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

The Alpine fault is the Pacific-Australian plate boundary in the South Isla nd of New Zealand. This study analyzes 195 earthquakes recorded during the 6 month duration of the Southern Alps Passive Seismic Experiment (SAPSE) in 1995/1996 and two M-L 5.0 earthquakes and aftershocks in 1997, which occur red close to the central part of the Alpine fault. Precise earthquake locat ions are derived by simultaneous inversion for hypocenter parameters, a one -dimensional velocity model, and station corrections. Together with focal m echanisms calculated using a first motion and amplitude ratio method, these results provide a picture of the seismotectonics in the central South Isla nd over a 6 month period. Moment tensor inversions of three earthquakes pro vide an independent means of comparison to the focal mechanisms derived usi ng the amplitude/first motion method. To validate our observations over tim e, 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 t hat the Alpine fault releases elastic strain seismically from the surface d own 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 comparabl e to locked sections of the San Andreas fault, with large earthquakes expec ted. Seismicity decreases north of Bruce Bay at the Alpine fault and within a triangular region along the Alpine fault located between the Hope and Po rters Pass fault zones. We interpret this as the result of deformation dist ributed on the Alpine fault and the Hope and Porters Pass fault zones. The base of the seismogenic zone is fairly uniform at 12 km +/- 2km over large parts of the South Island. The high Alps region has a shallower base of the seismogenic zone, indicating localized elevated temperatures east of the A lpine fault. Most of the study region deforms under a uniform stress field with a maximum principal horizontal shortening direction of 110 degrees -12 0 degrees, similar to geodetic observations and plate motions. This confirm s that the region is not undergoing strain partitioning. The earthquake dat a show that the deformation away from the Alpine fault is distributed on ma inly NNE trending thrust faults and strike-slip transfer faults with a maxi mum seismogenic depth of 12 km.