Full-waveform inversion imaging of the slow slip region at the shallow Hikurangi Subduction Margin reveals complex fault and hydrogeological systems

Abstract:

The northern Hikurangi subduction margin hosts shallow slow-slip events (SSEs) as well as multiple historic tsunami earthquakes. The physical mechanisms and properties of the subduction interface which enable these dual modes of fault rupture remain largely enigmatic. In 2017-2018, the NZ3D seismic experiment was conducted offshore of Gisborne to image the structure of the overriding plate and subduction interface and to infer the physical properties within the region of shallow SSEs. This experiment included a 3D seismic volume collected with four 6 km long streamers, ocean-bottom seismometers and land stations.

Here, we present the results of 2D full-waveform inversion performed on the collected streamer data, preliminary to the processing of the full 3D dataset. The resulting high-resolution velocity models and their vertical differentiation reveal a complex system of thrust faulting, horst and graben like structures and bottom-simulating reflectors within the accretionary prism, as well as the decollement below the accretionary prism. Velocity inversions across the imaged thrust faults in the accretionary prism indicate the presence of fluids, potentially supporting the hypothesis that the subduction interface has elevated pore-fluid pressures, which are drained along some of the thrust faults. Velocity inversions are also observed across bottom-simulating reflectors which indicate the presence of free-gas. Imaging of the decollement reveals an acoustically transparent region of low velocity contrasts in the inferred location of a subducted seamount, previously identified by magnetic and seismic data, which is thought to entrain fluids on the down going plate. Interestingly, new 3D travel-time tomography results also reveal a low-velocity anomaly in the location of the interpreted seamount which, when combined with our observations, raises questions about its nature.