Keywords:S-wave velocity structure, phase velocity, northern marginal fault zone of the Saga Plain, ambient noise
We conducted microtremor array surveys to estimate S-wave velocity structure beneath the Saga Plain and Shiroishi Plain, Saga prefecture, Kyushu, Japan. Previous microtremor arrays performed along a P-wave reflection survey line have shown that the basement depth of the Saga Plain changes at the northern marginal fault zone of the Saga Plain and increases toward the Ariake Sea up to 1 km depth in the coast (Yoshimi et al., 2019, Yoshimi & Shigefuji, 2019) . On the November 2019, we performed microtremor array observation in the western part of the Saga plain and whole Shiroishi plain at eight sites: in the center of Saga city, southern part of Saga city, western part of Saga city, Ogi city, and four places in Shiroishi town (including KiK-net station SAGH05). Eight set of three-component velocity microtremor (Tokyo Sokushin SE-321, natural period 10 seconds) were used for the observation. To cover wider range of the wavelength, five to six arrays with different radius were set for each site. The maximum radius ranges 510m to 1100m. Vertical component of the microtremor data are analyzed with the SPAC method (Okada, 2003) to obtain phase velocity. By averaging phase velocities of every triangle array, unified phase velocities of Rayleigh wave are obtained for each site. Then, 1D S-wave velocity (Vs) structure of each array site was estimated from the observed phase velocity with the inversion technique using a GA method. We found that the velocity structure of the western part of the Saga plain is similar to that of the central part. Details will be shown in the presentation.