Crustal heterogeneity in South Australia, earthquake evidence

Abstract

Lateral velocity variations in the South Australian crust have been mapped using the relatively new technique of seismic tomography. The input data comprised P1 travel times from 1340 local earthquakes recorded on the South Australian seismic network during the period 1978–1983. To reduce errors in the earthquake locations, focal depths and origin times, we used an averaging technique of dividing the area under consideration into a series of overlapping cylindrical cells. All earthquakes and seismic stations were assigned to the centre of the nearest cell. Travel times between all centres were computed by averaging over all relevant cell pairs, after first scaling all travel times by a factor equal to the distance between cell centres divided by the actual ray path length. Only ray paths appropriate to the P1 phase were considered. Starting with a first guess model (based on the number of ray intersections with each cell), the travel times were computed along all ray paths and compared with the observed (averaged) values. The errors and residuals were back projected for each ray over all the chords along the ray and the average cell slowness adjusted in an iterative sense until the computed and observed travel times agreed within a specified error tolerance. The areas with poor raypath coverage (lack of earthquakes and/or stations) could not be imaged. The area covered lies mostly within the Adelaide Geosyncline, where there is believed to be a thick cover of Proterozoic sediments. The velocity image is of the upper 20 km of the crust, so no direct correlation between the surface geology and the crustal velocity is to be expected. Results indicate a significant variation in crustal velocity from 5.9 to 6.5 km s−1. The velocities correlate remarkably with time terms computed in a separate study of Moho refracted arrivals (Pn). Low time terms correspond to high crustal velocity and high time terms reflect anomalously low crustal velocity. The residual component of the time terms can be explained in terms of variations in depth to the Moho throughout the Geosyncline of the order of 10 km. The prominent features of the tomographic velocity map are a N-S trending velocity low over the Spencer Gulf graben, and a ridge of high velocity beneath the Flinders Ranges. The velocity low can be associated with a region of low gravity aligned parallel to the Torrens Hinge Zone, which marks the western boundary of the Geosyncline. The northern end of the high velocity ridge coincides with a region of positive Bouguer gravity. Teleseismic residuals from both earthquakes and nuclear explosions show a similar trend to the Pn time-terms, and indicate lateral variability in crustal thickness and velocity. A component of the residual must be attributed to mantle velocity inhomogeneity which extends to depths of 300 km.