The 2012 Newcastle and Sydney SPAC microtremor surveys using Geopsy software and comparisons

Abstract

Since the 1989 Newcastle ML5.5 earthquake, the city of Newcastle, Australia, has become a focus for earthquake risk assessment. Surficial geology in the area varies from deeper alluvial deposits near the Hunter River, to shallower soils overlying weathered rock on the valley margins. Ambient vibration techniques, based on the dispersion property of surface waves in layered media, are promising for assessing the subsurface geophysical structure, in particular the shear-wave velocity (Vs). Using one such technique, the Spatial Auto-Correlation (SPAC) method, we characterise soil deposits at 23 sites in and around the city of Newcastle. Results show that values for soil overlying bedrock ranges from 200 m/s to 1000 m/s, with the higher velocity values observed in shallow soils which are relatively consolidated and distal to the river. Bedrock depth varies from 6 to 56 m, but an accurate quantification is hampered by the low frequency picks (< 2 Hz) which are either unavailable or of dubious quality. Some Vs profiles show two abrupt changes, the first at ~ 4-15 m depth and the second ~19-56 m. Low Vs values are of particular interest as they may indicate areas of higher seismic hazard.