An electrical resistivity image of the Hikurangi subduction margin

Along the Hikurangi subduction margin, on the east coast of New Zealand's North Island, the interplate coupling changes from locked in the south to weakly coupled in the north. New magnetotelluric (MT) data from 151 locations linking previous MT surveys into a single contiguous data set that encompasses the weakly coupled part of the margin are analysed. By inverting the combined data we have constructed a 3-D image of the electrical resistivity of the subduction interface shear zone along a 300-km-long segment of the margin. Our results show that the electrical resistivity of the subduction interface shear zone is heterogenous; the degree of heterogeneity decreasing from north to south. The resistivity heterogeneities correlate well with the distribution of near-plate interface seismicity, V-p/V-s values and the pattern of areal strain rate derived from GPS data. These correlations are consistent with variations in the fluid content of the subduction interface shear zone. In the norther n part of this segment, conductive areas adjacent to the interface are interpreted to be fluid rich areas where seismicity is sparse, V-p/V-s ratios are high and the areal strain rate is extensional. In contrast, where the areal strain rate is compressional the plate interface is more resistive, and seismicity is more abundant consistent with greater interplate friction. In the south, the resistivity of the plate interface is more homogenous, and the overlying plate is more resistive at shallower levels than in the north. Our results support the hypothesis that the fluid and/or hydrated clay content of the subduction interface shear zone are an important control on interplate coupling.