Dynamic site characterization of Christchurch strong motion stations

Abstract

This paper details efforts to characterize the small-strain dynamic properties of 13 strong motion station (SMS) sites in the greater Christchurch, New Zealand area. These SMS recorded a unique set of ground motions (GM) from the 2010-2011 Canterbury earthquakes. Currently, little information about the subsurface layering and dynamic characteristics at these 13 SMS is available. Information provided by GeoNet consists only of generalised layering based on regional geological characteristics and nearby well logs, with no information on dynamic properties. Consequently, the seismic site classifications of these sites were largely based on assumptions. To better define the site classifications, we performed active- and passive-source surface wave testing to obtain shear wave velocity (Vs) profiles at each site. The Vs profiles were used to calculate the average Vs over the top 30 m of the subsurface and to estimate the natural period of vibration (Tn). Additionally, estimates of Tn were obtained by computing the horizontal-to-vertical spectral ratios from recorded GM at each SMS. Based on this new information, we have updated the site classifications at the 13 SMS sites tested; 10 of which ended up with a slightly different site classification than the original assumption (often one site class lower).