observations and other source studies that suggest the occurrenceof at least one secondary event make this earthquakevery segmented for its magnitude. Such rupture segmentationhas already been observed regionally for the Darfield Mw 7.1earthquake, as well as in preliminary studies of another largeaftershock, the Mw 6.0 June earthquake (Beavan, personalcommunication 2011). Earthquakes in Canterbury are alsoparticularly energetic (larger ones all present a high Me/Mwratio (Fry and Gerstenberger 2011, page 833 of this issue). Theregion is characterized by the presence of a very dehydrated andbrittle structure, the Hikurangi plateau (Reyners and Cowan1993). This structure pushes the regional brittle-ductile transitiondeeper (35 km), fostering strain release following a largeevent through the generation of aftershocks rather than aseismicslip. The Christchurch area is also marked by the presenceof the intraplate volcanism formation of the now extinct BanksPeninsula volcano, about 11 My ago (Timm et al. 2009). Theintrusion of the volcano has not only highly segmented faultsin the region near Christchurch, but may have also broughtcloser to the surface the very brittle and dehydrated Hikurangiplateau. This segmented and energetic fault system may explainan event like the February earthquake.Finally, we hope to bypass issues arising from variable siteconditions by first obtaining a better defined local velocitymodel from aftershock studies (Reyners et al. 2011; Bannisteret al. 2011, page 839 of this issue), and second by using thelarge database of well-recorded aftershocks as empirical Green’sfunctions. This will allow us to increase the frequency bandwidthof the waveforms and hence define the slip history inmore details.ACKNOWLEDGMENTSThe author would like to acknowledge the anonymous reviewerfor significantly improving this manuscript. This study madeuse of SAC and GMT software.REFERENCESBannister, 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, 839–845.Beavan, J., E. Fielding, M. Motagh, S. Samsonov, and N. Donnelly(2011). Fault location and slip distribution of the 22 February 2011M W 6.2 Christchurch, New Zealand, earthquake from geodeticdata. Seismological Research Letters 82, 789–799.Berrill, J., H. Avery, M. B. Dewe, A. Chanerley, N. Alexander, C. Dyer,C. Holden, and B. Fry (2011). The Canterbury AccelerographNetwork (CanNet) and some results from the September 2010, M7.1 Darfield earthquake. In Proceedings, Ninth Pacific Conference onEarthquake Engineering, NZSEE, Auckland, New Zealand paperno. 181.Bouchon, M. (1981). A simple method to calculate Green’s functions forelastic layered media. Bulletin of the Seismological Society of America71, 959–971.Cousins, J., and G. McVerry (2010). Overview of strong motion datafrom the Darfield earthquake. Bulletin of the New Zealand Societyfor Earthquake Engineering 43 (4), 222–227.Di Carli, S., C. François-Holden, S. Peyrat, and R. Madariaga (2010).Dynamic inversion of the 2000 Tottori earthquake based on ellipticalsubfault approximations. Journal of Geophysical Research 115,B12328; doi:10.1029/2009JB006358.Fry, B., R. Benites, M. Reyners, C. Holden, A. Kaiser, S. Bannister, M.Gerstenberger, C. Williams, J. Ristau, and J. Beavan (2011). Verystrong shaking in the New Zealand earthquakes. Submitted to Eos.Fry, B., and M. Gerstenberger (2011). Large apparent stresses from theCanterbury earthquakes of 2010 and 2011. Seismological ResearchLetters 82, 833–838.Gledhill, K., J. Ristau, M. Reyners, B. Fry, and C. Holden (2011).The Darfield (Canterbury, New Zealand) Mw 7.1 earthquake ofSeptember 2010: A preliminary seismological report. SeismologicalResearch Letters 82 (3), 378–386; doi:10.1785/gssrl.82.6.378.Hancox, G., C. Massey, and N. Perrin (2011). Landslides and relatedground damage caused by the Mw 6.3 Christchurch earthquakeof 22 February 2011. Geomechanics News (New Zealand) 81 (June2011), 53–67.Kaiser, A. E., R. A. Benites, A. I. Chung, A. J. Haines, E. Cochran, andB. Fry (2011). Estimating seismic site response in Christchurch city(New Zealand) from dense low-cost aftershock arrays. ExtendedAbstract of the Fourth IASPEI/IAEE International Symposium onthe Effects of Surface Geology on Seismic Motion, August 23–26,Santa Barbara, California.Kaiser, A., C. Holden, J. Beavan, D. Beetham, R. Benites, A. Celentano,D. Collett et al. (2011). The February 2011 Christchurch earthquake:A preliminary report. Submitted to New Zealand Journal ofGeology and Geophysics.Reyners, M. E., and H. Cowan (1993). The transition from subductionto continental collision: Crustal structure in the north Canterburyregion, New Zealand. Geophysical Journal International 115 (3),1,124–1,136.Reyners, M., D. Eberhart-Phillips, and S. Bannister (2011). Trackingrepeated subduction of the Hikurangi Plateau beneath NewZealand. Earth and Planetary Science Letters; doi:10.1016/j.epsl.Sambridge, M. (1999). Geophysical inversion with a neighbourhoodalgorithm—I. Searching a parameter space. Geophysical JournalInternational 138, 479–494.Sibson, R., F. Ghisetti, and J. Ristau (2011). Stress control of an evolvingstrike-slip fault system during the 2010–2011 Canterbury, NewZealand, earthquake sequence. Seismological Research Letters 82,824–832.Timm, C., K. Hoernle, P. Bogaard, I. Bindeman, and S. Weaver (2009).Geochemical evolution of intraplate volcanism at Banks Peninsula,New Zealand: Interaction between asthenospheric and lithosphericmelts. Journal of Petrology 50 (6), 989–1,023.GNS Science1 Fairway DriveLower HuttAvalon 5010 New Zealandc.holden@gns.cri.nz788 Seismological Research Letters Volume 82, Number 6 November/December 2011
