Sub-parallel normal and thrust faulting in the same earthquake?
The Mw 7.8 Kaikoura Earthquake of 14th November 2016 was characterised by a surprising degree of spatial complexity in the surface displacement field in the Marlborough region, South Island, New Zealand. This complexity includes movement on up to 12 faults, besides a high degree of variability in apparent slip along strike of individual faults over relatively short distances. This pattern of surface rupture suggests that non-elastic processes are playing a significant role in controlling displacement.
The location and tectonic setting of the coastal Marlborough region is useful as it displays active deformation of a convergent margin on land (with consequent good access and visibility). The Marlborough Fault Zone (MFZ) is located at the transition from subduction along the Hikurangi Interface to the north, to a transpressional transform (Alpine Fault) to the south. It represents an accretionary wedge, subject to shape adjustments that balance the tendency to thicken by compressive thrusting with topographic reduction caused by gravitational loading, as a function of the frictional properties of the faults involved. The various mountain ranges of the Marlborough region have been constructed by thrusting on a series of sub-parallel thrust faults, that have subsequently evolved to accommodate strike-slip motion. While active uplift of the Papatea Block by up to 8m occurred during this event, it is perhaps surprising that this was accommodated by oblique normal slip on parts of the Kekerengu fault system, effectively producing a large scale pop-up/pop-out structure. Geomorphic evidence makes it clear that similar motion has also been sustained in past earthquakes on these fault segments. Results from field observations, TLS and photogrammetry will be compared to help elucidate the scale and significance of this phenomenon, and the tsunami hazard posed by similar structures in accretionary wedges underwater considered.