SUBSURFACE STRUCTURES BASED ON MICROTREMOR OBSERVATIONS IN LANDSLIDE AREA OF TANDIKAT, WEST SUMATRA, INDONESIA

Abstract

The 2009 West Sumatra Earthquake (M7.5) caused landslides in Padang Pariaman, West Sumatra, Indonesia, and severe damage in Tandikat. Previous studies of the damage reported that the sliding surfaces are 2-m thick pumice layers above hard clay layers. In this study, we performed microtremor investigations at the landslide areas to estimate ground motion characteristics from horizontal to vertical (H/V) spectral ratios and subsurface structures from phase velocity dispersion curves of array measurements. The predominant periods of H/V spectral ratios are distributed in the range of 0.1–0.4 s. The H/V spectral ratios in the damaged area exhibit sharp unimodal peaks and the peak values in areas without landslides are small. These results may reflect the difference in the velocity structures between the areas with and without landslides during the 2009 earthquake. Hard subsurface structures are suggested in the areas without landslides. However, soft surface layers with an S-wave velocity of 200 m/s or less are estimated over the entire area, even in areas without landslides, by subsurface structure models derived from array data. The layers, including pumice layers, represent remnants of prior landslides. We are concerned about future coseismic landslides caused by the deposits, even in the areas that did not experience landslides during this event. We hope the results will contribute to landslide disaster prevention in the area.