Seismicity and structure of the eastern Gawler Craton and Lake Eyre region
Date
2023
Authors
Agrawal, Shubham
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Kati Thanda-Lake Eyre represents the lowest point and the largest lake (when filled) in Australia. Located northeast of the Gawler Craton, the region is associated with pronounced intraplate seismic activity and thick sedimentary cover that potentially obscures the elusive boundary between older Proterozoic and younger Phanerozoic eastern Australia. Despite this, the area had limited seismic data coverage due to its remoteness and the harsh arid climate, creating a seismic coverage blindspot in the centre of the Australian continent. To address this, two seismic arrays were deployed from 2018 to 2022 - the Lake Eyre Basin array and AusArray-SA array, which provides the basis for this thesis.
One of the aims of this thesis is to understand the seismic signature of sedimentary layers within South Australia using receiver functions. Three-quarters of the Australian continent is covered by a blanket of sedimentary and regolith material, and estimates of sedimentary thickness are essential as the next frontier for mineral exploration in Australia is sediment-hosted minerals. However, active-seismic methods such as reflection imaging can be prohibitively expensive, particularly in remote areas. Therefore, to address these challenges, we utilize seismic stations in South Australia to determine basement depth based on the arrival time of the P-converted-to-S phase at the boundary between the crustal basement and sedimentary strata. By establishing a predictive relationship between Ps arrival time and basement depth using borehole data, we provide a way to obtain an initial estimate using relatively low-cost and portable seismic stations. Using the shallow structure information, the thesis investigates the Moho topography using two recent passive seismic deployments in Gawler Craton and Lake Eyre Basin, South Australia. However, more than half of the stations are located on low-velocity sedimentary rocks, leading to high-amplitude reverberations in the receiver function signal. We employ a resonance removal filter to reduce the sediment reverberation effect, which facilitates the detection of the P-to-S conversion from Moho, providing new insights into the crust of an unexplored region of South Australia. The study's approach to dealing with sediment-impacted receiver functions can serve as a valuable resource for future passive seismic experiments in Australia. Finally, the thesis focuses on understanding the occurrence of earthquakes in the Lake Eyre region. Intraplate seismicity can pose a significant risk as it is often non-periodic, poorly understood, and sporadically recorded by sparse seismic networks across vast continents. Within Australia, the distribution of intraplate seismicity is non-uniform but instead tends to concentrate along weak zones of increased activity, for example, the eastern margin of the Gawler Craton in South Australia. Over 140 new local events have been recorded that would otherwise have gone undetected by the national network. After relocation, the pattern of earthquakes becomes more spatially defined and appears closely tied to the edge of the Gawler Craton. The earthquakes are associated with a trans-crustal scale fault system that adds new constraints on the poorly defined craton boundary. Interestingly, they follow the natural spring system in the region, indicating a possibility of fluid-assisted hydrofracturing, as the Great Artesian Basin groundwater is found to circulate through the upper crust and interact with mantle-derived volatiles. Thus, the thesis provides new constraints on the crustal structure of a hitherto under-explored region in the heart of the Australian continent and contributes to the ever-increasing global understanding of intraplate earthquakes.
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