Investigation of shear wave velocity depth variability, site classification, and liquefaction vulnerability identification using a near-surface Vs model of Christchurch, New Zealand

Abstract

Following the companion work of McGann et al. [1], several applications of a regional surficial shear wave velocity (Vs) model developed for Christchurch, New Zealand are examined. Comparisons of time-averaged Vs over various profile depths are used to characterize the inherent depth variability of the soils in the region. The degree of correlation between 30m shear wave velocity (Vs30) and average velocity over shallower profile depths (Vsz) exhibited by the current model is compared to similar correlations developed for other locations, and consideration is given to differences between the four primary surficial geologic units present in the majority of the Christchurch urban area. The effects of the observed Vs depth variability on expected seismic response are assessed through the consideration of transfer functions developed for 30m typical Vs profiles for eight subregions of Christchurch. The regional Vs model is also used to develop site classification maps for Christchurch using current New Zealand and international site classification schemes, and these maps are used to comment on the applicability of these conventional schemes to the soil profiles typical to the region. Models of 5m shear wave velocity (Vs5) filtered by average soil behaviour type index are used to examine the relationship between Vs5 and observations of liquefaction-related surface damage made following the 22 February 2011 Christchurch earthquake. It is shown that when properly filtered to remove regions with soils that are less susceptible (or not susceptible) to liquefaction due to soil composition, there is a good correlation between Vs5 and severity of liquefaction-related damage.