The Mw 7.6 Dusky Sound earthquake of 2009

Abstract

The Mw 7.6 Dusky Sound earthquake of July 15th, 2009, was the largest magnitude earthquake in New Zealand since the devastating 1931 Hawke’s Bay event (Ms 7.8). The earthquake was sufficiently large to generate at least a 2.3 m wave at Passage Point. Despite its large magnitude, this event resulted in relatively minimal damage when compared to worldwide events of a similar size. This can be explained as a fortunate combination of the sparse population of the area and the specific physical characteristics of the earthquake. Centroid Moment Tensor (CMT) solutions define the rupture surface as a low-angle plane and finite fault inversions confirm the slip occurred on the interface between the eastward-subducting Australian plate and overriding Pacific plate, initiating at about 30 km depth and rupturing upward and southwestward to about 15 km depth. The oceanward rupture directivity likely contributed to the lower intensity of measured ground motion than might be expected for such a large, shallow event. The amount of radiated seismic energy from the earthquake was relatively small, and far fewer landslides were triggered from this event than from the 2003 Mw 7.2 Fiordland event.