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.

The Edgecumbe earthquake, 1987 March 2 (ML 6.3), was associated with renewed tectonic rupture on the Edgecumbe Fault, renewed movement on the Onepu and Rotoitipakau Faults, and several new surface breaks-the Awaiti, Otakiri, Te Teko, and OmeheuFaults. These northeast trending tectonic ruptures are widely distributed across the Rangitaiki Plains, range in length from 0.5 to 7 km, and are mostly downthrown to the northwest. They are associated with warping and prominent fissuring. Maximum displacement, 2.5 m vertical and 1.8 m extensional, occurred near the middle of the Edgecumbe Fault trace. Trenching investigations revealed that perhaps two faulting events have occurred in the past 1850 years in addition to the 1987 event; the earlier one is unsubstantiated, but may have occurred at about 1850 years B.P., and another occurred at about 800 years B.P. (the time of deposition of the Kaharoa Ash). The 800 year B.P. event was associated with warping, but little fissuring. An average slip vector, derived mainly from laterally offset cultural features, trends 330°, plunges 55°, and represents up to 3.1 m of normal fault slip on a plane of average strike about 055°. Our proposed fault model has a 55° dipping plane curving upwards to become almost vertical within 10 m of the ground surface. This 55° dip is probably representative to at least 100 m depth. The tectonic effects of the earthquake were influenced by the soft, wet sediments forming the Rangitaiki Plains and the relatively shallow hypocentre, but were otherwise typical of normal faulting events expected in the Taupo Volcanic Zone.

A previous geodetic estimate of 18 mm/yr horizontal extension for the Taupo Volcanic Zone (TVZ) immediately north of Lake Taupo for the period 1949-86 is re-examined for several reasons: this rate has not been confirmed by GPS surveys in the 1990s; newly compiled precise levelling data now allow us to estimate the extent of non-tectonic deformation attributable to the Wairakei geothermal field; and the precise levelling and lake-levelling data reveal a spatial variation in tectonic subsidence that casts doubt on the earlier assumption of homogeneous horizontal strain. We use the vertical and horizontal data to derive a Mogi point source model for the geothermal field, and this model allows us to correct the observed horizontal velocities of survey points. Statistical analysis of the corrected horizontal velocities shows that the strain across the TVZ is not homogeneous. When these factors are accounted for, an extension rate of 8 ± 2 mm/yr (1 SE) can be applicable for both 1949-86 and 1986-97. This is about half the previous estimate, which we now consider to be incorrect. The distribution of deformation differs between these periods, and the seismicity of the region shows temporal variations on a similar time-scale (decades). The extension rate is much greater than can be accounted for by seismic strain release, and the occurrence of historical earthquakes up to M = 6 indicates that a significant part of the measured extension represents seismic strain accumulation. The spatial heterogeneity of the strain partitions the region identically to that derived from geological studies of fault activity. In particular, there is a spatial concentration of extension and tilt about the Whangamata fault system.

The M,.6.3 Edgecumbe earthquake, March 2, 1987, was associated with a 7-km surface rupture of the NE striking Edgexumbe fault and 10 secondary fault ruptures. Average net slip of the main fault at the ground surface was 1.7 m with an azimuth of N30øW. Regional subsidence of up to 2 m affected a large part of the Rangitaiki Plains and the Whakatane graben widened by about 0.7 m in a NW-SE direction. Dislocation modelling suggests the fault has a dip of about 40 ø and extends to more than 6 km depth with 2.7 m normal slip, parameters consistent with seismological data presented in a companion paper. The slip was large for an earthquake of this magnitude in comparison with other notanal faulting events. There is preliminary evidence for both regional postseismic deformation and continued fault slip. The Edgecumbe earthquake illustrates active extension of the Taupo volcanic zone, a back arc basin associated with the Hikurangi subduction zone beneath the North Island of New Zealand. The faulting was in many ways typical of worldwide historical normal faulting earthquakes and suggests that back arc spreading, where onshore, is accommodated by tectonic processes that are similar to those in other extensional environments. INTRODUCTION The Edgecumbe earthquake occurred in New Zealand's Taupo volcanic zone, at the eastern side of the central volcanic region, at 0142 UT, March 2, 1987. This 8-km-deep [Smith and Oppenheimer, 1989; Anderson and Webb, 1989] ML6.3, Ms6.6 event was associated with surface rupture and regional deformation. Landslides, liquefaction, compaction, and moderate to severe structural damage, especially at the towns of Edgecumbe, Te Teko, and Kawerau (Figure 1), also occurred [Franks et al., 1989]. No deaths were directly attributed to the earthquake, and serious injuries were few owing to the largely rural nature of the region and the occurrence of a foreshock (ML5.2) 7 min before the mainshock, which caused people to move outside, particularly from some industrial plants. The most affected region was the low-lying alluvial Rangitaiki Plains, bounded by hilly terrain to the west, south, and east and by the Pacific Ocean to the north (Figure 1). Within a few days after the earthquake, detailed geological, geodetic, and seismological observations were made throughout the affected area [New Zealand Department of Scientific and Industrial Research, 1987] and are reported in vol. 32, 1989, a special edition of the New Zealand Journal of Geology and Geophysics. Important aspects of these studies include the observations of surface ruptures, the existence of previous regional geodetic data, and the rapid installation and survey of geodetic networks to monitor postseismic deformation across the fault ruptures. This paper presents a review of the geological and geodetic data, synthesizes them into a model of faulting consistent with seismological interpretations, and makes comparisons with similar data from other normal faulting earthquakes. The seismological data are reviewed and compared in a companionpaper,...

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