Advanced Seismic Imaging Techniques Characterize the Alpine Fault at Whataroa (New Zealand)

Abstract:

The plate-bounding Alpine Fault in New Zealand is a large transpressive continental fault zone that is late in its earthquake cycle. The Deep Fault Drilling Project (DFDP) aims to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth. We have acquired and processed reflection seismic data to image the subsurface around the drill site. The resulting velocity models and seismic images of the upper 5 km show complex subsurface structures around the Alpine Fault zone. The most prominent feature is a strong reflector at depths of 1.2-2.2 km with a dip of ~40° to the southeast below the DFDP-2 borehole, which we assume to be the main trace of the Alpine Fault. The reflector exhibits varying lateral reflectivity along its extent. Additionally, subparallel reflectors are imaged that we interpret as secondary branches of the main fault zone. The derived P-wave velocity models reveal a 400-600 m thick sedimentary layer with velocities of ~2.3 km/s above a schist basement with velocities of 4.5-5.5 km/s. A pronounced low-velocity layer with velocities of approximately 3.5 km/s can be observed within the basement at 0.8-2 km depth. Small-scale low-velocity anomalies appear at the top of the basement and can be correlated to the fault zone. The results provide a reliable basis for a seismic site characterization at the DFDP-2 drill site that can be used for further structural and geological investigations of the architecture of the Alpine Fault in this area.