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.

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 2009 July 15 M W 7.8 Dusky Sound earthquake ruptured a ∼80 km long section of the Puysegur subduction interface, where the Australian Plate is obliquely subducted beneath southwestern New Zealand (Fig. 1; Fry et al 2010). It was the largest earthquake in New Zealand since the 1931 February 3, M W 7.8 Napier earthquake and is the largest event ever recorded at the Puysegur subduction zone.…”

Background and delayed-triggered swarms in the central Southern Alps, South Island, New Zealand

“…Similar values were observed on strong motion accelerometers in the area [cf. Fry et al ., , Table ].…”

“…It caused about 2.6 cm of permanent westward coseismic displacement at Haast [ Mahesh et al ., ], approximately 85 km southeast of the center of the SAMBA network. This earthquake stands out from other earthquakes of comparable size because of its waves' large low‐frequency (0.01–0.1 Hz) and relatively small high‐frequency contents (>5 Hz) [ Fry et al ., ]. To calculate the peak static and dynamic stress changes caused by this event (see sections 4.2 and 4.3), we model the mainshock rupture using the Coulomb static stress triggering software and use available information on peak ground accelerations (PGA) and peak ground velocities (PGV) in the study area.…”

“…[]; Fry et al . []] earthquake, that occurred approximately 350 km to the south of the study area, the seismicity rate in the center of the SAMBA network increased significantly. The same was observed following the 3 September 2010 Darfield (Canterbury) earthquake (M W 7.1; Gledhill et al .…”

Spatially and temporally systematic hydrologic changes within large geoengineered landslides, Cromwell Gorge, New Zealand, induced by multiple regional earthquakes

“…Directivity effects, radiation patterns, and nonuniform attenuation structures are not taken into account in this formulation of seismic energy density, despite their likely importance for some of the earthquakes under analysis. The 2009 M w 7.8 Dusky Sound earthquake, for instance, ruptured toward the south‐southwest, away from Cromwell Gorge, such that relatively low accelerations and shaking intensities were experienced across the South Island given the large moment of this earthquake [ Fry et al ., ]. The effects of radiation patterns (i.e., position on the focal sphere) are considered below, but otherwise, the empirical relationship for seismic energy density is used without modification.…”