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Near-source Strong Ground MotionsObserved in the 22 February 2011 ChristchurchEarthquakeBrendon A. Bradley and Misko CubrinovskiBrendon A. Bradley and Misko CubrinovskiUniversity of CanterburyINTRODUCTIONOn 22 February 2011 at 12:51 p.m. local time, a momentmagnitude M w 6.3 earthquake occurred beneath the cityof Christchurch, New Zealand, causing an level of damageand human casualties unparalleled in the country’s history.Compared to the preceding 4 September 2010 M w 7.1 Darfieldearthquake, which occurred approximately 30 km to the westof Christchurch, the close proximity of the 22 February eventled to ground motions of significantly higher amplitude in thedensely populated regions of Christchurch. As a result of thesesignificantly larger ground motions, structures in general, andcommercial structures in the central business district in particular,were subjected to severe seismic demands and, combinedwith the event timing , structural collapses accounted for themajority of the 181 casualties (New Zealand Police 2011).This manuscript provides a preliminary assessment of thenear-source ground motions recorded in the Christchurchregion. Particular attention is given to the observed spatialdistribution of ground motions, which is interpreted based onsource, path, and site effects. Comparison is also made of theobserved ground motion response spectra with those of the 4September 2010 Darfield earthquake and those used in seismicdesign in order to emphasize the amplitude of the ground shakingand also elucidate the importance of local geotechnical anddeep geologic structure on surface ground motions.There are numerous identified faults in the Southern Alpsand eastern foothills (Stirling et al. 2007) and several significantearthquakes (i.e., M w > 6) have occurred in this regionin the past 150 years, most notably the M w 7.1 Darfield earthquakeon 04/09/2010 (New Zealand Society for EarthquakeEngineeering 2010). The M w 6.3 Christchurch earthquakeoccurred at 12:51 p.m. on Tuesday 22 February 2011 beneathChristchurch, New Zealand’s second-largest city, and representsthe most significant earthquake in the unfolding seismicsequence in the Canterbury region since the 4 September2010 Darfield earthquake. The 6.3 event occurred on a previouslyunrecognized steeply dipping blind fault, which trendsnortheast to southwest (the location relative to Christchurchis presented in the context of subsequently observed groundmotions). Figure 2 illustrates the inferred slip distribution onthe fault obtained by Beavan et al. (2011, page 789 of thisTECTONIC AND GEOLOGIC SETTINGNew Zealand resides on the boundary of the Pacific andAustralian plates (Figure 1) and its active tectonics are dominatedby: 1) oblique subduction of the Pacific plate beneaththe Australian plate along the Hikurangi trough in the NorthIsland; 2) oblique subduction of the Australian plate beneaththe Pacific plate along the Puysegur trench in the southwestof the South Island; and 3) oblique, right-lateral slip alongnumerous crustal faults in the axial tectonic belt, of which the650-km-long Alpine fault is inferred to accommodate approximately70–75% of the approximately 40 mm/yr plate motion(DeMets et al. 1994; Sutherland et al. 2006).▲ ▲ Figure 1. Tectonic setting of New Zealand (courtesy of J.Pettinga).doi: 10.1785/gssrl.82.6.853Seismological Research Letters Volume 82, Number 6 November/December 2011 853