Oblique slip on the Puysegur subduction interface in the 2009 July MW 7.8 Dusky Sound earthquake from GPS and InSAR observations: implications for the tectonics of southwestern New Zealand

The M-W 7.8 Dusky Sound earthquake of 2009 July 15 was the largest earthquake in New Zealand in the past similar to 80 yr and is the only major subduction interface earthquake in the New Zealand historical record. We have estimated the coseismic and early post-seismic slip distribution in the earthquake by inversion of GPS and differential interferometric synthetic aperture radar (DInSAR) observations. We show that slip during the earthquake was highly oblique and essentially in the Australia-Pacific relative plate motion direction. Failure occurred on a ca. 80 by 50 km patch of the subduction interface with maximum slip 5-6 m. In this region, the offshore Alpine Fault carries up to similar to 30 mm yr(-1) of slip before terminating at Resolution Ridge near the southern end of the 2009 slip patch. The southernmost part of the Alpine Fault trace lies near the up-dip end of the 2009 slip patch and the fault probably exists only within the crust of the over-riding plate in this region, terminating at shallow depth where it meets the subduction interface. There is no evidence for rupture of the Alpine Fault during the 2009 event. However, Coulomb stress changes from the subduction earthquake have brought this section of the Alpine Fault closer to failure. In contrast to many subduction zones undergoing oblique subduction, where the slip in major earthquakes is partitioned into more-or-less pure thrust earthquakes on the subduction interface and strike-slip earthquakes in the backarc, this is a case where the majority of the subduction interface slip is not partitioned, with only a shallow convergence zone west of the strike-slip Alpine Fault undergoing contraction approximately normal to subduction zone strike. North of the 2009 earthquake region the Alpine Fault lies further east relative to the subduction interface and partitioning between strike-slip on the Alpine Fault and trench-normal thrusting on the subduction interface takes place in a more typical fashion. Our results provide the first clear demonstration of non-partitioned oblique slip between the Australian and Pacific plates offshore of southern New Zealand.