Strain distribution across the Australian‐Pacific plate boundary in the central South Island, New Zealand, from 1992 GPS and earlier terrestrial observations

Abstract

As part of a geodetic experiment aimed at understanding the deformation associated with the Australian‐Pacific plate boundary in the South Island of New Zealand, in 1992 we reoccupied using Global Positioning System (GPS) techniques a first‐order triangulation and trilateration network established in 1978 between Christchurch on the east coast and Hokitika on the west. The network crosses the South Island a few tens of kilometers southwest of the region where the plate boundary changes from a single, throughgoing oblique slip fault, the Alpine fault, to a series of subparallel strike‐slip faults, the Marlborough faults. The GPS data have been analyzed as daily network solutions, with about 12 stations in each solution. RMS repeatabilities for stations with multiple occupations are 4 mm, 8 mm, and 15 mm in the north, east, and vertical components, respectively. The observed strain across the network is consistent with the entire NUVEL‐IA Pacific‐Australia plate velocity being accommodated on land across this part of the South Island. Shear strain rates derived from the GPS and terrestrial data show that the highest strain rate (> 0.4 μrad/yr) occurs in the region of the Southern Alps and Alpine fault. This rate is about two thirds of the rate predicted from the NUVEL‐IA plate velocity model assuming that all the plate boundary deformation occurs across this region, implying that about two thirds of the plate motion is accommodated in the vicinity of the Alpine fault. Significant shear strain rates greater than 0.2 μrad/yr are also observed farther east, particularly in the region of the Porters Pass‐Amberley fault zone, demonstrating that this zone accommodates a significant part of the plate boundary deformation. Using dislocation modeling, we show that the variation in fault‐parallel shear strain rates across the Alpine fault is reasonably consistent with a nonoptimized dislocation model involving a 50° dipping fault that is presently locked to a depth of ∼12 km. One explanation for this observation is that the upper ∼12 km of the crust in this area is storing most of the right‐lateral shear component of the relative plate motion as elastic energy that will be released in a future major earthquake. The variation in the other component of shear strain, which may be interpreted as fault‐normal relative contraction, is not explained by a simple dislocation model, implying that the normal component of plate motion is taken up in other ways.