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-43.45km0 10 200-43.50-50degrees N-43.5503000200-43.60100200 mm observed200 mm modelled172.55172.60172.65 172.70degrees E172.75172.80▲ ▲ Figure 7. Observed (blue arrows) and modeled (red arrows) vertical displacements for the model of Figure 6. Predicted model displacementsare also shown as contours with 50 mm spacing. Central Christchurch shown by solid black square. An extensive regioneast of central Christchurch shows subsidence exceeding the model predictions, probably as a result of ground failure due to liquefaction,lateral spreading, and compactionthe GPS data are strongly downweighted, the maximum slipdecreases by 10–15% and its depth increases by about 0.5 km.In either case the goodness of fit of the non-downweighted datadoes not change significantly from the original solution. Wetake this as evidence that the solution is not strongly dependenton a particular dataset.DISCUSSIONThe Christchurch earthquake occurred within the wider aftershockregion of the September 2010 Darfield earthquake, andvery close to a strongly felt M W 5.1 aftershock (http://www.geonet.org.nz/earthquake/quakes/3368445g.html) that occurredwithin a few days of the Darfield mainshock. This indicatesthat stress changes due to Darfield almost immediately causedsignificant earthquake activity in the vicinity of the futureFebruary earthquake. Calculations by ourselves and others(e.g., Zhan et al. 2011, page 800 of this issue) show positive butvery small Coulomb stress changes from Darfield in the regionof the February quake; these results do not highlight easternChristchurch as a region of large Coulomb stress increase. TheChristchurch event seems less complex than Darfield, withmost of the surface deformation (away from the liquefactionregions) explicable by slip on two sub-parallel fault planes; theDarfield event involved several reverse fault segments in additionto the main strike-slip fault.An inversion for fault slip kinematics using strong-motiondata is reported by Holden (2011, page 783 of this issue),using a fault geometry based on the geodetic solution. As wellas revealing details of the rupture process, she finds a similarfault slip distribution, depth, and magnitude, though a slightlyhigher ratio of reverse faulting to strike-slip (rake 135° comparedto 145°–150° on the main slip patch of the geodetic model) anda larger maximum slip (more than 4 m compared to 2.5–3 mfor the geodetic model). This provides a degree of confidencein both the geodetic and strong-motion models, but indicatesthere are still differences to be resolved with future work.Both the CSK ascending and descending datasets fit wellwith the majority of the GPS data, leading to generally lowresiduals between model and observations (Figures S1, S2). Theascending ALOS data have a slightly worse fit (Figure S3), butthis mostly occurs in regions where the ALOS data are coherentand the CSK data are not. Some of the GPS stations in thelow-lying areas between central Christchurch and the coastalso have large residuals to the model (Figures 6–7). The GPSdata also show a significant region of ground subsidence in centralChristchurch (Figure 7) amounting to tens of centimetersin excess of what is modeled, even in regions where the modelSeismological Research Letters Volume 82, Number 6 November/December 2011 797