Fine-scale Relocation of Aftershocks of the 22 February Mw 6.2 Christchurch Earthquake using Double-difference Tomography

Abstract

On 22 February 2011 New Zealand time (21 February UTC), the MW 6.2 Christchurch earthquake occurred just 7 km southeast of the center of Christchurch city, New Zealand (Fry et al. 2011, Holden 2011, page 783 of this issue). There were 181 confirmed fatalities, and the damage to Christchurch city is estimated to be NZ$15 billion–$NZ20 billion (US$12 billion–US$16 billion). The event was well-recorded by the broadband and strong-motion national-scale GeoNet network (Petersen et al. 2011) as well as by the Canterbury regional strong-motion network (Avery et al. 2004). Since the 22 February earthquake, more than 2,700 further aftershocks have been recorded up to 1 May 2011, including 21 events with local magnitude ( ML ) greater than 5. Here we describe the initial relocation analysis for these aftershocks. The Mw 6.2 Christchurch earthquake is part of the larger aftershock sequence of the Mw 7.1 Darfield earthquake, which occurred at 16:35 3 September UTC, 2010. Seismological, GPS, and InSAR data all suggest that the earthquake rupture process for the Mw 7.1 Darfield earthquake involved failure of multiple fault segments (Beavan et al. 2010; Gledhill et al. 2011). A surface rupture for that earthquake, now termed the Greendale fault, extending ∼29.5 km and located ∼4 km south of the epicenter, is consistent with strike-slip faulting with an average horizontal surface displacement of ∼2.5 m (Quigley et al. 2010). The vast majority of the 7,400+ aftershocks following the Darfield earthquake are shallow, at less than 15 km depth. Figure 1 shows that, although many of the aftershocks occurred near the surface trace of the Greendale fault, intense clusters of aftershock activity have also occurred at the western and eastern ends of the Darfield fault trace, as well as north-northwest from the Darfield epicenter. The distribution of …