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

Abstract

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.

3D Active Source Seismic Imaging of the Alpine Fault Zone and the Whataroa Glacial Valley in New Zealand

“…Extensive fracturing enables fluid flow and is observed to be both inherited from previous structures and fault‐related (Massiot et al., 2018; Simpson et al., 2020; Williams et al., 2018). Laboratory analyses of seismic P‐wave velocities (Adam et al., 2020; Jeppson & Tobin, 2020a, 2020b; Li et al., 2020; Simpson et al., 2020) are not directly transferable to seismic scales due to their small‐scale sensitivity (cm‐range resolution for laboratory samples instead of tens‐of‐meter‐range for seismic) but imply damage zones as thick as 1 km.…”

“…However, no appropriate anisotropic migration velocity model could be derived for the Alpine Fault given the generally low signal-to-noise ratio. Transferring laboratory measurements to field results is challenging as shown by Jeppson and Tobin (2020b). Even determining anisotropy in the shallow region of the Alpine Fault is very challenging according to Godfrey et al (2000).…”

“…Seismic anisotropy is expected from the analysis of elastic properties of rock samples collected near the Alpine Fault (Adam et al, 2020;Christensen & Okaya, 2007;Godfrey et al, 2000;Graham, 2020;Jeppson & Tobin, 2020b;Li et al, 2020;Simpson et al, 2020). This anisotropy has a great influence on seismic imaging as proven, for example, by modeling studies at the Alpine Fault conducted by Adam et al (2020).…”

Fracture Shape and Orientation Contributions to P-Wave Velocity and Anisotropy of Alpine Fault Mylonites

“…Previous studies using seismic data (Savage et al, 2007;Karalliyadda and Savage, 2013), core measurements (Okaya et al, 1995;Godfrey et al, 2000;Christensen and Okaya, 2007;Allen et al, 2017;Simpson et al, 2019;Adam et al, 2020;Jeppson and Tobin, 2020) and numerical modeling (Godfrey et al, 2002;Dempsey et al, 2011;Adam et al, 2020;Simpson et al, 2020) demonstrate that the rock in the vicinity of the Alpine Fault is strongly anisotropic. The anisotropy is attributed to many factors such as mineral crystallographic preferred orientation (CPO), grain shape preferred orientation (SPO), foliation and fractures.…”