The GPS velocity field in the North Island of New Zealand is dominated by the long‐term tectonic rotation of the eastern North Island and elastic strain from stress buildup on the subduction zone thrust fault. We simultaneously invert GPS velocities, earthquake slip vectors, and geological fault slip rates in the North Island for the angular velocities of elastic crustal blocks and the spatially variable degree of coupling on faults separating the blocks. This approach allows us to estimate the distribution of interseismic coupling on the subduction zone interface beneath the North Island and the kinematics of the tectonic block rotations. In agreement with previous studies we find that the subduction zone interface beneath the southern North Island has a high slip rate deficit during the interseismic period, and the slip rate deficit decreases northward along the margin. Much of the North Island is rotating as several, distinct tectonic blocks (clockwise at 0.5–3.8 deg Myr−1) about nearby axes relative to the Australian Plate. This rotation accommodates much of the margin‐parallel component of motion between the Pacific and Australian plates. On the basis of our estimation of the block kinematics we suggest that rotation of the eastern North Island occurs because of the southward increasing thickness of the subducting Hikurangi Plateau. These results have implications for our understanding of convergent margin plate boundary zones around the world, particularly with regard to our knowledge of mechanisms for rapid tectonic block rotations at convergent margins and the role of block rotations in the slip partitioning process.

The positions of 115 ground marks in a 150 × 100 km area of oblique continental collision in the central Southern Alps, New Zealand, have been measured by Global Positioning System (GPS) two to four times between 1994 and 1998. Contemporary velocity and strain rate fields derived from these observations are largely invariant along the northeasterly strike of the mountains and Alpine fault. Across strike, more than 60% of the strain occurs within a band from 5 km NW to 20 km SE of the Alpine fault, but significant strain continues at least a further 60 km SE to near the edge of the Southern Alps foothills. Projections of the fault‐parallel and fault‐normal components of velocity onto an Alpine faultnormal profile show that about 85% of the NUVEL‐1A model relative plate motion is observed within the GPS network. The surface displacements in the high strain rate region are well fit by a model in which stable slip or shearing is occurring at 50–70% of the relative plate rate in a region deeper than about 5–8 km on the down‐dip extension of the SE dipping Alpine fault. Material shallower than this is behaving elastically and thus storing elastic strain in the region of the Alpine fault. The longer‐wavelength displacements can be modeled either as distributed deformation beneath the Southern Alps, or by localization of elastic strain around the upper end of a discrete NW dipping fault or shear zone that is slipping stably below about 30 km depth and would outcrop near the SE boundary of the mountains if extrapolated to the surface. Strain determined from a small‐scale survey network crossing the Alpine fault indicates no significant near‐surface aseismic fault slip on the central Alpine fault over the past 25 years. Our results are consistent with independent geological evidence that the central section of the Alpine fault is capable of producing large to great earthquakes.

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The magnitude 8 Wairarapa, New Zealand, earthquake of 1855 was associated with surface rapture along the Wairarapa fault and regional uplift of the southwest of the North Island. Forward elastic dislocation modelling shows that movement on a steeply dipping Wairarapa fault alone cannot account for the recorded deformation data. Modelling of movement on the subduction interface that underlies the Wellington region as well as the Wairarapa fault also fails to produce a satisfactory fit to the data. Although a complex Wairarapa fault model may be able to explain the deformation pattem if its location, subsurface geometry, and slip distribution could be indcpea•denfly constrained, the best effort supix)ned by available data, a flexed model incorporating a left side step of 8 km at the surface, incorrectly locates the deformation. The best fit to the data is obtained from a listtic Wairarapa fault model involving rapture on 0 to 50 km width of the deeper part of the subducfion interface. The shallower part of the subducfion interface, east of the Wairarapa fault, apparently did not rupture in 1855, and the uplift mechmfism for the overlying Aorangi Range remains unexplained. Partitioning of strike-slip and dip-slip components of the relative plate motions may involve separate earthquakes. Seismological verification of listtic fault rapture mechanisms is required to determine the plausibility of the listtic model presented here, because its implications are tlmt the 1855 earthquake did not completely account for fi•e relative plate motion in the region. 12.375confirming the 12-m value. 12,378DARBY AND BEANLAND: • 1855 WAIRARAPA EARTHQUAKE, NEW ZEALAND

Geodetic measurements of deformation across the Taupo back arc and Hikurangi forearc regions of New Zealand are derived from Global Positioning System (GPS) measurements made in 1990 and 1991 and triangulation observations made in the 1920s, 1950s, and 1970s. The GPS horizontal coordinate differences have precisions of 5-6 mm and the triangulation observations have precisions of 0.7-1.2 arc sec. These different kinds of observation allow simultaneous estimation of strain parameters and single-epoch coordinates for a total of 184 stations, at 68 of which GPS observations were made. Under the assumption that no shift of the scale or orientation of the GPS reference frame is embodied in the data from 13 GPS stations within the Taupo Volcanic Zone (TVZ) common to the 1990 and 1991 surveys, the rotational, dilatational and shear components of the deformation rate tensor within the TVZ can be estimated for that 1year interval. The principal extension rate from 1990 to 1991 is 0.21 _+0.09 x 166/yr (68% confidence) at an azimuth of 124_+ 13 ø, corresponding to 8_+4 mm/yr extension over 40 km. Neither the orthogonal principal extension rate, 0.02_+ 0.03 x 10-6/yr, nor the rotation rate, 0.06+0.04/•rad/yr, is significant. The dilatation rate of 0.23+_0.09 x 10'6/yr is therefore produced by uniaxial extension. The data from 75 stations distributed from behind the back arc region and well into the forearc region, which were surveyed in at least two of the triangulation or GPS epochs between the 1920s and 1991, allow the spatial variation of the shear components of the deformation rate tensor to be estimated. The maximum engineering shear rates lie in the range 0.1-0.2 x 10-6/yr within the TVZ and maintain these values, with similar orientations, in the forearc region to the east. The western and southern margins of active extension are reasonably well determined by changes in orientation and magnitude of the shear component. These results provide confirmation of the previously less well-determined deformation field, and in comparison with subduction models indicate that the surface deformation is reflecting variation from north to south of coupling of the subduction plate interface, and that both trench suction and gravitational collapse probably contribute to the extension in the forearc region. Paper number 94JB03265 0148-0227/95/94JB-03265505.00 triangulation observed mainly in the 1920s, 1950s, and 1970s. The GPS measurements were made jointly by the Geology and Geophysics), Victoria University of Wellington, University of Colorado at Boulder, and the University NAVSTAR Consortium (UNAVCO). As well as establishing new GPS sites for future reoccupation, these surveys occupied many of the trigonometric stations established by the Department of Survey and Land Information (DOSLI) (formerly Department of Lands and Survey) for the original first-order survey in the 1920s and 1930s, second-order surveys in the 1950s, and a dedicated survey for crustal deformation in the 1970s. These terrestrial data have previously been analyz...