Microtremor exploration for shallow S-wave velocity structure in Bandung Basin, Indonesia
Andi Muhamad Pramatadie 1 6 Hiroaki Yamanaka 1 Kosuke Chimoto 1 Afnimar 2 Kazuki Koketsu 3 Minoru Sakaue 3 Hiroe Miyake 3 I Wayan Sengara 4 Imam A. Sadisun 51 The Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8502, Japan.
2 Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Jalan Ganesha no. 10, Bandung 40132, Indonesia.
3 Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
4 Faculty of Civil and Environment Engineering, Institut Teknologi Bandung, Jalan Ganesha no. 10, Bandung 40132, Indonesia.
5 Faculty of Earth Science and Technology, Institut Teknologi Bandung, Jalan Ganesha no. 10, Bandung 40132, Indonesia.
6 Corresponding author. Email: pramatadie.a.aa@m.titech.ac.jp
Exploration Geophysics 48(4) 401-412 https://doi.org/10.1071/EG16043
Submitted: 12 April 2016 Accepted: 18 April 2016 Published: 31 May 2016
Abstract
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 16 m. 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 30 Hz. 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 500 m/s. The S-wave velocities of the two layers are 120 and 280 m/s on average. The distribution of the averaged S-wave velocity in the top 30 m 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.
Key words: amplification, Bandung Basin, microtremor exploration, phase velocity, S-wave velocity, shallow soil.
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