The importance of shear wave velocity information of a soil site

Micro-tremor surveys based on the single-station horizontal/vertical spectral ratio (HVSR) method have typically been used to determine the potential seismic hazard of a soil site. This approach assumes that sites possessing similar natural periods have similar levels of hazard. In a micro-tremor survey undertaken by the authors, multiple dominant site periods varying in the order of between 0.3 sec and 1 sec were observed on a flexible soil site. The shear wave velocity model of the site obtained by the microtremor array method with the spatial auto-correlation (SPAC) processing technique revealed a 95 m thick layer of probable gravels underlying thin layers of sands and soft silts. Earthquake simulation analyses carried out in accordance with the modelled shear wave velocity profile indicate a 30 mm maximum seismic displacement demand of the site for a 500 year return period earthquake. Had the shear wave velocity information not been available, the site seismic hazard predicted in accordance with the measured site period of 1 sec would have been much more onerous.