A focused look at the Alpine fault, New Zealand: seismicity, focal mechanisms, and stress observations

Abstract. The Alpine fault is the Pacific-Australian plate boundary in the South Island of New Zealand. This study analyzes 195 earthquakes recorded uring the 6 month duration of the Southern Alps Passive Seismic Experiment (SAPSE) in 1995/1996 and two Mr. 5.0 earthquakes and aftershocks in 1997, which occurred close to the central part of the Alpine fault. Precise arthquake locations are derived by simultaneous inversion for hypocenter parameters, a one-dimensional velocity model, and station corrections. Together with focal mechanisms calculated using a first motion and amplitude ratio method, these results provide a picture of the seismotectonics in the central South Island over a 6 month period. Moment tensor inversions of three earthquakes provide an independent means of comparison to the focal mechanisms derived using the amplitude/first motion method. To validate our observations over time, we compare the SAPSE seismicity with the seismicity recorded by the New Zealand National Seismic Network (NZNSN) and a local network at Lake Pukaki east of the Southern Alps (6 months versus 8 years). Our study indicates that the Alpine fault releases elastic strain seismically from the surface down to 10-12 km depth between Milford Sound in the south and the Hope fault in the north. The seismicity rate of the Alpine fault is low but comparable to locked sections of the San Andreas fault, with large earthquakes expected