Spatial and temporal strain rate variations at the northern Hikurangi margin, New Zealand

We have calculated shear strain rates in the northeastern part of the North Island of New Zealand, using geodetic data collected in the mid-1920s, and 1995. In addition we use observations from a part of the network across Raukumara Peninsula measured in 1976, to look for temporal variation in shear strain rates in that area. The strain rates were estimated using a simultaneous reduction method, assuming uniform strain in space and time within each subnetwork. We find the largest spatial variation in shear strain rates from the mid-1920s to 1995 occurs across Raukumara Peninsula, increasing from 0.06 +/- 0.04 ppm/yr in the west to 0.18 +/- 0.03 ppm/yr in the east (1 sigma confidence). The orientation of the azimuth of relative extension shows significant temporal variation across Raukumara Peninsula. From the mid-1920s to 1976 the maximum shear strain rate was 0.22 +/- 0.05 ppm/yr with the azimuth of relative extension oriented at 129 degrees +/- 7 degrees. From 1976 to 1995 the maximum shear strain rate was 0.14 +/- 0.05 ppm/yr with the azimuth of relative extension oriented at 13 degrees +/- 12 degrees. We show that a model where the subduction interface is locked down to 15 or 30 km depth, and slipping at NUVEL-1A plate rates below that depth, call explain the strain rates and orientation observed during 1976-1995. To fit the observations from the mid-1920s to 1976, we suggest that additional dip slip occurred on the shallow part of the subduction zone, above 30 km. From this we conclude that the plate boundary below Raukumara Peninsula is locked down to 15 or 30 km depth and the aseismic shallow slip episode that occurred during the mid-1920s-1976 was an isolated event, possibly induced by M greater than or equal to 7 historical earthquakes in the area.