Fracturing and pore-fluid distribution in the Marlborough region, New Zealand from body-wave tomography: Implications for regional understanding of the Kaikōura area

Relative to more mature fault zones, immature fault zones that have accumulated smaller total displacement are characterized by less efficient strain localization and more complicated earthquake ruptures. How differences in maturation are reflected in regional-scale upper-crustal fracturing is not well known. Recently, complicated earthquake ruptures associated with immature fault zones, such as the 2016 Kaikōura earthquake in New Zealand, have occurred in areas that are in regional proximity (<100 km away) to more mature faults. Here we examine whether inefficient strain localization in less mature fault zones is associated with a broader distribution and anomalously elevated concentration of fractures over distances of tens of kilometers. We use regional seismic arrival-time tomography in a broad area around the Kaikōura earthquake to investigate lateral variations in Vp and Vp/Vs. Focusing on the extensively faulted but compositionally uniform Torlesse-Pahau terrane in the Marlborough region where the earthquake occurred, we attribute lateral variations in Vp and Vp/Vs to differences in concentration of fluid-filled fractures. Using numerical models relating seismic velocities and fracturing, we solve for the lateral variation in concentration of ∼0.01 aspect ratio fluid-filled fractures. We find that areas near the Kaikōura rupture have >3% elevated fracture porosity compared to the adjacent area to the north. The elevated regional fracturing in the Kaikōura area is interpreted to result from more distributed deformation, and broader distribution of earthquakes, due to inefficient localization of strain from a regionally uniform strain rate field, highlighting the relationship between relative maturity of upper-crustal fault zones and lateral variability of regional upper-crustal properties. Plain language summary: When earthquakes occur in immature fault zones (areas that have accumulated small total displacement), they tend to rupture multiple poorly developed faults of diverse orientations. In contrast, more mature fault zones are associated with more developed, smoother faults and more localized earthquake activity. How these differences in maturation are reflected in the regional distribution of fractures is not well known. Here we use the arrival times of P and S waves from earthquakes to constrain seismic velocities (Vp and Vp/Vs) and use numerical models to relate seismic velocities to fracture concentration. We focus on the Marlborough region of the South Island, New Zealand, where immature fault zones recently ruptured during the 2016 Kaikōura earthquake but which also has more mature fault zones <100 km away. We find elevated fracture concentrations (3% higher fracture porosity), indicative of more distributed deformation along immature fault zones relative to more mature fault zones. In immature settings, fracturing is not as effectively localized along individual fault traces, leading to a broadly distributed fracture network. • We image upper-crustal Vp and Vp/Vs, and interpret their variations in terms of amount of fluid-filled fractures. • The Marlborough region, which experiences near-uniform shear, has variable fluid-filled fracture concentration. • Within the Marlborough region, areas of less mature faulting have more fluid-filled fractures than more mature areas.