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

Five new seismic monitoring stations have been added to the Australian National Seismograph Network (ANSN) in Western Australia with the aim of better locating the small earthquakes of the Southwest Seismic Zone of southwest Western Australia. The Southwest Seismic Zone, one of the most seismically active onshore areas in Australia, is an area known for its seismicity by the local people who regularly experience small cupboard rattlers. These often occur in swarms as documented by Dent (e.g. 2009, 2014, 2017, 2019, 2020). It is more widely known for the M6.5 Meckering earthquake of 1968 which destroyed many houses in Meckering, produced a 37 km long and 2 m high fault scarp and dramatically bent the railway line. There is no obvious tectonic reason for PREVIEW AUGUST 2021 earthquakes to be occurring in this area as it is in the middle of an Archean cratonic region although the scarps from surface rupturing events, such as the Meckering event, mainly trend approximately north-south and appear to lie along trends in the magnetic data (Dentith et al 2009). Until mid-2020 the ANSN had a fairly sparse coverage of the area. A secondary Public Seismic Network (PSN) is also monitoring in the area. Inclusion of the PSN data has quite an effect on the calculated locations, as exampled by the tighter clustering of the Beacon swarm of 2009 (Dent, 2009) rather than the linear trend from just using the ANSN data. In mid-2020 four more semi-permanent stations were installed in the area and one in the Goldfields and added to the data streaming into the National Earthquake Alerts Centre (NEAC) and incorporated into the location algorithm. This presentation looks at seismicity of the area, the small swarms that have been detected since then and the relationship of the seismicity to the geology. Presented at the 2021 Australasian Exploration Geoscience Conference (AEGC)