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Fine-scale Relocation of Aftershocks of the22 February M w 6.2 Christchurch EarthquakeUsing Double-difference TomographyStephen Bannister, Bill Fry, Martin Reyners, John Ristau, and Haijiang ZhangStephen Bannister, 1 Bill Fry, 1 Martin Reyners, 1 John Ristau, 1 andHaijiang Zhang 2EOnline material: Hypocenters of 2,177 earthquakes recordedduring 21 February 21–31 March 2011INTRODUCTIONOn 22 February 2011 New Zealand time (21 February UTC),the M W 6.2 Christchurch earthquake occurred just 7 kmsoutheast of the center of Christchurch city, New Zealand (Fryet al. 2011, Holden 2011, page 783 of this issue). There were181 confirmed fatalities, and the damage to Christchurch cityis estimated to be NZ$15 billion–$NZ20 billion (US$12 billion–US$16billion). The event was well-recorded by thebroadband and strong-motion national-scale GeoNet network(Petersen et al. 2011) as well as by the Canterbury regionalstrong-motion network (Avery et al. 2004). Since the 22February earthquake, more than 2,700 further aftershockshave been recorded up to 1 May 2011, including 21 events withlocal magnitude (M L ) greater than 5. Here we describe the initialrelocation analysis for these aftershocks.The M w 6.2 Christchurch earthquake is part of the largeraftershock sequence of the M w 7.1 Darfield earthquake, whichoccurred at 16:35 3 September UTC, 2010. Seismological,GPS, and InSAR data all suggest that the earthquake ruptureprocess for the M w 7.1 Darfield earthquake involved failureof multiple fault segments (Beavan et al. 2010; Gledhill et al.2011). A surface rupture for that earthquake, now termedthe Greendale fault, extending ~29.5 km and located ~4 kmsouth of the epicenter, is consistent with strike-slip faultingwith an average horizontal surface displacement of ~2.5 m(Quigley et al. 2010). The vast majority of the 7,400+ aftershocksfollowing the Darfield earthquake are shallow, at lessthan 15 km depth. Figure 1 shows that, although many of theaftershocks occurred near the surface trace of the Greendalefault, intense clusters of aftershock activity have also occurred1. GNS Science, Lower Hutt, New Zealand2. Dept. of Earth, Atmospheric, and Planetary Sciences, MassachusettsInstitute of Technology, Cambridge, MA, U.S.A.at the western and eastern ends of the Darfield fault trace, aswell as north-northwest from the Darfield epicenter.The distribution of the aftershocks (Figure 1A) indicatesa complex fault system, most of which was previously undetected;prior to the 2010–2011 activity there was negligiblerecorded seismicity in the region, as illustrated in Figure 1Bfor the time period 2000–2010. However, some faults havebeen inferred from geological mapping studies (Howard et al.2005; Pettinga et al. 2001) as well as from onshore and offshoreseismic reflection work (Dorn et al. 2010; Barnes 1995, 1996;Barnes et al. 2011; Wood et al. 1989).Following the destructive 22 February earthquake newstudies have begun to characterize the location and geometryof possible hidden faults beneath the region. This new workincludes a combination of offshore marine seismic surveys(Barnes et al. 2011), additional gravity acquisition and interpretation,new aeromagnetic surveys across sections of theCanterbury Plains (Figure 1B), some active-seismic reflectionsurveys, and relocation analysis of the existing earthquakeaftershock data (this study). Here we provide relocation analysisfor more than 2,100 aftershocks that have occurred since theM w 6.2 February earthquake. Separate analysis and processingof the aeromagnetic and reflection seismic survey data is underway.SEISMIC DATASeismic waveform data for the 22 February earthquake,and aftershocks, recorded by the New Zealand national seismographnetwork (Petersen et al. 2011) and the CanNet(Canterbury) strong-motion network (Avery et al. 2004) arepublicly available through GeoNet (http://www.geonet.org.nz).The strong-motion data coverage is excellent, with 15 strongground-motionrecorders (Figure 2) within 8 km of the top ofthe fault plane of the February M w 6.2 earthquake, 13 of theserecording vertical ground accelerations greater than 0.2 g.Many of the strong-motion recorders usually triggered foraftershocks of M L > 4 (90 events since 21 February to 1 May),doi: 10.1785/gssrl.82.6.839Seismological Research Letters Volume 82, Number 6 November/December 2011 839