High-velocity frictional properties of Alpine Fault rocks: Mechanical data, microstructural analysis, and implications for rupture propagation

The Alpine Fault in New Zealand is a major plate-bounding structure that typically slips in similar to MS earthquakes every c. 330 years. To investigate the near-surface, high-velocity frictional behavior of surface and borehole-derived Alpine Fault gouges and cataclasites, twenty-one rotary shear experiments were conducted at 1 MPa normal stress and 1 m/s equivalent slip velocity under both room-dry and water saturated (wet) conditions. In the room-dry experiments, the peak friction coefficient (mu(p) = T-p/sigma(n)) of Alpine Fault cataclasites and fault gouges was consistently high (mean mu(p) = 0.67 +/- 0.07). In the wet experiments, the fault gouge peak friction coefficients were lower (mean mu(p) = 0.20 +/- 0.12) than the cataclasite peak friction coefficients (mean mu(p) = 0.64 +/- 0.04). All fault rocks exhibited very low steady-state friction coefficients (mu(ss)) (room-dry experiments mean mu(ss) = 0.16 +/- 0.05; wet experiments mean mu ss = 0.09 +/- 0.04). Of all the experiments performed, six experiments conducted on wet smectite-bearing principal slip zone (PSZ) fault gouges yielded the lowest peak friction coefficients (mu(p) = 0.10-0.20), the lowest steady-state friction coefficients (mu(ss) = 0.03-0.09), and, commonly, the lowest specific fracture energy values (E-G = 0.01-0.69 MJ/m(2)). Microstructures produced during room-dry and wet experiments on a smectite-bearing PSZ fault gouge were compared with microstructures in the same material recovered from the Deep Fault Drilling Project (DFDP-1) drill cores. The near-absence of localized shear bands with a strong crystallographic preferred orientation in the natural samples most resembles microstructures formed during wet experiments. Mechanical data and microstructural observations suggest that Alpine Fault ruptures propagate preferentially through water-saturated smectite-bearing fault gouges that exhibit low peak and steady-state friction coefficients. (C) 2017 The Authors. Published by Elsevier Ltd.