In the general framework of seismic hazard analysis for quantifying site amplification, knowledge of the near surface shear wave velocity profile is crucial. The non-destructive and affordable ambient vibration array technique has been shown to be a superior method for estimating this shear wave velocity. The present study seeks to investigate the robustness of shear wave velocity profiles obtained using the ambient vibration array technique at sites exhibiting various (low to high) impedance contrasts by investigating a site in the central business district of Adelaide, South Australia. The city is founded on a regolith, which generally exhibits a range of impedance contrasts, and it is accepted that it is one of the Australian capital cities with the highest seismic risk. Eight ambient vibration array measurements were undertaken across the city. The acquired data were used to invert the shear wave velocity profiles, which are currently very limited, through the analysis of the dispersion characteristics of the surface waves by means of the spatial autocorrelation (SPAC) method, from which a further adjustment was carried out to suit the measured horizontal vertical spectral ratio (HVSR) curve ellipticity. Validation of the proposed shear wave velocity models was carried using previous studies and forward modeling techniques. The present study demonstrates that the SPAC method provides reliable results at regolith sites which are subject to various (low to high) impedance contrasts, provided the frequency band of the captured dispersion curve is wide enough to resolve the investigated depth. The primary innovations of this paper are to: (1) investigate the applicability of the microtremor SPAC method to obtain the near surface shear wave velocity of a low to high impedance contrast site; and (2) propose a new near surface shear wave velocity profile for Adelaide, South Australia.

The data presented in this article contain datasets of passive noise measurements at regolith sites in Adelaide, South Australia. The data were acquired using three component (3C) LE-3Dlite Lennartz seismometers with an eigenfrequency of 1 Hz. The data were acquired at eight sites across Adelaide׳s regolith in a hexagonal array layout. Four tests, each with a duration of 30 min, were conducted at different times. The ambient noise data can be used for both horizontal to vertical spectral ratio (HVSR) analysis and array analyses, which are essential to obtain the site fundamental frequency and the ellipticity of the fundamental mode Rayleigh waves at the measured site. The array analyses are useful to obtain the dispersion curves, which are needed to estimate the shear wave velocity profile.

Subsurface geometry, particularly the depth of bedrock, is crucial in seismic hazard studies because the basin geometry has been shown to play an important role in the altering of seismic waves. Estimating the bedrock surface using ambient seismic noise analysis has been undertaken by many researchers, with most studies focusing on sites with a strong impedance contrast between the bedrock and the overlying materials. The application of this technique at regolith sites, which is subjected to impedance contrasts in the low to high range is underdeveloped and requires further attention. This study seeks to address this need and is focused on the city of Adelaide in South Australia, which exhibits site amplification and is associated with various impedance contrasts. Analyses of ambient noise data are carried out using the generic function (GF) of the classic horizontal vertical spectral ratio (HVSR) method and the spatial autocorrelation (SPAC) technique to estimate the depth to bedrock. Comparison of the bedrock depth predictions from the seismic methods with boreholes drilled in close proximity to the measured sites demonstrate that the SPAC method provides superior estimates especially to those obtained from the other approach. This work demonstrates that the microtremor SPAC method is an effective tool for estimating bedrock structure at regolith sites.

Seismic site classification is the most widely accepted practical method in the design of seismic resistant infrastructure. The horizontal vertical spectral ratio (HVSR) technique for analyzing ambient noise data has been successfully applied to quantify site effects in the estimation of seismic site classes associated with seismic hazards. This successful application was mainly carried out in high impedance contrast sites. The present paper focuses on the application of the HVSR technique to regolith sites which were suggested by previous studies to be low in impedance contrast between the upper and underlying bedrock layers (< 4–5). A case study is examined which explores the central business district of Adelaide, South Australia and incorporates 10 in situ ambient noise measurements carried out across the city. Adelaide experienced more medium-sized earthquakes than any other capital city in Australia in the past half of the last century. Site amplification was also observed to occur in Adelaide. Ambient noise data were used to estimate the site predominant period and to infer the site shear wave profile, after establishing that the data were free from noises from an industrial source, checking the reliability of the HVSR ellipticity curve and validating the appropriateness of the adopted method and resulting shear wave models. The results show that the predominant fundamental period for Adelaide is 0.8s or higher, which suggests a subsoil class D according to the Australian Standard. Results of the inversion for the upper 30m shear wave velocities of Adelaide's subsoil layers varies from 194m/s to 418m/s, which are related to classes D to C (NEHRP classification system), classes D to B (Australian Standard classification system) or classes D/DE to C (regolith case classification system). These results are in a good agreement with several previous studies. This suggests a promising application of the HVSR analysis for seismic assessment at regolith sites.

In this paper we present a hypothesis for localized, intraplate deformation in the continental crust of south-central Australia that involves fluid-assisted reactivation of faults in the mid- to lower crust. Using data from a temporary seismometer deployment in the Flinders Ranges, we show that earthquakes, relocated in a 3D velocity model, cluster in elongated low vp/vs anomalies that extend to depths exceeding 20 km, and are aligned with the axis of the Flinders Ranges. In the northern Flinders Ranges these low vp/vs anomalies can be interpreted as fractured Neoproterozoic to Cambrian sediments that separate two cratonic blocks, the Gawler Craton to the west and the Curnamona Province in the east. Previous studies of Helium isotopes in springs to the north of the area provide evidence of mantle-derived fluids that may influence faulting at depth. Our focal mechanism and stress inversion results show a regionally compressive stress field that provides no evidence for stress concentration. We also argue that mechanisms for localized faulting such as thermal weakening and isostatic rebound also fail to account for the occurrence of earthquakes at mid- to lower crustal depth in this area of high heat flow and that the focused seismicity can only be explained by high pore fluid pressure in the lower crust.