The 1987 to 2019 Tennant Creek, Australia, earthquake sequence: a protracted intraplate multi-mainshock sequence

The 1987 to 2019 Tennant Creek earthquake sequence comprises three 1988 surface-rupturing mainshocks (moment magnitude (Mw 6.2, 6.3, and 6.5) that occurred within a 12-hour period, a preceding foreshock sequence commencing in 1987, and a prolonged aftershock sequence including a Mw 5.0 earthquake on the 1st August 2019. Each surface rupturing event produced a distinct scarp; the south-dipping Kunayungku scarp, north-dipping Lake Surprise west scarp and south-dipping Lake Surprise east scarp. Fault geometries were confirmed by trenches across the rupture traces, levelling surveys across the rupture traces, newly acquired satellite-derived high-resolution elevation data, and well-located aftershocks. Focal mechanisms and modelling using available seismic data support the hypothesis that the first mainshock ruptured the Kunayungku fault, the second mainshock ruptured the Lake Surprise west fault (and potentially rupturing across multiple other blind faults), and the third mainshock ruptured the Lake Surprise east fault. Trenching across all three ruptures found no evidence of prior rupture along the Lake Surprise east and Kunayungku faults. Potential evidence of prior rupture on the Lake Surprise west scarp has been reported. However, we consider this evidence to be circumstantial and to equally support an alternative interpretation; that the pre-1988 topography relates to a paleo-channel along underlying bedrock topography. Surface rupture locations and orientations are strongly aligned to underlying linear geophysical anomalies, suggesting strong control of bedrock structure on contemporary seismicity. Almost 31 years after the initial sequence, a Mw 5.0 aftershock was recorded near the western tip of the West Lake Surprise rupture. InSAR fault modelling suggests this occurred on a shallow blind fault (< 2 km depth to top of fault). This structure is also aligned with linear geophysical anomalies, providing further support that pre-existing basement structures are providing strong controls on the location and geometry of faulting in this intraplate stable continental region.