Microtremor exploration for shallow S-wave velocity structure in Bandung Basin, Indonesia
Pramatadie A.M.a, Yamanaka H.a, Chimoto K.a, Afnimarb, Koketsu K.c, Sakaue M.c, Miyake H.b, Sengara I.W.b, Sadisun I.A.b
a Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Midori-ku, Yokohama, Kanagawa, 226-8502, Japan
b Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Bandung, 40132, Indonesia
c Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo, 113-0032, Japan
Abstract
© 2017 ASEG.We have conducted a microtremor survey for shallow S-wave velocity profiles to be used for seismic hazard evaluation in the Bandung Basin, Indonesia. In the survey, two arrays were deployed temporarily at each of 29 sites, by installing seven vertical sensors in triangular configurations with side lengths from 1 to 16m. Records of vertical microtremors from each array were used to estimate Rayleigh wave phase velocity spectra using the spatial autocorrelation method, as well as the horizontal-to-vertical spectral ratio obtained at the centre of the arrays. Phase velocities at sites on the basin margin exhibit higher values than those obtained in the central part of the basin, in a frequency range of 7 to 30Hz. The phase velocity data were used to deduce S-wave velocity profiles of shallow soil using a hybrid heuristic inversion method. We validated our inversion models by comparing observed horizontal-to-vertical spectral ratios with ellipticities of the fundamental mode of Rayleigh waves, calculated for the inversion models. The S-wave velocity profiles in the area can be characterised by two soft layers over a firm engineering basement that has an S-wave velocity of 500m/s. The S-wave velocities of the two layers are 120 and 280m/s on average. The distribution of the averaged S-wave velocity in the top 30m clearly indicates low values in the eastern central part and high values in the edge of the basin. The amplification is large in the areas with low velocity layers. In addition, we have proposed an empirical relation between the amplification factor and the topographical slope in the area.
Author keywords
Bandung Basin,Horizontal-to-vertical spectral ratios,Microtremors,Rayleigh-wave phase velocity,S-wave velocity,S-wave velocity structure,Shallow soils,Spatial autocorrelation methods
Indexed keywords
amplification,Bandung Basin,microtremor exploration,phase velocity,S-wave velocity,shallow soil
Funding details
The authors thank two anonymous reviewers and Prof. Nobuyuki Yamada as an editor for their careful comment on the manuscript and their suggestions. We also thank to Lindsay Thomas for careful reading and comments to improve the manuscript. A part of this study was supported by Science and Technology Research Partnership for Sustainable Development (SATREPS) in the project theme ‘Multi-disciplinary Hazard Reduction from Earthquakes and Volcanoes in Indonesia’. We also thank the graduate students at Tokyo Institute of Technology and Institut Teknologi Bandung for the field surveys. We acknowledge financial support from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan. Some of the figures in this paper were made using GMT software (Wessel and Smith, 1991).