Imaging the Shallow Subsurface Structure of the North Hikurangi Subduction Zone, New Zealand, Using 2‐D Full‐Waveform Inversion

Abstract

The northern Hikurangi plate boundary fault hosts a range of seismic behaviors, of which the physical mechanisms controlling seismicity are poorly understood, but often related to high pore fluid pressures and conditionally stable frictional conditions. Using 2‐D marine seismic streamer data, we employ full‐waveform inversion (FWI) to obtain a high‐resolution 2‐D P wave velocity model across the Hikurangi margin down to depths of ~2 km. The validity of the FWI velocity model is investigated through comparison with the prestack depth‐migrated seismic reflection image, sonic well data, and the match between observed and synthetic waveforms. Our model reveals the shallow structure of the overriding plate, including the fault plumbing system above the zone of slow‐slip events to theoretical resolution of a half seismic wavelength. We find that the hanging walls of thrust faults often have substantially higher velocities than footwalls, consistent with higher compaction. In some cases, intrawedge faults identified from reflection data are associated with low‐velocity anomalies, which may suggest that they are high‐porosity zones acting as conduits for fluid flow. The continuity of velocity structure away from International Ocean Discovery Program drill site U1520 suggests that lithological variations in the incoming sedimentary stratigraphy observed at this site continue to the deformation front and are likely important in controlling seismic behavior. This investigation provides a high‐resolution insight into the shallow parts of subduction zones, which shows promise for the extension of modeling to 3‐D using a recently acquired, longer‐offset, seismic data set.

Slow slip source characterized by lithological and geometric heterogeneity

“…Moreover, a recent full waveform inversion (FWI) of seismic line 05CM-04 demonstrates that the marked variations in Vp observed in the core and borehole data from this interval ( Fig. 4) are not limited to centimeter to meter scales, but also vary at larger scales away from the drilling sites as multi-kilometer patches with Vp varying laterally by >1 km/s (30). These protoliths to plate interface rocks comprise mainly carbonates and volcaniclastic sediments (widely altered to smectite clay), with minor amounts of siltstone, silty claystone, limestone, and basalt.…”

Three‐Dimensional P Wave Velocity Structure of the Northern Hikurangi Margin From the NZ3D Experiment: Evidence for Fault‐Bound Anisotropy

“…Multichannel seismic (MCS) reflection and seafloor magnetic studies by Bell et al (2010) and Barker et al (2018) suggest that a large (30 × 10 km) seamount is subducting in this region and shallow tectonic tremors detected by Todd et al (2018) are mostly located just above and landward of the suspected seamount ( Figure 1c). Using the same reflection data as Barker et al (2018), Gray et al (2019) derived a fine-scale velocity structure of the uppermost 1.5-2 km below the seafloor by applying a full-waveform inversion (FWI) technique and revealed sudden velocity changes within the frontal accretionary wedge indicating low-velocity zones along the splay faults, which may be related to fluid flow.…”

“…Yellow squares are the locations of four IODP Exp372/375 drill sites (Wallace et al, 2019). Brown line shows the multichannel seismic reflection profile of 05CM-04 (Bell et al, 2010;Barker et al, 2018;Gray et al, 2019; Figure 1d). The red dashed line marks the outline of location of the inferred subducting seamount (Barker et al, 2018).…”

Overpressured Underthrust Sediment in the Nankai Trough Forearc Inferred From Transdimensional Inversion of High‐Frequency Teleseismic Waveforms

“…Active‐source seismic surveys have been widely used to infer pore pressure distribution (e.g., Tsuji, Kamei, et al, 2014). High‐resolution (∼0.1–1 km) P ‐wave velocity ( V p ) structure or reflection profiles have been acquired for many subduction zones (Bell et al, 2010; Canales et al, 2017; Gray et al, 2019; Kamei et al, 2012; Li et al, 2015; Shiraishi et al, 2019). In these studies, low V p or highly reflective zones are often associated with high pore fluid pressure; however, interpreting rock properties from only V p (or impedance) information is somewhat subjective.…”