Abstract
In 2010-2011, the city of Christchurch, New Zealand was devastated by a series of powerful earthquakes, the most destructive being the 22 February 2011 M_w6.2 Christchurch Earthquake. This event resulted in 185 casualties, thousands of injuries, and widespread soil liquefaction that caused billions of dollars in damage to buildings, homes and infrastructure. Ultimately, as a result, approximately 7,500 homes have been abandoned and an estimated 80% of the structures within the Christchurch central business district have been demolished.

A network of 19 seismic recording stations in the greater Christchurch area captured an extensive and unique set of ground motions (GM) during the 2010-2011 earthquakes. Potentially, these GM can be used for back-analyses aimed at understanding the spatial variability of the ground shaking (particularly site and basin effects) during each event, followed by accurate forward-estimates aimed at quantifying the amplitude and frequency content of future design GM. However, detailed GM analyses cannot presently be conducted because little information exists on the shear wave velocity (V_s) structure of the greater-than-400-m deep interlayered alluvial sand and gravel deposits that underlie Christchurch.

My research team recently conducted deep V_s profiling at 15 strategic locations throughout Christchurch with the goal of developing a seismic velocity model for the Canterbury basin. The deep V_s profiling was performed using a combination of large active-source and ambient-wavefield surface wave testing. The dispersion estimates from the active-source and ambient-wavefield data at each site were combined to produce an inter-method composite dispersion curve with associated data uncertainty bounds. After which, the open-source software package Geopsy was used to perform a multi-mode, joint inversion of the dispersion data, the SPAC curves and the peak frequency of the mean horizontal-to-vertical (H/V) spectral ratio curve for the site. Rather than providing a single, deterministic V_s profile for each site, these inversions have been used to provide a suite of profiles that fit the experimental data equally well, given the estimated data uncertainty. While results are still preliminary, this work has resulted in V_s profiles that extend a minimum of several hundred meters and a maximum of greater than 1km below the ground surface. This research has more than tripled the available comparisons between large active-source and ambient-wavefield surface wave methods utilized for deep V_s profiling. These comparisons are needed before confidence in utilizing passive-wavefield methods independently can be achieved.