Simplified models of the Alpine Fault seismic cycle: stress transfer in the mid-crust

Abstract

SUMMARY We examine the expected elastic and inelastic strain accumulation and release across the transpressive Alpine Fault of New Zealand using a finite element model, which incorporates a frictional fault and material behaviour in the crust based on geophysical measurements and rock mechanics experiments. A zone of localized ductile creeping is predicted beneath the part of the fault that slips seismically. Localized creeping results from (a) thermal weakening along the fault ramp and (b) enhanced stresses due to interaction with the shallow, brittle fault above. The creeping zone controls the shorter-wavelength feature seen in the GPS-determined surface velocities. Using the model we show that: (1) seismic faulting loads the creeping shear zone elastically, transferring stress to the mid-crust, and enhancing creep localization in the lower crust; (2) seismic loading and additional thermal weakening, due to long-term advection and exhumation of rocks along the Alpine Fault, can explain the shallow depth and localization of creeping below the Alpine Fault several hundred years after an earthquake and (3) model surface velocities are in good agreement with present-day velocities determined from campaign GPS measurements. Model strain rates show a transient response after faulting that lasts for the first ca. 20 per cent of the interseismic period (ca. 100 yr), with a gradual broadening in the surface velocity signal and almost constant strain rates thereafter until the next faulting event occurs.