MODELING THE SHALLOW SUBSURFACE STRUCTURE IN THE URBAN AREA OF KOCHI CITY

Abstract

We have constructed a shallow underground structural model of Kochi City for an area 10.5 km east-west by 5.5 km south-north with a resolution of 125 m, which could control seismic amplification in the short period up to 1 s. By compiling the geological information given by the Committee of Kochi Geo-Hazard Evaluation (2011), we constructed a multilayered model overlying the engineering bedrock with an S-wave velocity of 700 m/s. Based on our newly developed model, the dispersion characteristics of theoretical surface waves at two sites were calculated and verified by comparison with observed data derived from our microtremor array experiments. The predominant periods along two north-south lines calculated from our model were coincident with previous observations derived from microtremor H/V spectral ratios by Mori et al. (2001). We also conducted a seismic response analysis using the synthesized ground motion data from the Central Disaster Management Council of Japan (2003) as the input motion applied to the basement of the developed subsurface structural model. We confirmed that the area where relatively high seismic intensity was predicted corresponded well with the area that was severely damaged during the 1946 Nankai Earthquake.