Interacting Intraplate Fault Systems in Australia: The 2012 Thorpdale, Victoria Seismic Sequences

Using a new seismic waveform data set, we locate 234 earthquakes and estimate source parameters (focal mechanisms, magnitude, and stress drop) of the two largest earthquakes (E1 and E2) in the 2012 seismic sequence in Thorpdale, Victoria. The focal mechanisms suggest thrust faulting, consistent with previous observations in southeast Australia. The estimated magnitudes are M-w 4.9 +/- 0.14 (E1) and 4.3 +/- 0.1 (E2). The estimates of stress drop 57 +/- 7.4 MPa (E1) and 28 +/- 2.4 MPa (E2) reflect strength of faults in an intraplate environment. By analyzing spatiotemporal distribution of aftershocks, we show that E1 and E2 reflect two separate seismic sequences about a month apart. E1 and E2 ruptured two adjacent faults with orientations 218 degrees/78 degrees/78 degrees (Fault 1) and 134 degrees/27 degrees/171 degrees (Fault 2), respectively, where angles indicate strike/dip/slip rake. To test causal mechanism of fault interaction, we performed Coulomb stress modeling, which shows very weak unloading of E2 by E1. Thus, we infer that fault interaction might instead reflect remotely triggered fluid diffusion. Also, a local rotation of the stress field proceeding E1 might have favorably reoriented Fault 2 for failure. Finally, we identify an apparent correlation between high heat flow and seismicity in southeastern Australia, suggesting a combination of mechanisms including transient stress perturbations and lithospheric thermal weakening associated with high heat flow as the principal factors localizing intraplate seismicity. Plain Language Summary The 2012 earthquake sequence in Thorpdale is the most recent and significant seismic activity in southeast Australia, during which two moderate-sized earthquakes were felt in the region with the second earthquake thought to be an aftershock of the first. Using a newly assembled data set, we estimate source parameters of these earthquakes and locate 232 smaller earthquakes associated with this seismic sequence. In this study, we show that faults ruptured with thrust motion, the moment magnitude of the two largest earthquakes are 4.9 and 4.3, and the stress release per fault length (57 and 28 MPa) is larger than the global average. By locating aftershocks, we also show that the two largest earthquakes ruptured two separate faults rather than one. The absence of any plausible Coulomb stress loading suggests that these different faults interacted with each other through other mechanisms such as a fluid diffusion.