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 km2. On 16 September 2018, an ML5.7 earthquake occurred near Lake Muir in the southwest of Western Australia and was followed by an ML5.4 aftershock. The main earthquake formed a mainly northtrending fault scarp ~5 km in length and with a maximum vertical displacement of ~40 cm. The main event was followed by a series of aftershocks, one of which had a magnitude of ML5.4. Using high-resolution aeromagnetic data, we analyse bedrock geology in a wide area surrounding the new scarp and map a series of major 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. <b>Citation: </b>S. Standen, M. Dentith & D. Clark (2021) A geophysical investigation of the 2018 Lake Muir earthquake sequence: reactivated Precambrian structures controlling modern seismicity,<i>Australian Journal of Earth Sciences</i>, 68:5, 717-730, DOI: 10.1080/08120099.2021.1848924

A shallow MW 5.3 earthquake near Lake Muir in southwest Western Australia on the 16 September 2018 was followed on the 8 November by a co-located MW 5.2 event in the same region. Sentinel-1 synthetic aperture radar interferograms (InSAR) allowed for the timely identification and mapping of the surface deformation relating to both earthquakes. Field mapping, guided by the InSAR observations, revealed that the first event produced an approximately 3 km-long and up to 0.4 m-high west-facing surface rupture. Five seismic rapid deployment kits (RDKs) were installed in the epicentral region within three days of the 16 September event. These data, telemetered to Geoscience Australia’s National Earthquake Alerts Centre, have enabled the detection and location of more than 750 dependent events up to ML 4.6. Preliminary joint hypocentre relocation of aftershocks using data from RDKs confirms an easterly dipping rupture plane for the first MW 5.3 event. The main shocks were recorded throughout the Australian National Seismic Network, in addition to a local broadband network in the Perth Basin operated by University of Texas at Dallas and the University of Western Australia. These data indicate large long-period ground-motions due to Rg phases and basin amplification. The two main shocks were widely felt within the region, including the Perth metro region (300 km away), with over 2400 online felt reports for the 8 November event. The Lake Muir sequence represents the ninth recorded surface rupturing earthquake in Australia in the past 50 years. All of these events have occurred in the Precambrian cratonic terranes of western and central Australia, in unanticipated locations. Paleoseismic studies of these ruptures found no evidence for regular recurrence of large events on the underlying faults. The events might therefore be considered “one-offs” at timescales of significance to typical probabilistic seismic hazard studies. Presented at 2019 Seismological Society of America Conference, Seattle in the special session on “Central and Eastern North America and Intraplate Regions Worldwide”

This poster presents a summary of discussion topics following the 2018 Lake Muir, WA, Earthquake Sequence Community Engagement Workshop held in Frankland River, WA, on 28 November 2018