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Natural seismicity and tectonic geomorphology reveal regional transpressive strain in northwestern Australia

A temporary seismic network deployed in northwestern Australia between October 2005 and March 2008 recorded 28 earthquakes ranging in magnitude between M L 2.0 and M L 5.3. Twenty-three of these events occurred in areas underlain by continental crust previously affected by Mesozoic continental rifting (Carnarvon and Perth basins), with five occurring within the area underlain by the Precambrian craton. Focal mechanisms for 26 of the earthquakes show dominantly strike-slip solutions, regardless of the type of underlying crust (craton or extended margin). Geomorphological investigations in the Gascoyne region have identified several youthful fault scarps, especially along the Mt. Narryer fault zone in west-central Western Australia. This fault zone lies in the epicentral region of the 1941M L 6.8 to 7.3 Meeberrie earthquake, the largest historical earthquake to have affected Australia. The 120 km-long north-trending Mt Narryer fault zone comprises five left-stepping en echelon segments. The two southern segments display west-side-up reverse displacements across the Roderick and Sanford river alluvial channels, and have captured and diverted active stream flow, formed sag ponds, and impounded Lake Wooleen. The geomorphic expression of the northern three segments, and the en echelon stepping nature of the fault zone as a whole, suggests a component of strike-slip deformation, which is consistent with the sense of faulting determined from the focal mechanism solutions in our seismicity analyses. The higher proportion of events recorded along the extended continental margin compared with the craton suggests that the dense network of late Mesozoic rift-related faults along thecontinental margin may be reactivating and responding to far-field stress conditions. However, the presence of the Mt Narryer fault system indicates that structures within the craton also accommodate some of the regional stress and can be sources of future large magnitude earthquakes, such as the1941 Meeberrie event. The differences in activity rates between the extended margin and cratonic provinces may reflect the differences in crustal architecture, and the way that faults in these two different terranes respond to far-field stresses.