Fault damage and pore fluid distribution regionally around Kaikōura, New Zealand from body-wave tomography

Abstract:

Individual earthquake ruptures are usually assumed to occur on individual faults and are often associated with narrow regions (< 5 km) of altered physical properties, such as areas of increased fracturing and/or increased pore fluids. Recently, earthquakes such as the 2016 Kaikōura, New Zealand earthquake have ruptured multiple faults with different orientations over regions with widths spanning > 25 km. Here we test whether such complicated earthquakes are associated with non-uniform, spatially-variable physical properties. Focusing on ­­aftershocks of the Kaikōura event, we use seismic arrival-time tomography in the Kaikōura region to infer variation in crustal properties. We relate perturbations in Vp and Vp/Vs to perturbations in fracture and pore fluid distribution in areas within 30 km of faults that ruptured the surface during the 2016 Kaikōura earthquake, focusing on the association of altered physical properties to distance from fault. We find that fault zones are on average characterized by decreased Vp and elevated Vp/Vs, features that decay towards background levels over lateral distances of < 10 km. Drops in Vp are linearly related to increases in Vp/Vs, consistent with elevated fracture density and/or increased pore fluids. Aftershocks from the Kaikōura earthquake fall predominately within 5 km of active faults, within the zone of altered material properties. The alteration is likely the result of distributed long-term deformation, as well as the recent seismic activity occurring in these pre-existing weaker regions. These results provide evidence that the region ruptured by the Kaikōura earthquake is characterized by both increased fault damage and increased pore fluids, highlighting the role of altered physical properties in aiding the localization of complicated fault ruptures.