The 2018 Lake Muir earthquake sequence, southwest Western Australia: rethinking the relationship between magnitude and surface rupture length for Australian stable continental region earthquakes

Modern geodetic and seismic monitoring tools are enabling the study of moderate-sized earthquake sequences in unprecedented detail. Discrepancies are apparent between the surface deformation envelopes ‘detectable’ using these tools, and ‘visible’ to traditional ground-based methods of observation. As an example, we compare the detectible and visible surface deformation caused by a sequence of earthquakes near Lake Muir in southwest Western Australia in 2018. A shallow MW 5.3 earthquake on the 16th of September 2018 was followed on the 8th of November 2018 by a MW 5.2 event in the same region. Focal mechanisms for the events suggest reverse and strike-slip rupture, respectively. Interferometric Synthetic Aperture Radar (InSAR) analysis of the events suggests that the ruptures are in part spatially coincident and deformed the Earth’s surface over ~ 12 km in an east-west direction and ~ 8 km in a north-south direction. Field mapping, guided by the InSAR results, reveals that the first event produced an approximately 3 km long and up to 0.5 m high west-facing surface rupture, consistent with slip on a moderately east-dipping fault. No surface deformation unique to the second event was identifiable on the ground. New rupture length versus magnitude scaling relationships developed for non-extended cratonic regions as part of this study allow for the distinction between ‘visible’ surface rupture lengths (VSRL) from field-mapping and ‘detectable’ surface rupture lengths (DSRL) from remote sensing techniques such as InSAR, and suggest longer ruptures for a given magnitude than implied by commonly used scaling relationships.