Elastic Properties and Seismic Anisotropy Across the Alpine Fault, New Zealand

Seismically detected low‐velocity zones are commonly associated with major crustal faults. In order to accurately interpret these low‐velocity zones and understand processes that produce them, direct measurements of seismic wave speed through fault zone rocks are needed. The Alpine Fault dominates the active transpressional plate boundary of the South Island, New Zealand. We examine heterogeneity by determining elastic properties of the Alpine Fault using Deep Fault Drilling Project (DFDP)‐1 drill core and borehole logs, and outcrop samples from central Alpine Fault field localities. We measured P and S wave velocities on saturated rock samples, in three orthogonal directions, over a range of effective pressures. P and S wave velocities in fault rock range from 2.5 to 5.0 km/s and 1.3 to 2.7 km/s, respectively, consistent with the borehole sonic log. These velocities are slower than those in the surrounding host rock corresponding to a 20% to 40% decrease in velocity, caused by mechanical and chemical alteration processes, extending at least 30 m from the principal slip surface. Hanging wall host rocks are strongly anisotropic, while brittle fault rock and footwall host rock are mostly isotropic. Fault zone trapped wave observations are geometrically and quantitatively consistent with field observations and laboratory measurements of the damage zone. Scale independence of the velocity measurement in this zone across core, log, and field scales shows that drill core and outcrop samples are good proxies for in situ rocks, suggesting that the low velocity is due to microscale features, such as microfractures and clay content.