[1] The Hikurangi subduction margin, New Zealand, has not experienced any significant (>M w 7.2) subduction interface earthquakes since historical records began $170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrust's seismogenic zone.Components: 20,324 words, 15 figures.

The Wellington region lies on the border between the Pacific and Australian plates, the former being subducted. The plate interface lies at shallow depth under the region (10-60 km) so that detailed and long-term monitoring of the seismicity of the zone where large thrust earthquakes are expected t o occur is possible. Hypocentres for events recorded by a 12 station seismic network for the period 1978-1982 (7231 events) have been refined by the use of a threedimensional velocity model derived from a simultaneous inversion of arrival-time data from 129 local events for both hypocentres and velocity. The hypocentres show that the subducting Pacific Plate is defined by a NW dipping zone of relatively intense activity. The plate interface, as given by the upper envelope of this activity, is offset vertically, by 7 km on average, along a NW-SE line through Cook Strait parallel to the found in the plate interface might serve to limit the rupture areas of such events, as may have been the case in 1855. It might also be the focus of possible precursory activity. These results will provide a basis for long-term monitoring of the seismicity of the Wellington region so that comparisons of activity throughout the seismic cycle can be made.

SUMMAR YThis report summarizes a variety of issues concerning the development of statistical versions of the so-called`accelerated moment release model' (AMR model). Until such statistical versions are developed, it is not possible to develop satisfactory procedures for simulating, ®tting or forecasting the model. We propose a hierarchy of simulation models, in which the increase in moment is apportioned in varying degrees between an increase in the average size of events and an increase in their frequency. To control the size distribution, we propose a version of the Gutenberg±Richter power law with exponential fall-off, as suggested in recent papers by Kagan. The mean size is controlled by the location of the fall-off, which in turn may be related to the closeness to criticality of the underlying seismic region. Other points touched on concern the logical structure of the model, in particular the identi®ability of the parameter assumed to control the size of the main shock, and appropriate procedures to use for simulation and estimation. An appendix summarizes properties of the Kagan distribution. The simulations highlight the dif®culty in identifying an AMR episode with only limited data.

Abstract. The aftershock distribution of the 1994 Arthur's Pass earthquake, Mw6.7, is unusual for a reverse faulting event in that it extends 12 km NNW and 30 km SSE of the actual fault plane, which strikes NE-SW. We have used several methods to infer the regional stress field in the region, including geodetic results, earthquake mechanisms, and inversion of P wave polarity data for the stress tensor orientation. The inversion method is new and does not require the focal mechanisms of the events used. It also incorporates the Coulomb failure criterion. All results point to a stress field favoring strike-slip faulting, not thrusting, with near-horizontal cvl and cv3 principal axes striking at 298 ø and 28 ø . Using dislocation theory, we calculate the stress induced

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.