Ground motions observed in the Darfield and Christchurch earthquakes and the importance of local site response effects

This manuscript provides a brief comparison of the nature of ground motions observed in Christchurch city during the 4 September 2010 Darfield and 22 February 2011 Christchurch earthquakes. Focus is given to the spatial distribution of ground motion intensity from both events on a regional and location-specific basis. Particular emphasis is also given to the importance of local site effects, that is, the seismic response of the shallow soil layers directly underlying the ground surface. Not only are such effects clearly prevalent at several locations in both events, but they can also be significantly different for different sites which have the same nominal 'site class'. The observations highlight the importance of, and insight which can be gained from, the use of site-specific response analysis for deep alluvial soil sites, which is presently not adequately accounted for in New Zealand seismic design guidelines.