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

Abstract

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°/78°/78°(Fault 1) and 134°/27°/171°(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. Key Points:• The two largest earthquakes (M w 4.9 and 4.3) in the seismic sequence indicate thrust faulting in southeast Australia • Higher than average stress drop (57 and 28 MPa) suggests the rupture of strong faults with longer recurrence intervals • Adjacent fault planes ruptured, implying fault interaction through interplay between stress conditions and fluid diffusion Supporting Information:• Supporting Information S1

Lateral Variations in Lithospheric Mantle Structure Control the Location of Intracontinental Seismicity in Australia

“…So and Capitanio () found that the recurrence interval for faults embedded in a viscous plate was inversely proportional to the viscosity of the host plate. Our hypothesis is also consistent with observed correlations between crustal heat flow and seismicity in SE Australia (Attanayake et al, ; Holford et al, ), as a thinner lithosphere would result in higher crustal heat flow. Notably, one of the most active areas of seismicity in Australia, the Flinders range, does not occur within a high‐attenuation zone.…”

Holistic Seismic Monitoring of CCS with Active and Passive Data: A synthetic feasibility study based on Pelican site, Australia

“…Earthquakes are less likely to occur in the area as seismicity of the region was studied before selection of Pelican field as a storage site. A baseline passive seismic monitoring study (Attanayake et al, 2023) of the area revealed that no seismic activity was recorded within a 10 km radius of the site. Microseismic events are more likely to be recorded when a fluid is injected into an underground reservoir.…”

Slip Tendency Analysis From Sparse Stress and Satellite Data Using Physics‐Guided Deep Neural Networks

“…However, much lower friction values are suggested in studies of fault reactivation, often less than 0.3 and possibly lower than 0.1 (Middleton & Copley, 2013). For a general study in Australia, one might choose a default value of 0.4 (Attanayake et al., 2019). However, our analysis focuses on the relative influence of stress on fault behavior, eliminating the need for an additional constant term.…”