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−43.25°OXZ0 10 20km−43.5°−43.75°172° 172.5° 173°▲ ▲ Figure 2. Seismometer stations on the Canterbury Plains. Open triangles show the sites of temporary stations operating duringSeptember–October 2010; filled triangles show short-period and broadband seismometer sites; filled circles show strong-motion seismometersites.but also triggered for many of the 100+ M L > 3 aftershocksnear the city.Additional waveform data were also collected by a temporaryseismic array that was deployed immediately after the 22February event. This array (Figure 2), consisting of six shortperiodand three strong-motion seismometer sites, was placedto the south and north of Christchurch to provide greaterazimuthal coverage of the aftershock region and to assist withlocation and seismic tomography analysis.RELOCATION ANALYSISWe relocate aftershocks of the Christchurch M w 6.2 earthquakeusing the waveform and travel-time data currently available.We anticipate that additional waveform data currentlybeing collected (Yoshihisa Iio, personal communication 2011)will assist in further improving the regional velocity modelfor the Canterbury Plains (Figure 1B), with subsequent iterativeimprovement also expected for the aftershock locations.Additional travel-time picking by GeoNet analysts over thenext few months will also assist in iterative improvement of theaftershock locations.Initial event locations and phase picks were obtained fromGeoNet, with some additional travel time picking carried outfor the temporary stations deployed in late February. Most ofthe initial GeoNet event locations, before the relocation analysisdescribed here, have event depths at standard “fixed” depthsof 2, 5, or 12 km. Most of the larger-magnitude events thattriggered the strong-motion recorders had 20 or more phasepicks. Travel-time picking was carried out by GeoNet analysts.During phase picking it was noted that waveform data for someearthquake-station paths indicate multiple phase arrivals, inparticular to GeoNet station OXZ located approximately 40km to the west of Christchurch. This suggests strong multipatheffects from high-velocity basement or sub-basement structure.In our analysis we specifically downweight S-phase travel-timepicks from station OXZ to allow for possible ambiguities of theS-phase arrivals.We invert data from aftershocks recorded following theChristchurch M w 6.2 earthquake, using the double-differencetomography approach of Zhang et al. (2009), which buildson earlier work by Zhang and Thurber (2003). The techniqueminimizes the residuals between observed and calculatedarrival-time differences for pairs of closely located earthquakes,while also minimizing the residuals of absolute arrival times.The algorithm solves for the hypocentral parameters of theearthquakes, also allowing some modification of the P-waveand S-wave velocity structure used for travel-time calculation.Catalog-based differential times CBDT were calculatedbetween events initially separated by less than 10 km, for allSeismological Research Letters Volume 82, Number 6 November/December 2011 841