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Kinematic Source Model of the 22 February2011 M w 6.2 Christchurch Earthquake UsingStrong Motion DataCaroline HoldenCaroline HoldenGNS ScienceINTRODUCTIONThe Canterbury earthquake sequence began in September2010 with the Mw 7.1 (source: GeoNet catalog, http://geonet.org.nz/canterbury-quakes/) Darfield earthquake that rupturedthe previously unknown 40-km-long Greendale fault 30 kmwest of Christchurch (Gledhill et al. 2011). Extreme groundaccelerations as high as 1.8 g near the epicenter were recorded.The event caused intense liquefaction in the eastern suburbs ofChristchurch as well as closer to downtown, near the courseof the Avon River. The Darfield earthquake was followed bya major aftershock on 22 February local time (21 FebruaryUTC) of magnitude Mw 6.2 (source: GeoNet), but Me 6.7(source: USGS, http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usb0001igm/neic_b0001igm_e.php). Thisearthquake was centred only a few kilometers south of theChristchurch city center. Extremely high accelerations (as highas 2.2 g) were also recorded near the epicenter (Kaiser, Beniteset al. 2011). In addition to the extreme liquefaction seen afterthe Darfield earthquake, this event also caused landslides,large rockfalls, widespread damage to earthquake-risk buildingsin Christchurch, and, most tragically, about 180 casualties.Another large aftershock of Mw 6.0 (source: GeoNet), butwith Me 6.7 (source: USGS), subsequently occurred on 13 Junelocal time (12 June UTC) just a few kilometers south of theFebruary event, causing further damage, landslides, rockfalls,and liquefaction.Following the Darfield earthquake, the GeoNet network(New Zealand National Hazard Monitoring Network) and itsregional component the CanNet network (Berrill et al. 2011)was supplemented by the deployment of 13 additional strongmotion instruments regionally (and another nine following theFebruary earthquake). We used this dense network of strongmotion instruments to constrain the source kinematics of theFebruary event. We present the inversion scheme and discussits limitations. These results are preliminary, since more thoroughdata processing is needed; however, they already provide akey model that will help in understanding the sequence of largeaftershocks that has developed near Christchurch. This work isstrongly dependent on other studies by Beavan et al. 2011, page789 of this issue; Fry et al. 2011; Bannister et al. 2011, page839 of this issue; Sibson et al. 2011, page 824 of this issue;and Kaiser, Benites et al. (2011).THE STRONG MOTION DATASETAt the time of the February earthquake there were 14 strongmotion GeoNet sites, from both the national and the regionalCanterbury network CanNet, within 20 km of the epicenter(Figure 1). However, there were strong site effects at stationsPRPC, SHLC, and HPSC, each of which sits on very softground and suffered intense liquefaction from the earthquake;therefore those three were excluded, leaving 11 stations to beincluded in the inversion scheme. The source-station distanceranges from 2 to 20 km.All of the recordings used in this study suffered from siteeffects to some degree. Stations on rock sites are found only onthe hills of Banks Peninsula (south of Christchurch) wherestrong topographic effects are the likely cause of an intensedamage pattern over the hills of Banks Peninsula as describedby Hancox et al. (2011). Stations on the plains suffered fromvery soft shallow layers inducing non-linear amplificationsand extreme phenomena such as liquefaction and trampolineeffects (Fry et al. 2011). Unfortunately, ground conditionswithin Christchurch are highly variable and will require furtherstudies for stations in this region to be included in themodeling (Kaiser, Holden et al. 2011).For our inversion study, the acceleration data has beenintegrated into velocity and filtered using a Butterworth bandpassfilter from 0.1 to 1.0 Hz. Since we are interested in thepolarity and amplitude of the first onset we used a causal filter.We applied the same filter to observed and synthetic data. Thedata from the CanNet stations were rotated from their originalorientation to north-south and east-west components.INVERSION SCHEMEWe inverted three-component data for 11 well-distributedstrong motion stations within 20 km of the epicenter. We useda fixed fault plane geometry of strike 59 and dip 67 as definedby Beavan et al. 2011 (page 789 of this issue) since processedInSAR data clearly shows deformation fringes resulting fromdoi: 10.1785/gssrl.82.6.783Seismological Research Letters Volume 82, Number 6 November/December 2011 783