Surface faulting associated with the 2016 Mw 7.8 Kaikoura earthquake: complexity of ruptures, 3D structure, geological history and fault source definition

Well-studied historic large to great earthquakes have typically had a complex surface fault rupture pattern (e.g. Gobi-Altai, 1957; Denali, 2002) and the surface faulting associated with the 2016 Kaikoura earthquake follows this trend with perhaps even greater observed complexity. Multiple ruptures with widely varying slip types occurred discontinuously over a region c. 200 km long and 30 km wide in northeastern South Island, New Zealand. The rupture propagated from southwest to northeast across many separate faults (13 with surface slip of >1.5m) with evidence for a shallowly dipping detachment fault, possibly the plate interface, also slipping co-seismically. In earthquake hazard studies, surface faulting patterns, including fault bends and step overs and relations between fault length and single event displacement, are often used to define fault rupture sources. The Kaikoura surface rupture data appear to have broken many if not most of these 'rules'. However, in the Kaikoura case, consideration of the regional tectonic history coupled with crustal geophysics suggest more fault connections at seismogenic depths than are apparent from the surface rupture pattern. Four distinct phases of deformation between the late Mesozoic and present have resulted in a complex array of fault types and geometries. Between the currently active strike-slip and reverse faults, the greywacke basement is generally steeply dipping with steep zones of intense broken formation/melange. Listric faults that have imbricated these strata are implied by the uniform metamorphic grade across great thicknesses. Some major fault dislocations are also indicated by seismic tomography velocity variations in the mid-upper crust. These factors need to be considered, in addition to surface fault discontinuity criteria, for fault rupture source characterisation in domains of low fault dips and tectonic overprinting with the possibility of listric geometries or crustal detachments at seismogenic depths.