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“Product CSK © ASI, (ItalianSpace Agency), year ofacquisition, 2011, distributedby e-GEOS (an ASI/TelespazioCompany).”10 kmGround displacementaway from satelliteChristchurchcentral businessdistrictFlight directionRadar look directionIncidence angle = 36°Ground displacementtowards satellite▲ ▲ Figure 2. The colored image shows an interferogram derived from CSK X-band radar images acquired on 19 and 23 February 2011,overlaid on shaded topography. Each color cycle (“fringe”) represents 1.55 cm of ground displacement in the line-of-sight (LOS) directionfrom the ground to the satellite. The total LOS displacement between the western edge of the image and central Christchurch ismore than 20 cm. The image is incoherent over most of Christchurch city due to extensive ground and building damage. The order ofthe colors in the fringes indicates whether ground displacement is toward or away from the satellite, as shown by text in two regions.See Figure 3 and Figures S1–S4 for other interferograms.and along-track displacements with a spatial resolution of about100 m. This technique measures the horizontal displacementsin the along-track direction and the line-of-sight displacements(same component as InSAR) with a precision of about 5–20 cm(see Figure 4).EARTHQUAKE SOURCE MODELING FROMDISPLACEMENT DATAGPS data provide a 3D coseismic displacement vector at a setof points, while DInSAR provides coseismic ground displacementin the line-of-sight from the ground to the satellite atpoints throughout the interference pattern, provided the patternremains coherent. For the modeling we define five differentdatasets: 1) 4-hr and 24-hr GPS; 2) 1-hr GPS; 3) CSK ascendingimage; 4) ALOS ascending image; and 5) CSK descendingimage. We apply an overall weighting to each dataset so thateach one has approximately the same misfit ( χ 2 per degree offreedom) in the best-fitting model.Single-fault ModelsThe displacement data are modeled in two steps (Arnadottirand Segall 1994) to solve for the location and geometry of thefault plane and the amount and direction of slip. First, we usednon-linear least-squares inversion software “disloc99” (Darbyand Beavan 2001) to solve for the best-fitting uniform-slip,792 Seismological Research Letters Volume 82, Number 6 November/December 2011