High‐resolution relocation of aftershocks of the Mw 7.1 Darfield, New Zealand, earthquake and implications for fault activity

Abstract

AbstractLow‐slip‐rate regions often represent under‐recognized hazards, and understanding the progression of seismicity when faults in such areas rupture will help us to better understand earthquake rupture patterns. The 3 September 2010 (UTC) Mw 7.1 Darfield earthquake revealed a formerly unrecognized set of faults in the Canterbury region of New Zealand, an area that had previously been mapped as one of the lower‐hazard areas in the country. In this study, we analyze the first four months of its aftershock sequence to identify active faults and temporal changes in seismicity along them. We jointly invert for three‐dimensional P wave and S wave velocities and hypocentral locations, using data for 2840 aftershocks recorded at 36 temporary and permanent seismic stations within 70 km of the main shock epicenter. These relocations delineate eight individual faults active prior to the 22 February 2011 Mw 6.3 Christchurch earthquake, the largest aftershock of the Darfield earthquake. Two of these faults are in the Christchurch region, one of which corresponds to geodetically determined rupture planes of the Christchurch earthquake. Using focal mechanisms calculated from first‐motion polarities, we find mainly strike‐slip faulting events, with some reverse and normal faulting events as well. We compare the orientations of these faults to the prevailing regional stress directions to identify which faults may have been active prior to the Darfield earthquake and which may be newly developed.