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. Plain Language Summary Our knowledge of fault zone geometry and properties at depth is commonly based on changes in seismic wave velocity. However, the relationship between seismic velocity and material properties and conditions at depth is poorly constrained in fault zone materials. We use laboratory and borehole measurements of wave speed through rocks surrounding the Alpine Fault in New Zealand to examine how deformation and alteration influence the seismic properties of rocks. We found that a 20% to 40% reduction in velocity persists at least 30 m from the fault corresponding to a region that has experienced significant mechanical deformation and chemical alteration. Key Points Average seismic wave velocity reductions of 30% relative to host rock extend tens of meters away from the active Alpine Fault trace Progressive disruption of