Development of an engineering bedrock map beneath Jakarta based on microtremor array measurements

Abstract

Abstract Jakarta has been selected as a study area for seismic microzonation by considering its population and its infrastructure growth, seismicity and geological setting. One of the important factors in seismic hazard analysis is the characteristics of the ground overlying bedrock, and, as yet, this has not been studied for Jakarta. This study was intended to estimate the depth to bedrock by applying microtremor array exploration. The phase velocity of microtremors was estimated by the spatial autocorrelation (SPAC) method, whereas the S-wave velocity profile was estimated by inversion using a genetic algorithm. Microtremor array analysis has been conducted at 55 sites that covered the whole of Jakarta. The result of one-dimensional (1D) S-wave velocity profile estimation indicates that the engineering bedrock depth has pronounced differences between northern (>700 m) and southern Jakarta (about 300 m). A 3D S-wave velocity model was constructed from 1D profiles resulting from a second inversion in which the depth of constant-velocity layers, with velocities of 500, 700 and 900 m s −1 , was determined. The constructed 3D velocity structures show a bedrock morphology that has a depth range of 350–725 m, with depths increasing towards northern Jakarta. The implication of differences in bedrock depth is that estimated seismic amplification for the thicker sediment will be higher.