Seismic source parameters in New Zealand from broad-band data

Abstract

SUMMARY The Leeds Tararua array of nine broad-band, three-component seismometers was deployed in southern North Island, New Zealand, from 1991 January to 1992 September and recorded many local events. We selected 111 events with local magnitude 12.5 and good station coverage, and attempted to find their source mechanisms by using conventional first motions and by matching of the vertical component waveforms to Rayleigh-mode synthetics. 90 source mechanisms were found: 32 from the method of first motions, 18 by waveform matching, and 40 using both methods. A comparison of mechanisms for events using both methods suggests that waveform matching is reliable even for small events where the coverage on the focal sphere does not allow a firstmotion mechanism to be found. The mechanisms are of all types but with a predominance of normal faulting with one of the planes aligned approximately NE-SW. There are many dip-slip events. Deep (60-80 km) events are mostly thrusts, and there is some evidence of a trend from normal faulting in the shallow part of the subducted slab and upper seismic plane to thrusts in the deeper part, although the pattern is not clear cut. A complex stress pattern is to be expected in this region, which is near the southern end of the Tonga-Kermadec-Hikurangi subduction zone. The magnitudemoment relation ML = (0.84 f 0.23) log Mo - (7.53 f 1.48) is found for the region, where ML is the local magnitude and M, the seismic moment in N m.