Low Frictional Strength of Alpine Fault Rocks of DFDP-1 at Elevated Temperature and Low Slip Rate

Abstract:

The Alpine Fault, New Zealand, is a major plate-bounding fault that accommodates 65-75% of the total relative motion between the Australian and Pacific plates. The absence of measurable contemporary surface deformation has been interpreted to indicate that the fault slips mostly in quasi-periodic large earthquakes (< M_w 8.0). In order to understand the mechanics of earthquakes, it is important to study the evolution of frictional properties of the fault rocks under conditions representative of the potential hypocentral depth. Here, we present experimental data obtained on DFDP-1 drill core samples of rocks that surround the principal slip zone(s) (PSZ) of the Alpine Fault and the PSZ itself. Simulated fault gouges were sheared under high pressure and temperature conditions in a hydrothermal ring shear apparatus. We performed experiments at temperatures of 25, 150, 300, 450 and 600 °C. Results show the occurrence of potentially unstable, velocity-weakening behaviour at low slip velocity at 150 °C. With increasing temperature, velocity-weakening occurs over a wider range of slip velocities; notably it is restricted to higher velocities at temperatures of 450 and 600 ºC. Additionally, the frictional strength decreases markedly for very low slip velocities (<0.3 μm/s) for the sample derived from the PSZ. Moreover, shear stress depends linearly on effective normal stress, indicating shearing is essentially frictional. No transition to ductile (normal stress independent) flow was observed and we speculate that it is postponed to higher temperature or lower slip rates due to the simultaneous operation of thermally activated processes and granular flow in the porous gouges. The AF should be able to nucleate earthquakes at temperatures up to at least 300 °C. The low frictional strength observed at higher temperatures and low velocity indicates that the AF is able to creep at low levels of differential stress at deeper levels, but can become unstable if slip accelerates.