Microtremor exploration for shallow S-wave velocity structure in Bandung Basin, Indonesia

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.