- © 2011 by the Seismological Society of America
Large earthquakes within seismogenic crust are generally thought to require the pre-existence of large fault structures. Such fault structures appear to evolve by the progressive growth and amalgamation of smaller faults and fractures (Cowie and Scholz 1992). In the course of their evolution some components of an evolving fault system may be inherited from previous tectonic episodes while others may be newly formed in the prevailing tectonic stress field. With increasing displacement and amalgamation of sub-structures, fault structures tend to become “smoother,” less complex, and perhaps weaker (Wesnousky 1988).
The 2010–2011 Canterbury earthquake sequence occurred within the upper crust of the South Island of New Zealand around 100 km southeast from the fast-moving (20–30 mm/yr) Alpine and Hope fault strike-slip components of the Pacific-Australia transform fault system linking into the southern Hikurangi Margin subduction zone (Figure 1). As of 15 July 2011, the sequence has included three major shocks: the Mw 7.1 Darfield earthquake (3 September 2010 UTC) followed by an Mw 6.2 event on 21 February 2011 UTC and an Mw 6.0 event on 13 June 2011 UTC, along with a rich aftershock sequence that includes 27 shocks with Mw > 5.0. Rupturing occurred on previously unrecognized faults that appear to be components of a highly segmented E-W structure concealed beneath alluvial cover and/or Neogene volcanics. Some subsurface information is, however, available from seismic reflection lines and gravity surveys (e.g., Field et al. 1989).
Here we seek to demonstrate how this complex sequence has likely arisen through reactivation under the contemporary tectonic stress field of a mixture of comparatively newly formed and older inherited fault structures.
The 2010–2011 Canterbury earthquakes occurred within 30 ± 5 km thick continental crust belonging to the buoyant Chatham Rise plateau contained within the Pacific plate (Eberhart-Phillips …