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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. 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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