INCREMENTAL EVOLUTION OF AN IMMATURE FAULT SYSTEM BY CONJUGATE STRIKE-SLIP FAULTING: THE 2010-2011 CANTERBURY, NEW ZEALAND, EARTHQUAKE SEQUENCE
Upper crustal fault structures evolve through progressive growth and amalgamation of smaller faults and fractures, some inherited from previous tectonic episodes while others may be new-formed in the prevailing stress field. The 2010-2011 Canterbury earthquake sequence includes the Mw7.1 Darfield earthquake (09/03/2010 UTC), an Mw6.2 event (02/21/2011 UTC), and an Mw6.0 event (06/13/2011 UTC) within a protracted aftershock sequence. The epicentral area in the central South Island lies about 100 km SE of the fast-moving strike-slip fault system defining the Pacific-Australia transform boundary. Previously unrecognised faults hidden below alluvial cover and/or Neogene volcanics have been activated within a wrench regime (σv = σ2) with horizontal maximum compressive stress σ1 trending 115±5°.
The Mw7.1 Darfield mainshock gave rise to right-lateral strike-slip of up to 5 m along the segmented rupture trace of a subvertical fault striking 085±5°, mapped for c. 30 km across the Canterbury Plains and lying at c. 30° to inferred σ1. The Mw6.2 aftershock involved dextral-reverse oblique slip on an ENE-trending en echelon aftershock lineament linked to the tip of the mainshock rupture by a dilational stepover. Left-lateral strike-slip occurs on a subordinate set of SE-SSE aftershock lineaments conjugate to and at 50-70° to the principal dextral rupture, one being responsible for the Mw6.0 aftershock crosscutting the main E-W structure. Continued ‘Andersonian’ conjugate faulting implies that displacement weakening leading to preferential failure has yet to reach the stage where one fault set dominates the other. Amalgamation of inherited and new-formed fault components, together with the creation of contractional jogs by mutual crosscutting of the conjugate fault sets, has led to an immature, segmented fault system prone to high stress-drop rupturing.