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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 (Vsahs) 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 Vsahs 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.
Highlights
The growth of the global population over the past century, combined with the accelerating pace of urbanization, has resulted in the explosive growth in the number of megacities
Since the true Vs profile in the basin is not that of a half-space but has Vs increasing with depth, our estimates of “near-surface” Vs are an estimate of apparent half-space Vs, which we denote as Vs ahs in what follows
The azimuth is 230◦ and the epicentral distance is 231 km. (a) Time windowing used for principal azimuth is 230° and the epicentral distance is 231 km. (a) Time windowing used for principal component analysis (PCA), with window lengths of 1, 2, and 4 seconds indicated by red, green, component analysis (PCA), with window lengths of 1, 2, and 4 seconds indicated by red, green, and and yellow shades, respectively. (b) Distribution of P-wave polarizations calculated from different yellow shades, respectively. (b) Distribution of P-wave polarizations calculated from different lengths lengths of the time window, in increments of 0.1 s
Summary
The growth of the global population over the past century, combined with the accelerating pace of urbanization, has resulted in the explosive growth in the number of megacities (population over million). Geosciences 2019, 9, 386 cause amplification and resonance of seismic wave motion The observation of such effects in recent destructive earthquakes—the 2015 Kathmandu [2] and 2017 Mexico City earthquakes [3]—imparts some urgency to the evaluation of basin effects in other megacities. These are often too expensive to cover the entire area of a large city Other approaches exist such as seismic refraction or reflection surveys, but these use active sources like explosives and require large, regularly-spaced sensor arrays that are impractical in built-up urban areas. It is possible to estimate near-surface Vs and Vp by observing the polarization directions of P-waves and S-waves generated by earthquakes at a seismic station This approach requires no artificial source or other expensive equipment, and it can be applied with a minimum of computational effort. The results are benchmarked against borehole data and shallow Vs estimated from other studies
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