Shear behavior of DFDP-1 borehole samples from the Alpine Fault, New Zealand, under a wide range of experimental conditions

Abstract

The Alpine Fault is a major plate-boundary fault zone that poses a major seismic hazard in southern New Zealand. The initial stage of the Deep Fault Drilling Project has provided sample material from the major lithological constituents of the Alpine Fault from two pilot boreholes. We use laboratory shearing experiments to show that the friction coefficient µ of fault-related rocks and their precursors varies between 0.38 and 0.80 depending on the lithology, presence of pore fluid, effective normal stress, and temperature. Under conditions appropriate for several kilometers depth on the Alpine Fault (100 MPa, 160 °C, fluid-saturated), a gouge sample located very near to the principal slip zone exhibits µ = 0.67, which is high compared with other major fault zones targeted by scientific drilling, and suggests the capacity for large shear stresses at depth. A consistent observation is that every major lithological unit tested exhibits positive and negative values of friction velocity dependence. Critical nucleation patch lengths estimated using representative values of the friction velocity-dependent parameter a−b and the critical slip distance Dc, combined with previously documented elastic properties of the wall rock, may be as low as ~3 m. This small value, consistent with a seismic moment Mo = ~4 × 1010 for an Mw = ~1 earthquake, suggests that events of this size or larger are expected to occur as ordinary earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3–7 km.