The role of seamounts, fluids, and normal faults in slow slip regions: seismological insights from the northern Hikurangi margin

At the northern Hikurangi margin, Aotearoa New Zealand, slow slip events (SSEs) recur every 6-24 months to ~30 km depth. While shallow SSEs (0-10 km) are well-studied offshore, the deeper portion (10-30 km) remains poorly understood, limiting insight into SSE initiation. In Woodward et al. 2026 we investigate the relationships between newly resolved SSEs and seismicity. We combine passive seismological, geodetic, geochemical and seismic reflection data to analyse the relationships between seismicity and slow slip events, and the mechanisms that invoke them. Using time-dependent inversions, we resolve two small SSEs (MW 6.2 and 6.4), one of which extends unusually deeply from 15 to 30 km depth. Using data from a dense onshore seismograph network, deployed directly above this deeper portion from December 2017 to October 2018, we construct a catalog of 3,071 high-quality earthquakes with hypocentral uncertainties ≤5 km, located with a 3-D velocity model and our new 1-D model. Focal mechanisms reveal numerous normal-faulting earthquakes, including some within the slab mantle. Seismicity distributions and normal-faulting earthquakes occur along vertically aligned pathways that link the subducting slab mantle to surface seeps, where fluids show mantle-derived signatures. We infer that normal faults form due to slab bending and localized uplift of subducting seamounts, which enhance plate interface roughness, damage the upper plate, and promote fluid migration. Landward of ~100 km from the trench, both surface seeps and normal-faulting mechanisms cease, coinciding with the downdip limit of shallow SSEs. Together, these results suggest that the Hikurangi margin’s rough subducting plate interface exerts strong control on forearc dewatering and SSE genesis.