Imaging Site and Basin Structure beneath Greater Jakarta, Indonesia and its Implications for Earthquake-Induced Resonance
Creators: name: Ry, Rexha
Publisher: name: The Australian National University
Year: 2023

description: Greater Jakarta, Indonesia's capital city and its metropolitan area (Depok, Tangerang, Bekasi, and Bogor), is inhabited by more than 31 million residents. The high population density in an area of very active tectonics that experiences a high level of earthquake activity makes the region a hotspot of seismic risk. This problem is made even more acute by (1) standards of residential construction in Java and elsewhere in Indonesia that do not prioritize earthquake resilience; and (2) Jakarta, like many other cities in Indonesia, resides on a sedimentary basin filled with geologically young sediments that may lead to amplification and resonance of seismic waves when subject to strong earthquake ground motion. My Ph.D. thesis research attempts to address the latter problem by improving our understanding of the seismic velocity structure of the Jakarta Basin. It is hoped that this will enable simulations of earthquake ground motion that accurately account for basin effects and thereby improve our understanding of the potential impacts of future earthquakes. This thesis aims to develop a new 3D model of the Jakarta Basin's shear-wave velocity (Vs) architecture based on several measures of earthquake-generated waves and ambient seismic noise. To complete this, a new temporary seismic network was deployed in Greater Jakarta to extend the spatial coverage of a previous deployment that was limited within Jakarta's administrative boundary. Three different approaches to estimating the Vs structure of the Jakarta Basin were explored: (1) using earthquake-generated P-wave polarization to estimate an apparent half-space velocity that should reflect the average Vs of the top 150 meters; (2) phase velocity inversion of Rayleigh wave Green's functions obtained from ambient noise, which can potentially yield much better resolution in the spatial variation of Vs both horizontally and with depth, and; (3) joint inversion of the phase velocity with Rayleigh wave ellipticity data, which can improve estimates of Vs in near-surface layers. The latter two approaches make extensive use of trans-dimensional inversion to allow the data itself to determine details in the model without the need for regularisation. The new knowledge about the 3D structure of the Jakarta basin obtained as a result of my thesis research includes: (1) a clear indication of how far the basin extends southward beyond the city limits of Jakarta, which was the limit of the previous array deployment; (2) details of the internal layering of the sediment fill of the Jakarta Basin, which appears to be in excellent agreement with the limited amount of borehole data; (3) estimates of the low Vs and the thickness of the Pleistocene-Holocene sediments that form the shallowmost layer of the Jakarta Basin fill, which should be important for constraining the amplification of seismic waves, and ; (4) tentative evidence that the Baribis Fault, or part of a similar back-arc thrust system, extends beneath the southern edge of the Jakarta Basin, deforming the boundary between mid- and late-Miocene sediments, and possibly layers of more shallow, younger sediments. To conclude, I believe the 3D model of the Jakarta basin presented in this thesis is suitable for scenario ground motion simulations to appraise the importance of basin amplification and resonance on earthquake risk in Greater Jakarta.