Analysis of intraplate eathquake dynamics on the North West Shelf Australia: new strike slip focal mechanisms

The recent 6.6 Mww magnitude earthquake on the North West Shelf of Australia (14 July 2019), and its dozens of aftershocks, including three of magnitude 5 and above, highlights the need to increase our understanding of earthquake causes and implications in this region. The North West Shelf hosts one of Australia’s most prolific seismic provinces, with over 650 earthquake events recorded since 1970, 170 of them this century alone. Data reduction for earthquake events usually includes epicentre and depth determination, but focal mechanisms, a crucial aspect of tectonic determinations, remain significantly under-represented in seismicity studies for this area. Historically, poor data quality is due to sparse station spacing and large distances between offshore events and seismic recording stations. Prior to this study, results from 1979 to present show only eight published focal mechanism determinations for earthquakes on the North West Shelf for events greater than magnitude 5mb/ML. All showed strike-slip focal mechanism solutions. This paper presents eight new focal mechanism determinations, derived from earthquake events in the Browse and Canning basins. All eight solutions indicate strike-slip focal mechanisms. Our new contribution doubles the available focal mechanism data set for the region, and concludes that with present-day stress orientations the north-western part of Australia is experiencing strike-slip stress conditions, supporting the results of previous seismic and geomorphological studies.