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▲ ▲ Figure 2. Distribution of fault slip inferred in the 22 February 2011 Christchurch earthquake (Beavan et al. 2011, this issue). Arrowsindicate the slip vector. The inferred hypocenter is indicated by a star.issue). It can be seen that slip on the fault occurred obliquelywith both significant up-dip and along-strike components(average rake, λ = 146°). The steeply dipping nature of the fault(δ = 69°), as well as the large up-dip component of slip, contributedto the large observed vertical accelerations discussed inthe next section. For the purpose of the subsequent engineeringanalysis of strong ground motion, the Beavan et al. (2011, page789 of this issue) finite fault model was “trimmed” using themethodology of Somerville et al. (1999), which resulted in theremoval of 1 km from the northeast and southwest extents ofFigure 2. The resulting “trimmed” fault therefore has dimensionsof 15 km along-strike and 8 km down-dip, giving a totalarea of 120 km 2 .Christchurch is located on the Canterbury Plains, a fandeposit resulting from the numerous rivers flowing eastwardfrom the foothills of the Southern Alps (Brown and Weeber1992). In the vicinity of Christchurch, the Canterbury Plains arecomprised of a complex sequence of gravels interbedded with silt,clay, peat, and shelly sands. The fine sediments form aquicludesand aquitards between the gravel aquifers, and with the nearbycoastline to the east, result in the majority of Christchurch havinga water table less than 5 m depth, with the majority of thearea including, and to the east of, the central business districthaving a water table less than 1 m from the surface (Brown andWeeber 1992). The postglacial Christchurch Formation createdby estuarine, lagoonal, dune, and coastal swamp deposits (containinggravel, sand, silt, clay, shell, and peat) is the predominantsurface geology layer in the Christchurch area, which outcropsup to 11 km west of the coast and has a depth of approximately40 km along the coast itself (Brown and Weeber 1992). At thesoutheast edge of Christchurch lies the extinct Banks Peninsulavolcanic complex.STRONG MOTION RECORD PROCESSINGVolume 1 ground motion records were obtained from GeoNet(http://www.geonet.org.nz/) and processed on a record-byrecordbasis. The overall processing methodology adopted iselaborated in Chiou et al. (2008, Figure 4). All ground motionswere processed with a low-pass causal Butterworth filter of50 Hz, and while the corner frequency of the high-pass filterwas record-specific, a frequency of less than 0.05 Hz providedphysically realistic Fourier spectra amplitudes and integrateddisplacement histories for all the near-source ground motions.Owing to the digital nature of all of the instruments, baselinecorrections were found to be unnecessary following theabove filtering. As a result, the processed ground motions canbe considered to provide reliable estimates of peak groundaccelerations (PGA) and spectral ordinates over the range0.01–10 seconds (Douglas and Boore 2010), which are typicallyof engineering interest. It should be noted that the aboveprocedure does not lend itself to the computation of residualdisplacements, which may be non-zero for near-source locations.However, as a result of possible instrument tilting, whichmay be significant at sites where liquefaction occurred, reliablecomputation of such residual displacements may not be possible(Graizer 2005) and is left for future study.854 Seismological Research Letters Volume 82, Number 6 November/December 2011