2009 July 15 Fiordland, New Zealand, Earthquake

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July 17, 2009
Strong Motion Seismology Group

An earthquake of Mw 7.8 (Global CMT Project) occurred near the west coast of the South Island of New Zealand (region known as Fiordland) at 09:22:29 UTC on July 15, 2009. According to the USGS preliminary report, the earthquake epicenter is located at the boundary between the Australian and Pacific plates in a complex region defined by the transition from an oblique subduction of Australian plate beneath the Pacific plate in the south (along the Puysegur Trench) to the strike-slip motion further north along the South Island (Alpine fault). The USGS reported that the event was generated by a thrust mechanism on the subduction interface between the Pacific and Australian plate. The preliminary location of the hypocenter is lat = 45.750 S, lon = 166.577 E, and depth = 12 km.

To investigate the source process of this earthquake, we have performed the teleseismic waveform inversion of Kikuchi and Kanamori (1991) using displacement records observed at FDSN stations from IRIS DMC. The 42 vertical components of the P-wave data at epicentral distances of 35-100 deg. were used in the inversion (Fig. 1). We fixed the epicenter determined by USGS and search for the focal depth to minimize the waveform fit. The residuals of both the point source and the finite fault inversion prefer a low-angle east-dipping fault plane (strike = 27 deg., dip = 32 deg.) This is also supported by the aftershock distribution (Fig. 2). The focal depth was estimated to be 30 km.

In the obtained slip distribution over the fault plane, we can find a single large asperity. The peak slip of 4.8 m appears on the shallow part of the fault plane, at the depth of 10-15 km. The results of the inversion are summarized in Fig. 3.

Fig. 1. Stations used for the teleseismic waveform inversion.

Fig. 2. Distribution of aftershocks (black dots) and cross sections along the lines A-A' (red dots).

The location of the July 15, 2009 New Zealand earthquake (red star). Orange circles correspond to the aftershocks reported by USGS. Heavy gray line correspond to the plate boundaries of the Australian (AU) and Pacific (PA) plates after Bird (2003).

Fig. 3. Source time function, point source solution, slip distribution, and comparison of the obsereved and synthetic displacement waveforms.

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