Geological constraints on Mmax values from Western Australia: Implications for seismic hazard assessments

Beau B. Whitney and James V. Hengesh

Although Western Australia is commonly viewed as a Stable Continental Region with low rates of earthquake activity, geological and geomorphological evidence indicates that active tectonic processes are occurring. A zone of intraplate transpressional shear (the Western Australia Transpressional Belt—WATB), extends southwest from near Savu Island, Indonesia across the North West Shelf through the Cape Range and the Gascoyne alluvial plain to the Mt. Narryer fault zone (~-27.5˚S). The North West Shelf is accommodating crustal flexure and shallow faulting due to the collision with the Banda Arc, the central west coast exhibits evidence of active fold growth since Marine Oxygen Isotope Stage (MIS) 5e and the Murchison region has evidence of Quaternary tectonic deformation and a record of two large magnitude historical earthquakes.

Geological data to constrain maximum earthquake magnitude (Mmax) estimates for individual seismogenic sources within Stable Continental Regions (SCRs) are lacking. However, within the Stable Continental Region (SCR) of Western Australia, individual morphotectonic structures exist that can be parameterized for inclusion in seismic hazard analyses. Recent studies of the WATB have identified a number of morphotectonics structures that provide structure specific seismic source data. Reverse reactivation of normal faults is manifest as surface expression of faultpropagation folds in the Murchison region. These individual blind-reverse faults are capable of generating earthquakes in the range of Mw6.4-7.4, which is consistent with the largest historically observed earthquake in the region, the 1941 Mw 7.1 Meeberrie event. The rupture of combined fault segments could push these Mmax values up to Mw 7.4-7.7, which would be similar to the 2001 Mw 7.7 Bhuj India event. Ongoing characterization of these features within the WATB has implications for probabilistic seismic hazard analyses (PSHA) in the region. Where these types of structures exist, Mmax estimates may differ from the Mmax values previously suggested in the Brown and Gibson (2004) and Clark et al. (2011) domain models.