Deformation at the Australia-Pacific Boundary in the North Island of New Zealand: Comparison of 20 Years of GPS-recorded Average Deformation With 6.5 Years Time Series of SSEs and Transients

Abstract:

The North Island of New Zealand occupies the boundary between the obliquely converging Australian and Pacific Plates. The Pacific Plate subducts beneath the eastern North Island along the Hikurangi Trough. Crustal deformation is strongly affected by coupling on the subduction interface, and strike-slip faulting and back-arc volcanism in the upper plate. We present strain rates over the last 20 years using a new methodology based on Vertical Derivatives of Horizontal Stress (VDoHS) rates. VDoHS rates, calculated by solving the force balance equations at the Earth’s surface, are the most spatially compact surface expressions of subsurface deformation sources. We assess our results in the context of distribution of interseismic coupling on the subduction interface and highlight areas of forearc extension related to possible forearc gravitational collapse. Instead of a expected rifting signal, we find strong contraction within parts of the Taupo Volcanic Zone, indicating that non-tectonic effects from magmatic and/or hydrothermal processes must influence deformation.

We compare the 20 years deformation with the pattern of SSEs and transients. SSEs characteristics vary along the Hikurangi margin: in depth, duration, size and frequency. Recently small, deep SSEs have been identified downdip from larger ones at the central margin. Are there other small SSEs at Hikurangi that have gone undetected? We map all SSEs and transients spanning the last 6.5 years detectable using VDoHS rates.

VDoHS rates also reveal the boundaries between the locked and unlocked portions of the megathrust, and we can track how this varies throughout the SSE cycle. Above the locked interface, the pull of the subducted plate generates shear tractions in the overlying plate in the direction of subduction, while above the creeping section shear tractions are in the opposite direction. We show a pattern of locking and unlocking signal along the Hikurangi margin and how this pattern changes with SSEs.