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Numerical Study on the Role of Basin Geometryand Kinematic Seismic Source in 3D GroundMotion Simulation of the 22 February 2011M W 6.2 Christchurch EarthquakeRoberto Guidotti, Marco Stupazzini, Chiara Smerzini, Roberto Paolucci, and Paolo RamieriRoberto Guidotti, 1 Marco Stupazzini, 2 Chiara Smerzini, 1Roberto Paolucci, 1 and Paolo Ramieri 3INTRODUCTIONAlmost six months after the M W 7.1 Darfield (Canterbury)earthquake, on 22 February 2011 at 12:51 p.m. (localtime), an M W 6.2 earthquake struck the city and suburbs ofChristchurch—the largest city on the South Island of NewZealand, with about 400,000 inhabitants. The Christchurchearthquake can be considered one of the greatest natural disastersrecorded in New Zealand. The death toll was more than180, with around 2,000 people injured, and structures alreadyweakened by the Darfield event and its aftershocks were badlyaffected (Cubrinovski and Green 2010; Tonkin and Taylor Ltd.2010; Kam et al. 2011). The earthquake was generated by anoblique thrust fault located between the Australian and Pacificplates, within about 6 km of the city center. It is worth recallingthat prior to the Darfield event there was no surface evidenceof the fault that generated the Christchurch earthquakeon February 2011, nor of the Greendale fault, recognized asresponsible for the September 2010 earthquake (Quigley et al.2010). During the last decade a set of seismic surveys across theCanterbury Plains had been carried out (Green et al. 2010), butthey did not reveal any convincing evidence of the Greendalefault and there was no clear indication that a major earthquakewas imminent in this particular region. Beyond the effects andthe consequences of the seismic event, the attention of the scientificcommunity was drawn to two aspects that had a primaryrole in the Christchurch earthquake: 1) the extremelysevere, strong ground shaking observed, especially on the verticalcomponent; and 2) the widespread liquefaction phenomenaacross the city (Cubrinovski and Green 2010; Green et al.2011, page 927 of this issue).1. Department of Structural Engineering, Politecnico di Milano,Milan, Italy2. Munich RE, Munich, Germany3. Consorzio Interuniversitario Lombardo per l’ElaborazioneAutomatica (CILEA), Segrate, Milan, ItalyBetween September 2010 and June 2011 the Canterburyarea experienced three major earthquakes with M W ≥ 6.0 anda large number of aftershocks (Gledhill et al. 2011; Bannisteret al. 2011, page 839 of this issue). The Christchurch earthquakewas recorded by several digital stations of the permanentnetwork operated by the Institute of Geological and NuclearSciences (GNS; data available at the GeoNET Data Centre:http://www.geonet.org.nz/). Peak ground motion accelerationsin the epicentral region of the earthquake range up to 1.261 gon the horizontal component and up to 1.629 g on the verticalcomponent. Table 1 shows a list of the accelerometric stationslocated within a 40-km-radius from the epicenter withthe corresponding values of peak ground acceleration (PGA)and peak ground velocity (PGV) on both the horizontal andvertical components (data from the Center for EngineeringStrong Motion Data, CESMD: http://www.strongmotioncenter.org/;bandpass filter transition bands are 0.1–0.25 Hz and24.50–25.50 Hz). The ground accelerations recorded withinthe city of Christchurch are among the largest ever recordedfor a New Zealand earthquake, with exceptionally high verticalground acceleration (Bradley and Cubrinovski 2011, page853 of this issue). The unusual severity of the ground shakingcan be explained as a combination of four major effects: 1) theproximity of the causative fault to the city, 2) the directivity ofground motion toward the urban area, 3) the strong amplificationeffects of the soft alluvial sediments beneath the city, and4) the hanging wedge effect, causing a significant increase ofground shaking on the hanging wall.The availability of this unprecedented dataset of nearfaultstrong ground motion, combined with the peculiargeological configuration of the Christchurch area, makes theChristchurch earthquake a relevant benchmark to test theeffectiveness of 3D numerical tools for the prediction of thevariability of strong ground motion in near-fault conditions.To this end, numerical simulation of seismic wave propagationwithin the Canterbury Plains, extending from the northwesternportion of the Southern Alps mountain range to thedoi: 10.1785/gssrl.82.6.767Seismological Research Letters Volume 82, Number 6 November/December 2011 767