Slow Slip Accommodates the Full Plate Convergence Budget at the Northern Hikurangi Subduction Zone

Accurately assessing strain accumulation and release in subduction zones is contingent upon robust detection and characterization of locking and slip along the megathrust. However, the distribution of slip on shallow, offshore plate boundaries is not well-resolved with onshore GNSS networks. At the Hikurangi Subduction Zone offshore Aotearoa-New Zealand, extensive investment has been made into seafloor geodetic techniques such as seafloor pressure and GNSS-acoustic, which have significantly improved observation and characterization of offshore SSEs. Despite their utility, oceanographic noise limits the ability of these seafloor techniques to detect SSEs. Formation pore pressure changes (as a proxy for volumetric strain) detected in borehole observatories have an enhanced signal-to-noise ratio and can reliably resolve deformation at the 10s of nanostrain-level, providing an improved view of shallow crustal deformation offshore.

Here, we report on a suite of SSEs observed in two IODP borehole observatories in the northern Hikurangi Subduction Zone between 2018 and 2023 and model their slip distribution and magnitude. During this time, five SSEs were clearly recorded in the borehole pore pressure data. Four of these occurred spontaneously, and the borehole pressure changes correlate with surface displacement observed at onshore GNSS stations. In contrast, in early 2021, the M_w 7.2 East Cape earthquake triggered a near-trench SSE that was only captured by the observatories. We jointly invert changes in pore pressure with onshore GNSS displacements and seafloor pressure (when available) for slip distribution along a 2D transect for each of the events. Our inversions incorporate realistic elastic properties constrained by high-resolution seismic velocity models and logging-while-drilling data, which is crucial for accurately resolving slip distribution and magnitude. We find large differences in slip initiation and evolution characteristics during the 2021 triggered SSE compared to the spontaneous events. We also find that, in total, SSEs accommodate most (>80%) of the plate convergence budget along the shallow (<10 km) megathrust. The 2021 triggered event was particularly important for filling in a slip deficit near the trench. Our results have implications for the role of SSEs in accommodating the megathrust strain budget near the trench at subduction zones.