Seismotectonic of a Locked Subduction Patch: The Southern Hikurangi Margin
Subduction zones produce the largest earthquakes on the planet, where rupture along the plate interface can result in the release of stress over large areas, with up to tens of meters of slip extending from below the surface to the trench. The regional stress field is a primary control on the faulting process, ergo understanding the regional stress field leads to a better understanding of the current and future faulting in the area. Abundant new seismic and continuous Global Positioning System (cGPS) data in the southern North and northern South Island, New Zealand, make it possible to characterize stress and strain parameters throughout the southern Hikurangi subduction zone. Stress orientations calculated within the subducting plate, the overriding Australian plate, and due to gravitational forces reveal that stress throughout the subducting system varies across the southern North Island. Margin parallel motion is being accommodated by shear deformation west of theWairarapa fault, whereas margin perpendicular motion is being accommodated east of theWairarapa fault. Stress parameters within the double Benioff zone (DBZ) were characterized in term of two bands of seismicity. In the deep region of the DBZ, inversion the upper band of seismicity shows down-dip tension, while the lower band shows compression. Tension in the upper band and compression in the lower band is consistent with bending stresses. In the shallow region of the DBZ, the inversion of both the upper and lower bands seismicity showed tension; this is indicative of slab pull. Shear-wave splitting of stacked waveforms of local earthquakes recorded on 291 three-component stations showed an average fast azimuth of N-S to NNE-SSW, west of theWairarapa fault. A fast azimuth orientation of N-S to NNE-SSW is sub-parallel to the local major faults. This indicates that the observed anisotropy west of theWairarapa fault is structurally derived. East of the Wairarapa fault, within the Wairarapa Basin, the average fast azimuth orientation isNNW-SSE. Because the fast azimuth orientation showed no dependence on station-earthquake distance, depth, or back azimuth and is perpendicular to major local faults; it has been interpreted as being reflective of the SHmax orientation. cGPS daily solutions for long-term and inter-slow slip events (inter-SSE) time periods show distinctly differing regions of shear strain rate in the southern North Island and northern South Island. Compression and positive (clockwise) rotation in the southern North and northern South Island was observed using both datasets. Inter-SSE time periods resulted in lower magnitude strain parameters than those calculated during time periods including SSEs. These datasets shows that strain parameters change on time scales of SSEs (< 10 years).