Fault Location and Slip Distribution of the22 February 2011 M W 6.2 Christchurch, NewZealand, Earthquake from Geodetic DataJohn Beavan, Eric Fielding, Mahdi Motagh, Sergey Samsonov, and Nic DonnellyJohn Beavan, 1 Eric Fielding, 2 Mahdi Motagh, 3 Sergey Samsonov, 4 andNic Donnelly 5EOnline material: Additional figures showing interferogramsand fault models; data tablesINTRODUCTIONThe 22 February (local time) M W ~6.2 Christchurch earthquakeoccurred within the aftershock region of the 4 September2010 M W 7.1 Darfield (Canterbury) earthquake (Gledhill etal. 2011). Both the Darfield and Christchurch earthquakesoccurred on previously unknown faults in a region of historicallylow seismicity, but within the zone of plate boundarydeformation between the Pacific and Australian plates. TheDarfield earthquake caused surface rupture up to 5 m (Quigleyet al. 2010, forthcoming), but none has been observed associatedwith the Christchurch earthquake. Geodetic dataindicate that strain has been slowly accumulating within theregion (Wallace et al. 2007; Beavan et al. 2002), and the presenceof active subsurface faults was known or suspected (e.g.,Pettinga et al. 2001). Earthquakes of magnitude up to 7.2 inthis region had been allowed for in the national seismic hazardmodel (Stirling et al. 2002), but the observed high apparentstresses (Fry and Gerstenberger 2011, page 833 of this issue)and high ground accelerations (Fry et al. 2011, page 846 ofthis issue) had not been anticipated, particularly those experiencedin the Christchurch event. These and other factors(Fry and Gerstenberger 2011, page 833 of this issue; Fry et al.2011, page 846 of this issue; Holden 2011, page 783 of thisissue), plus the close proximity of the February earthquake to1. GNS Science, Lower Hutt, New Zealand2. Jet Propulsion Laboratory/Caltech, Pasadena, California, U.S.A.3. Helmholtz Centre Potsdam, GFZ German Research Centre forGeosciences, Potsdam, Germany; also at Department of Geomaticsand Surveying Engineering, University of Tehran, Tehran, Iran4. European Center for Geodynamics and Seismology, Walferdange,Luxembourg; now at Canada Centre for Remote Sensing, Ottawa,Canada5. Land Information New Zealand, Wellington, New ZealandChristchurch city center, were responsible for the major damagecaused by the earthquake (e.g., Kaiser et al. 2011).A large amount of geodetic ground-displacement datais available to constrain the source of the earthquake, in partbecause we reoccupied nearly 200 GPS sites that had beenobserved following the Darfield earthquake, and in partbecause a number of space agencies collected synthetic apertureradar (SAR) data over the source area that we were able to usein differential interferometric SAR (DInSAR) processing. Thegeodetic data were collected one day to seven weeks followingthe February earthquake, so they include ground deformationdue to aftershocks, in particular the M W 5.8 and M W 5.9 eventsthat occurred within two hours of the mainshock.To first order, the earthquake source can be modeled as aplanar fault striking ~59° and dipping ~69° to the southeast.The peak slip of 2.5–3 m is a mixture of reverse and right-lateralslip and is located ~7 km east-southeast of Christchurchcity center at a depth of ~4 km. Slip of ~1 m reaches within~1 km of the ground surface. The slip near the southwest endof the plane is approximately right-lateral with magnitude ~1m. The geodetic data are significantly better fit by two faultplanes, a compact region of oblique slip on the fault describedabove, plus right-lateral strike slip on a near-vertical fault to itssouthwest that coincides with the locations of the two majoraftershocks and with a trend of smaller aftershocks. A lobe ofground uplift seen in some of the SAR data (e.g., Figure 4) justwest of the main slip patch is not well modeled, and suggestssome slip may also have occurred elsewhere, perhaps on a splayoff the main fault plane.GEODETIC DATAWe use campaign GPS data collected between 28 Februaryand 14 April from 57 sites (Figure 1) that were also occupiedfollowing the September 2010 Darfield earthquake(Beavan, Samsonov, Motagh, et al. 2010). We also use continuousGPS (cGPS) data from five regional sites operated byGeoNet (http://www.geonet.org.nz) for Land Informationdoi: 10.1785/gssrl.82.6.789Seismological Research Letters Volume 82, Number 6 November/December 2011 789
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Fine-scale Relocation of Aftershock
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REFERENCESAvery, H. R., J. B. Berri
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Near-source Strong Ground MotionsOb
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(A)Magnitude, M w876542009 NZdataba
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Vertical-to-horizontal PGA ratio543
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REFERENCESAagaard, B. T., J. F. Hal
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Spectral Acceleration (0.3 s), (g)I
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Spectral Acceleration (3 s), (g)In[
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TABLE 1Mean (μ LLH ) and standard
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Strong Ground Motions and Damage Co
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high, but there were no buildings d
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Soil Liquefaction Effects in the Ce
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Location of structures illustrated
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Comparison of Liquefaction Features
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Use of DCP and SASW Tests to Evalua
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TABLE 1Damage severity categories (
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only minor damage, mostly to their
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80°100°120°140°EXPLANATIONBorde
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Chang, K. H. (1997). Korean peninsu
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Wheeler, R. L. (2008). Paleoseismic
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A significant outcome of this study
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TABLE 1 (continued)Earthquakes for
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Meeting CalendarM E E T I N GC A L
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Seismological Research Letters (SRL
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Christa von Hillebrandt-Andrade, Pr