Tectonics of the taranaki region, New Zealand: Earthquake focal mechanisms and stress axes

Abstract

We present 39 well-determined focal mechanisms for crustal earthquakes from the Taranaki region in western North Island, New Zealand. Earthquake locations, azimuths, and take-off angles were calculated using a 3D velocity model and only those mechanisms with at least 15 clear first motions were considered. Principal stress axes were determined by inverting focal mechanisms and independently by inverting earthquake first motions. Based on misfit values and differences in seismicity and geology we interpret data east and west of Mt Taranaki separately. Lower crustal earthquakes in eastern Taranaki display both strike-slip and normal faulting mechanisms; σ3 is subhorizontal and aligned northwest-southeast, while the best fit σ1is aligned northeast-southwest with a dip of 27–38° to the horizontal. The principal stress directions in eastern Taranaki are similar to those near the southern Taupo Volcanic Zone, suggesting that the back-arc extension that characterises the Taupo Volcanic Zone continues into eastern Taranaki. Swarm earthquakes from the Cape Egmont Fault Zone, west of Mt Taranaki, have dominantly strike-slip focal mechanisms. The maximum (σ1) and minimum (σ3) compressive stresses west of Mt Taranaki are both subhorizontal, with σ1 aligned east-west and σ3 north-south. The focal mechanisms and principal stress directions do not agree with the geologically inferred northwest-southeast extension direction. We suggest that western Taranaki may be affected by stresses induced by magmatism beneath Mt Taranaki and that the normal faulting seen at the surface mainly occurs associated with significant eruptions from the volcano. The failure angle on faults in western Taranaki exceeds that expected for Byerlee friction and hydrostatic fluid pressure and this suggests that these faults have a relatively low coefficient of friction or relatively high pore fluid pressure. © 2006 Taylor & Francis Group, LLC.