Evidence from GPS measurements for contemporary interplate coupling on the southern Hikurangi subduction thrust and for partitioning of strain in the upper plate

Abstract

The spatial extent and strength of contemporary coupling between the Pacific and Australian plates on the Hikurangi subduction interface beneath the Wellington region of New Zealand are not well known. Consequently, the potential for the plate interface to produce great thrust earthquakes, or to load the major Wellington and Wairarapa faults in the overlying plate, is also unknown. To address these issues, a GPS network across the southern North Island has been partially or wholly surveyed six times between 1992 and 1999 and yields well‐determined relative velocities between 82 points. The relative velocity of the east coast with respect to the west coast across the 80‐km width of the southern end of the North Island is ∼14 mm yr−1 at an azimuth of 258°. This is about one third of, and closely parallel to, the NUVEL‐1A relative velocity of the Pacific with respect to the Australian Plate (39.4 mm yr−1 at 261°). Strain rate solutions indicate partitioning of contemporary strain, with a 35° change in principal axis orientations across the network. Using elastic dislocation theory, an inversion of the observed point velocities and their full variance‐covariance matrix determines the variation with depth of contemporary coupling at the subduction interface. If uniform coupling is assumed, then 100% coupling occurs from very shallow depths down to ∼29±1 km. However, if variable coupling is assumed, it reaches 100% at depths between 16 and 24 km. At greater depths, there is a transition zone at least 20 km wide, down to a depth of at least 40 km, over which contemporary coupling diminishes to zero. These depth ranges correlate moderately well with low‐angle thrust focal mechanisms recently obtained by other workers and indicate that the downdip width of the interface, over which a great thrust earthquake could occur, exceeds 100 km. Elastic dislocation models also explain the observed partitioning of strain, with Coulomb stress calculations indicating that the Wairarapa fault is being loaded at seismogenic depths.