Quantifying the seismic site amplification characteristics of Adelaide’s regolith

Abstract

The historical seismic events have clearly indicated that site amplification has played a principal role in defining the damage to structures founded on the regolith in the city of Adelaide, South Australia. An amplification factor of up to 3.7 is suggested. Thus, a seismic site amplification study in Adelaide city is needed. The use of the ambient noise method for quantifying seismic site amplification of Adelaide’s regolith was selected because of its advantages (non-destructive and affordable). The application of ambient noise analysis for the study of site effects at regolith locations was carried out as it is underdeveloped and requires further attention. A case study is examined which explores Adelaide’s regolith and incorporates 10 in situ ambient noise measurements carried out across the city of Adelaide for seismically classifying the site, estimating bedrock depth and obtaining the shear wave velocity profile. Seismic site classification was investigated using the horizontal vertical spectral ratio (HVSR) technique. The results show that Adelaide’s regolith varies from 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) and are in a good agreement with several previous studies. 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. Both the generic function (GF) of the classic HVSR method and the spatial autocorrelation (SPAC) technique were used to estimate the depth to bedrock. The bedrock depth predictions from the seismic methods were validated against boreholes drilled in close proximity to the measured sites. The comparison demonstrates that the SPAC method provides better estimates, especially to those obtained from another approach. In the general framework of seismic hazard analysis for quantifying site amplification, the knowledge of near surface shear wave velocity profile is crucial. New constraints by means of shear wave velocities for the study case site were developed and proposed. The proposed shear wave velocity models were compared and validated against previous studies and forward modelling techniques. On the basis of these validation results, the applicability of the array SPAC method at regolith sites is justified.