Shallow shear-wave velocity beneath jakarta, indonesia revealed by body-wave polarization analysis

Abstract

Noting the importance of evaluating near-surface geology in earthquake risk assessment, we explored the application to the Jakarta Basin of a relatively new and simple technique to map shallow seismic structure using body-wave polarization. The polarization directions of P-waves are sensitive to shear-wave velocities (Vs), while those of S-waves are sensitive to both body-wave velocities. Two dense, temporary broadband seismic networks covering Jakarta city and its vicinity were operated for several months, firstly, from October 2013 to February 2014 consisting of 96 stations, and secondly, between April and October 2018 consisting of 143 stations. By applying the polarization technique to earthquake signals recorded during these deployments, the apparent half-space shear-wave velocity (Vs ahs) beneath each station is obtained, providing spatially dense coverage of the sedimentary deposits and the edge of the basin. The results showed that spatial variations in Vs ahs obtained from polarization analysis are compatible with previous studies, and appear to reflect the average Vs of the top 150 m. The low Vs that characterizes sedimentary deposits dominates most of the area of Jakarta, and mainly reaches the outer part of its administrative margin to the southwest, more than 10 km away. Further study is required to obtain a complete geometry of the Jakarta Basin. In agreement with previous studies, we found that the polarization technique was indeed a simple and effective method for estimating near-surface Vs that can be implemented at very low-cost wherever three-component seismometers are operated, and it provides an alternative to the use of borehole and active source surveys for such measurements. However, we also found that for deep basins such as Jakarta, care must be taken in choosing window lengths to avoid contamination of basement converted phases.