HVSR,SSR, and VS30 Values at Three New Zealand Basin Arrays

Microtremors and small earthquakes have been recorded on three small soft-soil basins in New Zealand, and on the surrounding rock, using arrays of up to twenty velocity seismographs. Comparisons of the resultant HVSR (Horizontal to Vertical Spectral Ratio) and SSR (Standard Spectral Ratio) values show that near the centre of the basins HVSR values can constitute good proxies for SSR values, but that near the basin edges this relationship may fail. The failures involve HVSR both under-predicting and over-predicting SSR. The location of effective basin edges may not be immediately obvious. Furthermore, there appears to be no relationship between values of Vs30 and values of HVSR or SSR for the basins, suggesting that Vs30 may be of limited value in distinguishing between soil classes.