Structural controls on localized intraplate deformation and seismicity in Southern Australia: Insights from local earthquake tomography of the Flinders Ranges

Abstract

Data from an array of 24 seismometers are used to image the crust beneath the Flinders Ranges, southeast Australia, with the goal of improving our understanding of crustal structure, rheology, and the mechanism responsible for the localized intraplate deformation that characterizes this region. A subset of P‐ and S‐wave traveltimes is inverted to jointly recover earthquake hypocenters, P‐wave velocity structure and vp/vs anomalies. The P‐wave velocity model reveals a spatial correlation between major negative velocity perturbations and concentrations of seismicity. In particular, a cluster of seismicity is observed within a distinct low velocity region between the Archean‐Mesoproterozoic Gawler Craton and the Palaeo‐Mesoproterozoic Curnamona Province, from 7 to 20 km depth. We postulate that this may be associated with a pre‐existing structural weakness in the crust that arises primarily from rifting between the Curnamona Province and the Gawler Craton. Another area characterized by a high level of seismicity overlies a major sequence of N‐S trending Ross‐Delamerian thrust faults, which correspond to a band of low vp and particularly vp/vs. The lack of evidence for elevated heat flows in both of these seismogenic regions suggests that thermally induced weakness is unlikely to play a dominant role. Instead, the dynamic behavior of this intraplate region appears to be caused by a serendipitously oriented regional stress field, provided by far field forces that originate from the boundary between the Pacific and Australian plates, which acts upon preexisting structural weaknesses in the lithosphere.

Localization of intraplate deformation through fluid-assisted faulting in the lower-crust: The Flinders Ranges, South Australia

“…A previous study of seismic tomography by Pilia et al (2013) used data from the temporary deployment to show that a major change in crustal structure occurs in the central Flinders Ranges, involving a series of low v p (and low v p /v s ) anomalies aligned and elongated in the N-S direction that closely follow the axis of the Flinders Ranges.…”

“…For our study we used 427 events, which included arrival times from 2970 P-waves and 2809 S-waves (as opposed to 400, 2628, and 2370, respectively, used in Pilia et al, 2013). In contrast to Pilia et al (2013), who used only P-waves, we used both P-and S-arrivals for the earthquake relocations. Although this results in slightly larger residuals due to the inclusion of more data, it was important for better constraining the earthquake depths (see Supplementary material).…”

“…This study utilizes recordings of these earthquakes by a total of 27 stations, including both those of the temporary deployment and stations of the permanent networks in the region operated by GSSA and GA. All earthquakes in this dataset are quite small, with the largest earthquake having M L = 4.05. As discussed below, 65 focal mechanisms were determined from the dataset -a dramatic increase over the six Pilia et al (2013). Previous estimates of stress from Clark and Leonard (2003) and Hillis and Reynolds (2003) are shown.…”

“…We used exactly the same method as Pilia et al (2013), but expanded on the dataset and included all available P-and S-waves from both permanent and temporary network stations. For our study we used 427 events, which included arrival times from 2970 P-waves and 2809 S-waves (as opposed to 400, 2628, and 2370, respectively, used in Pilia et al, 2013).…”

Linking mainland Australia and Tasmania using ambient seismic noise tomography: Implications for the tectonic evolution of the east Gondwana margin

“…Therefore, the number, size, position and shape of the cells are implicitly controlled by the data, in which the noise is also treated as unknown in the inversion. This provides a parsimonious trade-off between data fit and model complexity (Bodin et al, 2012b), a more sophisticated pursuit than conventional ad hoc regularisation, predominantly utilised by linearised inversion techniques (Rawlinson et al, 2006;Saygin and Kennett, 2010;Pilia et al, 2013). The principal issue with locally linearised problems around a reference model is that, in general, there is no guarantee as to whether we appropriately tune the parameters (usually damping and smoothing), which can ultimately lead to misinterpretation (Bodin et al, 2012b).…”