TERRESTRIAL AND GPS MEASUREMENTS OF DEFORMATION ACROSS THE TAUPO BACK-ARC AND HIKURANGI FORE-ARC REGIONS IN NEW-ZEALAND

Geodetic measurements of deformation across the Taupo back are 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 are 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 tenser within the TVZ can be estimated for that 1-year interval. The principal extension rate from 1990 to 1991 is 0.21 +/- 0.09 x 10(-6)/yr (68% confidence) at an azimuth of 124 +/- 13 degrees, 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 mu 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 are 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 tenser 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.