Examination of seismicity in the central Alpine Fault region, South Island, New Zealand

Abstract

Abstract We have investigated seismicity in the Alpine Fault region of the South Island of New Zealand by relocating the New Zealand National Seismograph Network (NZNSN) data for the 3‐year period March 1991 through April 1994. Through simultaneous inversion for hypocentres, one‐dimensional velocity structure, and station corrections, we obtained velocity and station parameters appropriate for earthquake location in this region. The 3‐year period of relocated seismicity revealed nine earthquakes per year of magnitude 2.5–3.6 along the Alpine Fault. These earthquakes extend to 10 km depth, and the Alpine Fault provides a northwestern boundary to a region of diffuse seismicity extending 70 km to the southeast, with a base gradually deepening from 10 km along the Alpine Fault to 20 km at its southeastward extent. Within the diffuse seismicity there are numerous small clusters which include: the M 5.8 March 1992 Wilberforce River sequence; continued aftershocks of the M 6.1 June 1984 Godley River earthquake; and other clusters that have their largest events ranging from M 3.2 to M 4.6. The June 1994 Mw 6.7 Arthur's Pass earthquake occurred within the diffuse seismicity region, but there was no associated cluster in our 3‐year sample of background seismicity.