Plate coupling in the northern South Island and southernmost North Island, New Zealand, as illuminated by earthquake focal mechanisms

Abstract

Abstract. Subduction of the Pacific plate in the northern South Island and southernmost NorthIsland of New Zealand is transitional, insofar as the crustal thickness of the Pacific plate increases significantly along strike in the northern South Island. Focal mechanisms of 145 events shallower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes. The stress regime in the subducted plate appears to be dominated by slab pull. T axes in both the upper and lower planes of the dipping seismic zone generally parallel the local dip of the zone, and the average azimuth of these T axes is rotated some 25 ø clockwise out of the direction of dip of the subducted plate. This can be related to the asymmetrical shape of the subducted slab. In contrast, the stress regime in the overlying plate appears to be dominated by subhorizontal compression. Low-angle thrust events near the plate interface in Cook Strait and the southernmost North Island concentrate in two areas which may mark the updip and downdip edges of a locked region identified from Global Positioning System (GPS) observations. An absence of low-angle thrust events near the plate interface in the northern South Island and the tendency of P axes of events in the subducted plate to become more horizontal suggest that plate coupling there is stronger than in the southernmost North Island. Differential coupling at the plate interface provides a viable mechanism for producing the large tectonic rotations seen in the northern South Island.

The Ormond, New Zealand, earthquake of 1993 August 10: Rupture in the mantle of the subducted Pacific plate

“…Wang et al 1995). This situation is different from the northern South Island, where various lines of evidence suggest the plate interface is strongly coupled (Reyners et al 1997). There, the stress regime in the subducted plate is dominated by slab pull.…”

“…The approximate location of the plate interface can be inferred by taking the upper envelope of seismicity within this dipping seismic zone (cf. Reyners et al 1997). Furthermore, the arrival-time inversion for seismic velocity structure of Reyners et al (in prep.…”

“…The occurrence of such an event lends support to the inference that the plate interface in the region is relatively uncoupled (cf. Reyners et al 1997).…”

Plate coupling and the hazard of large subduction thrust earthquakes at the Hikurangi subduction zone, New Zealand

“…Focal mechanisms of individual earthquakes of M^ 2.7 -5.0 in the Marlborough region (Reyners et al 1997b) confi r m n that the plate interface there is currently strongly coupled. The seismic strain regime in the subducted plate is dominated by downdip tension, which can be related to slab pull from the deeper part of the plate.…”

“…The large number of earthquaki recorded, combined with the dense station coverage, h;n allowed arrival time inversions for three-dimensional (3-1 crustal structure (Eberhart-Phillips & Reyners 1997;Lia 1997;Reyners et al in press), while the use of both fir motion polarity data and amplitudes of seismogram envelop* has allowed the determination of individual focal mechanis n for over 250 earthquakes (Reyners et al 1997b; Reyners •- …”