Continental drilling through the shallow Alpine fault, New Zealand
Final Report Abstract
Gouge from seismogenic fault zones provide key information associated with earthquake mechanisms and were/are the focus of various ICDP initiatives. Most recently, the Alpine Fault, New Zealand, was subject to pilot hole drilling as a first step in ICDP deep drilling, the so called DFDP (Deep Fault Drilling Project). DFG funding served to drill a second hole (DFDP-1B) which resulted in (i) extra core material for mineralogical-petrological studies of the fault zone and wall rock composition, and (ii) whole round cores for geotechnical testing under seismogenic conditions. Initial results from geotechnical tests were gained from direct shear experiments on dry rock powders as well as rotary shear experiments on water-saturated gouge. The strength of the dry wall rock samples is uniform, with friction coefficients ranging from ~0.6-0.65. The fault core sample is slightly weaker with a coefficient of friction of ~0.5-0.6. Velocity-weakening behavior is consistently observed in the wall rock samples, however, the fault core sample exhibits velocity-strengthening. Significant rates of healing are observed in all samples, suggesting that the fault easily regains its strength during interseismic periods. The results of water-saturated rotary shear tests at effective normal loads of 15 MPa confirm the general observation that clay-rich gouge shows velocity-strengthening behaviour, and lower friction coefficients in the presence of water. Friction values of µ=0.31-0.58 and (a-b)= -0.001-0.015 for samples from from Fault Creek, Hare Mare and Martyr River match those reported by Boulton et al. (2012), while Saddle Creek is significantly weaker. While all samples show strain hardening during rotary testing (~45-60 shear strain), the phyllosilicate-rich gouge samples from Hare Mare and Gaunt Creek (base of gouge zone represents PSS) are weaker than the other materials and are characterised by velocity-strengthening. The data suggest that slip updip is likely aseismic, whereas earthquake nucleation at depth occurs within the cataclasites and mylonites that compose the majority of the Alpine Fault assemblage. Because surface outcrop samples, including the material used to represent the principal slip zone (PSZ), are velocity-strengthening, it is likely that earthquake nucleation and propagation along the PSZ will localize on the boundary between the fault gouge and velocity-weakening cataclasites.
