Large‐Scale Crustal Structure Beneath Singapore Using Receiver Functions From a Dense Urban Nodal Array.

Geophysics has a role to play in the development of "smart cities". This is particularly true for Singapore, one of the world's most densely populated countries. Imaging of Singapore's subsurface is required to identify geological faults, model shaking from future earthquakes, and provide a framework for underground development. A noninvasive geophysical technique that is well suited for urban areas is passive seismic surveys using compact seismic instruments called nodes. Here we image Singapore's crustal structure using receiver functions generated by a 40‐day deployment of a 88 station nodal array. We generate high resolution receiver functions, despite the noisy environment and short recording time and also create common‐conversion point images. Our results reveal a complex crustal structure, containing multiple discontinuities. Azimuthal variations indicate a distinct change in crustal structure on either side of the postulated Bukit Timah fault, which has implications for seismic hazard. Plain Language Summary: By 2050 over two thirds of the global population is expected to live in cities (United Nations, Department of Economic and Social Affairs, Population Division, 2019). Such dense population centers increase the exposure to nearby natural hazards such as earthquakes. Rapid urbanization also creates an ever pressing need to understand the subsurface of our cities, for example, for underground development. One technological advancement that allows us to image the subsurface of cities are passive seismic surveys using nodes. Nodes are small seismic instruments that can be deployed in urban areas such as schools and parks. We present results of a passive seismic survey using 88 nodes deployed in Singapore for 40 days. We generate high resolution receiver functions across the array, despite the short recording time, by using array stacking techniques. The dense nature of the array also allows continuous high frequency signals to be traced. Our results reveal that the structure of the crust is different on either side of the Bukit Timah fault, which agrees with geological information. Ancient faults in peninsula Malaysia have been reactivated in the recent past due to stress transfer from the Sumatran subduction zone; therefore, a geological fault under a densely populated urban center warrants further study for seismic hazard. Key Points: Robust receiver functions are generated from a temporary dense nodal array in a noisy urban environmentHigh frequency receiver functions reveal Singapore's crustal structure with direct implications for seismic hazard analysisAzimuthal variations reveal distinct crustal structure on either side of the Bukit Timah fault [ABSTRACT FROM AUTHOR]