Site and Basin Effects on Seismic Hazard in Indonesia:Sulawesi and Jakarta Case Studies

Abstract

Earthquakes are among the most costly, devastating and deadly natural hazards. The extent of the seismic hazard is often influenced by factors like the source location and site characteristics, while the susceptibility of assets is influenced by the population density, building design, infrastructure and urban planning. A comprehensive knowledge of the nature of source and local geology enables the establishment of an effective urban planning that takes into account the potential seismic hazard, which in turn may reduce the degree of vulnerability. The first probabilistic seismic hazard assessment (PSHA) incorporating the effects of local site characteristic for the island of Sulawesi in Indonesia has been conducted. Most of the island, with the exception of South Sulawesi, is undergoing rapid deformation. This leads to high hazard in most regions (such that PGA > 0.4g at 500 year return period including site effects) and extremely high hazard (like PGA > 0.8 g at 500 year return period) along fast-slipping crustal fault. On the other hand, a distant site relative to fault might suffer higher ground motion if that site is composed of soft soil. This research has proven that incorporating near-surface physical properties, in this case is represented by VS30, surface geology contribute significantly to ground motions, consequently, responsible for potential building damage. The PSHA study that took place in Sulawesi took us move further, investigate the effect of deep structure on seismic waves. Jakarta was chosen for its location sitting on less known deep sediment basin and economic and political importances. A dense portable-seismic-broadband network, comprising 96 stations, has been operated within four months covering the Jakarta. The seismic network sampled broadband seismic-noise mostly originating from ocean waves and anthropogenic activity. We used Horizontal-toVertical Spectral Ratio (HVSR) measurements of the ambient seismic noise to estimate the fundamental-mode Rayleigh wave ellipticity curves, which were used to infer the seismic velocity structure of the Jakarta Basin. By mapping and modeling the spatial variation of low-frequency (0.124{0.249 Hz) HVSR peaks, this study reveals variations in the depth to the Miocene basement. To map these velocity profiles of unknown complexity, we employ a Transdimensional-Bayesian framework for the inversion of HVSR curves for 1D profiles of velocity and density beneath each station. The inverted velocity profiles show a sudden change of basement depth from 400 to 1350 m along N-S profile through the center of the city, with an otherwise gentle increase in basin depth from south to north. Seismic wave modelings are conducted afterward and shows that for very deep basin of Jakarta, available ground motion prediction equation (GMPE) is less sufficient in capturing the effect of basin geometry on seismic waves. Earrthquake scenario modeling using SPECFEM2D is performed to comprehend the effect of deep basin on ground motions. This modeling reveals that the city may experience high peak ground velocity (PGV) during large megathrust earthquake. The complexity of the basin is responsible for magnifying ground motions observed in the basin.