Hypocenter, Fault Plane, and Rupture Characterization of Australian Earthquakes: Application to the September 2021 Mw 5.9 Woods Point Earthquake

Abstract

The Australian Seismometers in Schools (AuSIS) network operates 50 broadband seismic stations across Australia that are hosted at schools. The instruments augment the Australian National Seismograph Network providing valuable data from urban and regional Australia. The network coverage is quite sparse, but these vital records of rare, moderate Australian earthquakes can improve our understanding of the deformation within the stable continental region of Australia, especially for events with no surface rupture. In this study, we present the feasibility of identifying the fault plane of moderate earthquakes on the Australian continent, using data from the AuSIS network. We examine the fault plane of the September 2021 Mw 5.9 Woods Point earthquake that occurred about 130 km northeast of the Melbourne metropolitan area. We estimate the hypocenter and the centroid moment tensor (CMT) to identify the fault plane from the auxiliary plane in the focal mechanism. We explore a range of 1D models and a 3D Earth model to simulate seismic arrivals and full waveform data. The hypocenter is resolved using P- and S-wave arrivals in a probabilistic framework and the CMT is derived from full waveform modeling through grid search over a set of trial points around the hypocenter. Our solution suggests the mainshock ruptured the depth of 15 ± 4 km, with a strike-slip mechanism striking 348° north on a nearly vertical plane. The high double-couple percentage of this event indicates a simple rupture that propagated from the south (hypocenter) toward the north (centroid) and remained subsurface. This indicates that the causative fault had a deeper structure than the previously known shallow, northwest–southeast-striking faults of the region. The P and T axes deduced from our fault model are notably aligned with the maximum horizontal crustal stress in the region.