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The 22 February (local time) MW~ 6.2 Christchurch earthquake occurred within the aftershock region of the 4 September 2010 MW 7.1 Darfield (Canterbury) earthquake (Gledhill et al. 2011). Both the Darfield and Christchurch earthquakes occurred on previously unknown faults in a region of historically low seismicity, but within the zone of plate boundary deformation between the Pacific and Australian plates. The Darfield earthquake caused surface rupture up to 5 m (Quigley et al. 2010, forthcoming), but none has been observed associated with the Christchurch earthquake. Geodetic data indicate that strain has been slowly accumulating within the region (Wallace et al. 2007; Beavan et al. 2002), and the presence of active subsurface faults was known or suspected (eg, Pettinga et al. 2001). Earthquakes of magnitude up to 7.2 in this region had been allowed for in the national seismic hazard model (Stirling et al …