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Fault Location and Slip Distribution of the22 February 2011 M W 6.2 Christchurch, NewZealand, Earthquake from Geodetic DataJohn Beavan, Eric Fielding, Mahdi Motagh, Sergey Samsonov, and Nic DonnellyJohn Beavan, 1 Eric Fielding, 2 Mahdi Motagh, 3 Sergey Samsonov, 4 andNic Donnelly 5EOnline material: Additional figures showing interferogramsand fault models; data tablesINTRODUCTIONThe 22 February (local time) M W ~6.2 Christchurch earthquakeoccurred within the aftershock region of the 4 September2010 M W 7.1 Darfield (Canterbury) earthquake (Gledhill etal. 2011). Both the Darfield and Christchurch earthquakesoccurred on previously unknown faults in a region of historicallylow seismicity, but within the zone of plate boundarydeformation between the Pacific and Australian plates. TheDarfield earthquake caused surface rupture up to 5 m (Quigleyet al. 2010, forthcoming), but none has been observed associatedwith the Christchurch earthquake. Geodetic dataindicate that strain has been slowly accumulating within theregion (Wallace et al. 2007; Beavan et al. 2002), and the presenceof active subsurface faults was known or suspected (e.g.,Pettinga et al. 2001). Earthquakes of magnitude up to 7.2 inthis region had been allowed for in the national seismic hazardmodel (Stirling et al. 2002), but the observed high apparentstresses (Fry and Gerstenberger 2011, page 833 of this issue)and high ground accelerations (Fry et al. 2011, page 846 ofthis issue) had not been anticipated, particularly those experiencedin the Christchurch event. These and other factors(Fry and Gerstenberger 2011, page 833 of this issue; Fry et al.2011, page 846 of this issue; Holden 2011, page 783 of thisissue), plus the close proximity of the February earthquake to1. GNS Science, Lower Hutt, New Zealand2. Jet Propulsion Laboratory/Caltech, Pasadena, California, U.S.A.3. Helmholtz Centre Potsdam, GFZ German Research Centre forGeosciences, Potsdam, Germany; also at Department of Geomaticsand Surveying Engineering, University of Tehran, Tehran, Iran4. European Center for Geodynamics and Seismology, Walferdange,Luxembourg; now at Canada Centre for Remote Sensing, Ottawa,Canada5. Land Information New Zealand, Wellington, New ZealandChristchurch city center, were responsible for the major damagecaused by the earthquake (e.g., Kaiser et al. 2011).A large amount of geodetic ground-displacement datais available to constrain the source of the earthquake, in partbecause we reoccupied nearly 200 GPS sites that had beenobserved following the Darfield earthquake, and in partbecause a number of space agencies collected synthetic apertureradar (SAR) data over the source area that we were able to usein differential interferometric SAR (DInSAR) processing. Thegeodetic data were collected one day to seven weeks followingthe February earthquake, so they include ground deformationdue to aftershocks, in particular the M W 5.8 and M W 5.9 eventsthat occurred within two hours of the mainshock.To first order, the earthquake source can be modeled as aplanar fault striking ~59° and dipping ~69° to the southeast.The peak slip of 2.5–3 m is a mixture of reverse and right-lateralslip and is located ~7 km east-southeast of Christchurchcity center at a depth of ~4 km. Slip of ~1 m reaches within~1 km of the ground surface. The slip near the southwest endof the plane is approximately right-lateral with magnitude ~1m. The geodetic data are significantly better fit by two faultplanes, a compact region of oblique slip on the fault describedabove, plus right-lateral strike slip on a near-vertical fault to itssouthwest that coincides with the locations of the two majoraftershocks and with a trend of smaller aftershocks. A lobe ofground uplift seen in some of the SAR data (e.g., Figure 4) justwest of the main slip patch is not well modeled, and suggestssome slip may also have occurred elsewhere, perhaps on a splayoff the main fault plane.GEODETIC DATAWe use campaign GPS data collected between 28 Februaryand 14 April from 57 sites (Figure 1) that were also occupiedfollowing the September 2010 Darfield earthquake(Beavan, Samsonov, Motagh, et al. 2010). We also use continuousGPS (cGPS) data from five regional sites operated byGeoNet (http://www.geonet.org.nz) for Land Informationdoi: 10.1785/gssrl.82.6.789Seismological Research Letters Volume 82, Number 6 November/December 2011 789