Seismic source parameters in New Zealand from broad-band data

Abstract

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 M o -(7.53 f 1.48) is found for the region, where ML is the local magnitude and M , the seismic moment in N m.

Earthquake source mechanism analysis for events between 1992 and 1997 using sparse New Zealand broadband data

“…The waveform inversion in this study used the technique of Mao et al (1994), which is an extension of the Harvard centroid moment tensor (CMT) technique (Dziewonski et al 1981) to local and regional distances. Nformi et al (1996) found the results of this technique and their amplitude ratio method to be in agreement for large events. Some of the source-time functions reported may, however, have been affected by noise in the seismograms.…”

“…Most waveform modelling studies in New Zealand have used teleseismic bodywaves (e.g., Anderson et al 1993;Webb & Anderson 1998). In one study using local data, Nformi et al (1996) used data from a temporary broadband array and the New Zealand network to determine source parameters of 132 local events. The waveform inversion in this study used the technique of Mao et al (1994), which is an extension of the Harvard centroid moment tensor (CMT) technique (Dziewonski et al 1981) to local and regional distances.…”

A new approach for separating mixed model parameters: application to simultaneous inversion of earthquake source parameters

“…A general approach to solve a reduced set of parameters in a non-linear way is to use gradient methods. Such a method for the inversion of the hypocentral relocation, of a double couple, and of the source time function, has been developed by Mao et al (1994) and applied to local events by Nformi et al (1996). In order to take into account the discrepancies between the three classes of physical parameters, the use of a weighting matrix had been proposed by Mao et al (1994).…”