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Stress Control of an Evolving Strike-Slip FaultSystem during the 2010–2011 Canterbury, NewZealand, Earthquake SequenceRichard Sibson, Francesca Ghisetti, and John RistauRichard Sibson, 1 Francesca Ghisetti, 2 and John Ristau 3INTRODUCTIONLarge earthquakes within seismogenic crust are generallythought to require the pre-existence of large fault structures.Such fault structures appear to evolve by the progressivegrowth and amalgamation of smaller faults and fractures(Cowie and Scholz 1992). In the course of their evolution somecomponents of an evolving fault system may be inherited fromprevious tectonic episodes while others may be newly formedin the prevailing tectonic stress field. With increasing displacementand amalgamation of sub-structures, fault structurestend to become “smoother,” less complex, and perhaps weaker(Wesnousky 1988).The 2010–2011 Canterbury earthquake sequenceoccurred within the upper crust of the South Island of NewZealand around 100 km southeast from the fast-moving (20–30 mm/yr) Alpine and Hope fault strike-slip components ofthe Pacific-Australia transform fault system linking into thesouthern Hikurangi Margin subduction zone (Figure 1). Asof 15 July 2011, the sequence has included three major shocks:the M w 7.1 Darfield earthquake (3 September 2010 UTC) followedby an M w 6.2 event on 21 February 2011 UTC and an M w6.0 event on 13 June 2011 UTC, along with a rich aftershocksequence that includes 27 shocks with M w > 5.0. Rupturingoccurred on previously unrecognized faults that appear to becomponents of a highly segmented E-W structure concealedbeneath alluvial cover and/or Neogene volcanics. Some subsurfaceinformation is, however, available from seismic reflectionlines and gravity surveys (e.g., Field et al. 1989).Here we seek to demonstrate how this complex sequencehas likely arisen through reactivation under the contemporarytectonic stress field of a mixture of comparatively newly formedand older inherited fault structures.1. Department of Geology, University of Otago, P.O. Box 56, Dunedin9054, New Zealand2. Terrageologica, 129 Takamatua Bay Rd., RD1, Akaroa 7581, NewZealand3. GNS Science, Te Pu Ao, P.O. Box 30-368, Lower Hutt, New ZealandTECTONIC/GEOLOGIC SETTINGThe 2010–2011 Canterbury earthquakes occurred within30 ± 5 km thick continental crust belonging to the buoyantChatham Rise plateau contained within the Pacific plate(Eberhart-Phillips and Bannister 2002). Local geology (Figure2) comprises a basement of highly deformed Mesozoic Torlessemetagraywackes and their metamorphosed equivalents atgreater depth, unconformably overlain by a Late Cretaceous–Neogene cover sequence up to 2.5 km thick (Forsyth et al.2008). Polyphase deformation within this basement assemblageincludes accretion, folding and thrusting along the Gondwanamargin, extensional fault structures from Late Cretaceous riftingof the Zealandia microcontinent, and Neogene transpressionacross the Alpine fault system.The cover sequence consists of Late Cretaceous–Paleogeneterrestrial-marine sedimentary units (including varyingthicknesses of Late Cretaceous Mt. Somers calc-alkaline volcanicsand Eocene basalts) overlain by a regressive Miocene-Pliocene clastic sequence that contains the predominantlybasaltic Late Miocene (11–6 Ma) Banks Peninsula volcanics.Thickness variations are partly attributable to deposition asa Late Cretaceous–Paleocene syn-rift sequence accompanyingextensional rifting along the Gondwana margin, whichimposed an extensive fault fabric within the basement (Lairdand Bradshaw 2004). Neogene shortening has led to varyingreactivation of these inherited fault systems. Over the area ofthe Canterbury Plains the older units are largely obscured byPliocene and Quaternary alluvial gravels up to a few hundredmeters thick (Forsyth et al. 2008).CONTEMPORARY STRESS FIELDAvailable evidence on the contemporary regional stress fieldin the central South island (Sibson et al., forthcoming) comesfrom two principal sources summarized in Table 1: 1) stressinversions from earthquake focal mechanisms together withone breakout determination from the Galleon-1 borehole;and 2) axes of maximum contractional strain-rate derived824 Seismological Research Letters Volume 82, Number 6 November/December 2011 doi: 10.1785/gssrl.82.6.824