A geophysical investigation of the 2018 Lake Muir earthquake sequence: reactivated Precambrian structures controlling modern seismicity

Seismicity in the intraplate southwest of Western Australia is poorly understood, despite evidence for potentially damaging earthquakes of magnitude > M6. Identifying stress-focusing geological structures near significant earthquake sequences assists in understanding why these earthquakes occur in seemingly random locations across a region of more than 250 000 km(2). On 16 September 2018, an M(L)5.7 earthquake occurred near Lake Muir in the southwest of Western Australia and was followed by an M(L)5.4 aftershock. The main earthquake formed a mainly north-trending fault scarp similar to 5 km in length and with a maximum vertical displacement of similar to 40 cm. The main event was followed by a series of aftershocks, one of which had a magnitude of M(L)5.4. Using high-resolution aeromagnetic data, we analyse bedrock geology in a wide area surrounding the new scarp and map a series of major similar to east-west-trending faults segmenting eight distinct geological domains, as well as a network of less prominent northwest-trending faults, one of which aligns with the southern segment of the scarp. Surface faulting, surface deformation and earthquake focal mechanism studies suggest movements on north- and northeast-trending structures. The main shock, the aftershocks, surface faulting and changes in InSAR-derived surface elevation all occur in a region bounded to the south by a prominent northwest-trending fault and to the north by a west-northwest-trending domain-bounding structure. Thus, we interpret the north-trending thrust fault associated with the main Lake Muir event as due to local stress concentration of the regional east-west stress field at the intersection of these structures. Further, we propose that a particularly large west-northwest-trending structure may be broadly focusing stress in the Lake Muir area. These findings encourage similar studies to be undertaken in other areas of Australia's southwest to further the current understanding of seismic release in the region.

Automated summary

Seismic activity in the intraplate southwest of Western Australia is not well understood, but studying geological structures can deepen the understanding of the causes of these earthquakes.