EXAMINATION OF SEISMICITY IN THE CENTRAL ALPINE FAULT REGION, SOUTH-ISLAND, NEW-ZEALAND

We have investigated seismicity in the Alpine Fault region of the Sout h Island of New Zealand by relocating the New Zealand National Seismog raph Network (NZNSN) data for the 3-year period March 1991 through Apr il 1994. Through simultaneous inversion for hypocentres, one-dimension al velocity structure, and station corrections, we obtained velocity a nd station parameters appropriate for earthquake location in this regi on. The 3-year period of relocated seismicity revealed nine earthquake s per year of magnitude 2.5-3.6 along the Alpine Fault. These earthqua kes extend to 10 km depth, and the Alpine Fault provides a northwester n boundary to a region of diffuse seismicity extending 70 km to the so utheast, with a base gradually deepening from 10 km along the Alpine F ault to 20 km at its southeastward extent. Within the diffuse seismici ty there are numerous small clusters which include: the M 5.8 March 19 92 Wilberforce River sequence; continued aftershocks of the M 6.1 June 1984 Godley River earthquake; and other clusters that have their larg est events ranging from M 3.2 to M 4.6. The June 1994 M(w) 6.7 Arthur' s Pass earthquake occurred within the diffuse seismicity region, but t here was no associated cluster in our 3-year sample of background seis micity.