PLATE COUPLING IN THE NORTHERN SOUTH-ISLAND AND SOUTHERNMOST NORTH-ISLAND, NEW-ZEALAND, AS ILLUMINATED BY EARTHQUAKE FOCAL MECHANISMS

Subduction of the Pacific plate in the northern South Island and south ernmost North Island of New Zealand is transitional, insofar as the cr ustal thickness of the Pacific plate increases significantly along str ike in the northern South Island. Focal mechanisms of 145 events shall ower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes. The stres s regime in the subducted plate appears to be dominated by slab pull. Taxes in both the upper and lower planes of the dipping seismic zone g enerally parallel the local dip of the zone, and the average azimuth o f these T axes is rotated some 25 degrees clockwise out of the directi on of dip of the subducted plate. This can be related to the asymmetri cal 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 t he southernmost North Island concentrate in two areas which may mark t he updip and downdip edges of a locked region identified from Global P ositioning System (GPS) observations. An absence of low-angle thrust e vents near the plate interface in the northern South Island and the te ndency of P axes of events in the subducted plate to become more horiz ontal suggest that plate coupling there is stronger than in the southe rnmost North Island. Differential coupling at the plate interface prov ides a viable mechanism for producing the large tectonic rotations see n in the northern South Island.