Ground penetrating radar as a means of studying palaeofault scarps in a deeply weathered terrain, southwestern Western Australia

The southwest seismic zone is a region of concentrated intra-plate seismicity in the southwest of Western Australia. The regional geology consists of Archean granitoids covered by a thick, electrically conductive, mantle of in situ weathered material and transported cover. Numerous palaeofault scarps have been recognised in the region, primarily based on remote sensing data. These scarps are primarily caused by reverse faulting, which is consistent with the results of focal mechanism studies of the earthquakes in the area. The scarps occur across an area more extensive than the historic region of seismic activity, implying that large seismic events can occur outside the current seismic zone, which has obvious implications for the estimation of seismic hazard.Ground penetrating radar (GPR) has been used to study Quaternary faulting in various parts of the world, and has been shown to be capable of imaging features associated with such faults, e.g. colluvial wedges, disrupted and displaced strata. However, the majority of studies are of normal and/or strike-slip faults and there are no studies demonstrating the method is useful in electrically conductive, deeply weathered terrains.GPR data have been collected across known palaeofault scarps at Hyden and Dumbleyung, and also the scarp created by the 1968 Meckering earthquake, all located in southwestern Western Australia. In each case there is a nearby trench to allow GPR responses to be related to known geology. At Meckering and Hyden, where the near-surface material contains moderate amounts of clay and the groundwater is fresh, it has proved possible to collect high quality data that images colluvium and also disrupted bedrock features which allow faults to be inferred. At Dumbleyung, where the near surface is more conductive due to clay-rich alluvial deposits and saline groundwater, the GPR data were of poorer quality and no sub-surface features were confidently identified, although the general geometry of the stratigraphy was imaged. This may also be the result of the scarp comprising a monoclinic fold rather than consisting of a hangingwall fault block above a fault that extends to the surface.Our results demonstrate that GPR surveys can be a valuable tool for studying palaeofaults in deeply weathered terrains, although this is subject to the local ground conditions. Even in what is considered a hostile environment for the method, GPR data can confirm whether a topographic feature is of seismic origin, and image features in sufficient detail to enable siting of trenches for palaeoseismic studies.

Research Highlights►GPR surveys are a valuable tool for studying palaeofaults in deeply weathered terrains.