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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 M_w7.1 Darfield earthquake (09/03/2010 UTC), an M_w6.2 event (02/21/2011 UTC), and an M_w6.0 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 (σ_v = σ₂) with horizontal maximum compressive stress σ₁ trending 115±5°.

The M_w7.1 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 M_w6.2 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 M_w6.0 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.