Crustal deformation during 1994-1998 due to oblique continental collision in the central Southern Alps, New Zealand, and implications for seismic potential of the Alpine fault

The positions of 115 ground marks in a 150 x 100 km area of oblique contine ntal collision in the central Southern Alps, New Zealand, have been measure d by Global Positioning System (GPS) two to four times between 1994 and 199 8. Contemporary velocity and strain rate fields derived from these observat ions 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 Alp s foothills. Projections of the fault-parallel and fault-normal components of velocity onto an Alpine fault-normal profile show that about 85% of the NUVEL-1A model relative plate motion is observed within the GPS network. Th e surface displacements in the high strain rate region are well fit by a mo del 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 el astically 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 bound ary of the mountains if extrapolated to the surface. Strain determined from a small-scale survey network crossing the Alpine fault indicates no signif icant near-surface aseismic fault slip on the central Alpine fault over the past 25 years. Our results are consistent with independent geological evid ence that the central section of the Alpine fault is capable of producing l arge to great earthquakes.