Estimation of Underground Structure at Higashinada Ward, Kobe Using Surface Wave of Microtremor and Explosion Record

Seismic ground motion at a site is strongly affected by the underground structure, especially the S-wave velocity structure below. During the 1995 Hyogo-ken Nanbu earthquake, it is found that the basin structure with the thickness of about 1km played a major role to form the damage belt in Kobe. We estimate the S-wave velocity structure at Higashinada ward, Kobe where the damage belt runs through from west to east. We deployed two different sized arrays in southeastern part of Higashinada ward. The smaller array, named KUMM array, has a diameter of 200 meters, while the larger array, named M-array, has a diameter of 2000 meters. We recorded microtremors at each array with ten three-components velocity sensors simultaneously. Independently, a seismic refraction experiment was carried out on December 12 and 14, 1995 in Hyogo and Osaka prefectures. We also recorded seismic waves from the explosion in the Osaka port by using the same KUMM array as the microtremor observation. Phase velocities of Rayleigh waves not only for microtremors but also for explosion-induced seismic waves are used to estimate the S-wave structure. Estimated S-wave velocity structure is very close to the PS logging data which is obtained at a site close to the array. Seismic refraction survey using explosions is often carried out to estimate an underground structure, in which only initial motions are used and later arrivals are considered to be noise. In this paper, we show a possibility to use phase velocities of Rayleigh wave, which is induced by an explosion, just as the case of microtremors.