depths are from 2.0 to 2.5 km of the southern part of Okinawa one. Based on these one-dimensional structures, in case of four layers model, the averaged of S-wave velocities over all sites are 0.69, 1.10, 2.01 and 3.46 km/s, respectively, which are similar to those of the Sakishima islands. It suggests the feasibility of construction of subsurface 3D numerical model over all the Nansei Islands.