Friction experiments on Alpine Fault DFDP core samples: Implications for slip style on plate boundary faults

Abstract:

The Alpine Fault is a major plate-boundary fault zone that poses a significant seismic hazard in southern New Zealand, with the next major earthquake expected to be imminent. Core samples from the Alpine Fault were recovered from two Deep Fault Drilling Project pilot boreholes that penetrated the principal slip zone (PSZ). We show here that at room temperature and low effective stress (30 MPa), materials from within and very near the PSZ are weaker than the surrounding cataclasites (µ = 0.45), exhibit velocity-strengthening friction, and also tend to restrengthen (heal) rapidly. Under conditions appropriate for several kilometers depth on the Alpine Fault (100 MPa, 160 °C, fluid-saturated), a cataclasite/gouge sample located very near to the PSZ exhibits µ = 0.67, which is high compared to measurements performed at lower pressures and temperatures for the Alpine Fault and other major fault zones sampled by scientific drilling. Every major lithological unit tested under elevated P-T conditions exhibits both positive and negative values of friction velocity-dependence suggesting that they are all capable of earthquake nucleation. Using representative values of the friction velocity-dependent parameter a-b, the critical slip distance D_c, and previously documented elastic properties of the wall rock, estimated critical nucleation patch lengths may be as low as ~3 m. This small value is consistent with a seismic moment M_o = ~4x10¹⁰ or a M_w = ~1, which suggests that events of this size or larger are expected to occur as normal earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3-7 km. In conjunction with previous geodetic and seismologic observations, our results indicate that the Alpine Fault has a high potential for frictional instability throughout the brittle crust, in contrast with other major fault zones on which the uppermost portion is relatively stable.