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will not be formed if it is energetically more favorable to slip onnon-optimally oriented planes (Anderson 1951). Reactivationof non-Andersonian faults is observed in numerous tectonicenvironments including Iran (e.g., Byerlee 1978) and the Aegean(e.g., Berberian 1995). The steep dip of reverse faults observed inaftershock and mainshock focal mechanisms along with geodeticallyderived fault geometries for the Darfield and Christchurchearthquakes strongly suggest Cretaceous-Oligocene faults,formed during formation of the Torlesse terrain and laterbreakup of the Rangitata Orogen (Jackson 1994), were seismicallyreactivated during the 2010–2011 Canterbury earthquakesequence. Likewise, the high stress drops, particularly for theDarfield event, calculated for each event (Fry et al. 2011, page846 of this issue) suggest reactivation of high-friction faultsunder low strain rates compared to faults in the Marlboroughfault zone or Puysegur and Hikurangi subduction zones.The lack of many aftershocks west of the Darfield earthquakein the Southern Alps (Figure 1), where thrust faultsare oriented more N-S and dip at lower angles (