Here - Stuff

is still a good fit to the horizontal observations (Figure 6). Wesuspect this is due to liquefaction and compaction of underlyingsediments even in regions where major observable groundsurface damage did not occur.The longer-wavelength ALOS data are coherent in partsof central and eastern Christchurch where CSK lost coherencebecause of severe ground damage, including major liquefaction.In particular, there is a clear region of ALOS fringes(Figure 3) west of the Avon-Heathcote estuary and just west ofthe main slip patch in the models of Figures 5–6. We obtainedtwo ALOS pairs, one using a post-earthquake scene only twodays after the earthquake and the other from nearly a monthlater. The two images are quite similar to each other, suggestingthat ALOS is detecting real ground deformation (or at least realphase changes) in this area. In support of this, pixel-tracking(i.e., non-interferometric) analysis of the CSK ascending data(Figure 4) also shows a region of toward-satellite ground motionin this area. We have attempted to model these signals as a smallshallow fault splaying off the main fault plane. Although thisdoes improve the fit to the ALOS data without degrading fitsto the other data, we have not found a solution that provides aclearly significant improvement. More complicated fault geometrymay be necessary to fit all of the details of the surface deformationsouth and east of Christchurch. Also, the deformationdetected by satellite radar and GPS in these regions would nothave been purely fault related, but would include deformationdue to ground damage and phase changes due to variationsof water content in the near surface; the mixture of shallowground damage and deeper fault slip may be difficult to unravel.We have used a uniform elastic half-space for all our modeling.There is in fact significant topography in the region, andthere are both depth and lateral variations in structure. Northof the fault are flat lying gravels and muds over greywacke basement(Forsyth et al. 2008), beneath which is dehydrated oceanicplateau material (Reyners et al. 2011). To the south of thefault are hills that are the remnant of a late Miocene volcanothat formed through the oceanic plateau crust. Future geodeticmodeling should take into account this elastic structure andtopography. However, because there are so many near-field dataconstraining the fault location, we doubt that our conclusionson the fault geometry and slip will be greatly changed by moresophisticated modeling.The geodetic source model presented here is just one partof the still-unfolding story of the earthquake sequence thatbegan in September 2010 and is continuing at the time of thiswriting with a damaging M W 6 aftershock on 13 June 2011.Multiple different fault surfaces have been active so far, andeach of the larger earthquakes has produced radiated energywell above the average expected for the size of the fault. Howand why this large amount of energy has been released shouldbecome clearer with future research.ACKNOWLEDGMENTSWe thank GeoNet, Trimble Navigation NZ Ltd, GeosystemsNZ Ltd, and Global Survey Ltd for providing continuousGPS data, and Josh Thomas, Dave Collett, Paul Denys, KirbyMacLeod, and Linda Alblas for their assistance with the postearthquakeGPS surveys. We thank Stephen Bannister andCaroline Holden for providing comments on the manuscript,and an anonymous reviewer for a number of suggestions thathelped us improve the paper. CSK original data is copyright2011 Italian Space Agency; part was provided by e-GEOS, anASI/Telespazio company, and part was provided under CSKAO PI project 2271. ALOS original data is copyright 2010and 2011 METI and JAXA, distributed by GeoGRID andPASCO. The inversions used Igor Pro (http://www.wavemetrics.com/);figures were prepared using Igor Pro and GMT(http://gmt.soest.hawaii.edu/). Much of this research wasfunded by the New Zealand government. Part of this researchwas performed at the Jet Propulsion Laboratory, CaliforniaInstitute of Technology, under contract with the NationalAeronautics and Space Administration.REFERENCESArnadottir, T., and P. Segall (1994). The 1989 Loma Prieta earthquakeimaged from inversion of geodetic data. Journal of GeophysicalResearch 99, 21,835–21,855.Bannister, S., B. Fry, M. Reyners, J. Ristau, and H. Zhang (2011).Fine-scale relocation of aftershocks of the 22 February M w 6.2Christchurch earthquake using double-difference tomography.Seismological Research Letters 82 (6), 839–845.Beavan, J., S. Samsonov, P. Denys, R. Sutherland, N. Palmer, and M.Denham (2010). Oblique slip on the Puysegur subduction interfacein the 2009 July M W 7.8 Dusky Sound earthquake from GPSand InSAR observations: Implications for the tectonics of southwesternNew Zealand. Geophysical Journal International 183 (3),1,265–1,286; doi: 10.1111/j.1365-246X.2010.04798.x.Beavan, J., S. Samsonov, M. Motagh, L. Wallace, S. Ellis, and N. Palmer(2010). The Darfield (Canterbury) earthquake: Geodetic observationsand preliminary source model. Bulletin of the New ZealandSociety for Earthquake Engineering 43 (4), 228–235.Beavan, J., P. Tregoning, M. Bevis, T. Kato, and C. Meertens (2002).Motion and rigidity of the Pacific plate and implications for plateboundary deformation. Journal of Geophysical Research 107 (B10);doi:10.1029/2001JB000282.Bibby, H. M. (1982). Unbiased estimate of strain from triangulationdata using the method of simultaneous reduction. Tectonophysics82 (1–2), 161–174.Chen, C. W., and H. A. Zebker (2002). Phase unwrapping for large SARinterferograms: Statistical segmentation and generalized networkmodels. IEEE Transactions on Geoscience and Remote Sensing 40,1,709–1,719.Crook, C. N. (1992). ADJCOORD: A Fortran Program for SurveyAdjustment and Deformation Modelling. New Zealand GeologicalSurvey EDS Report 138, Department of Scientific and IndustrialResearch, Geology and Geophysics, Lower Hutt, New Zealand.Dach, R., U. Hugentobler, P. Fridez, and M. Meindl (2007). Bernese GPSSoftware Version 5.0. Bern, Switzerland: Astron. Inst., University ofBern, 612 pp.Darby, D. J., and R. J. Beavan (2001). Evidence from GPS measurementsfor contemporary interplate coupling on the southern Hikurangisubduction thrust and for partitioning of strain in the upper plate.Journal of Geophysical Research 106 (B12), 30,881–30,891.Forsyth, P. J., D. J. A. Barrell, and R. Jongens (2008). Geology of theChristchurch Area. Institute of Geological and Nuclear Sciences1:250,000 Geological Map 16, 1 sheet + 67 pp. Lower Hutt, NewZealand: GNS Science.798 Seismological Research Letters Volume 82, Number 6 November/December 2011