Seismological Characterization of Northern Hikurangi Margin Slow Slip Regions Associated With Normal Faults, Seamounts, and Seeps

Abstract

Abstract At the northern Hikurangi margin, Aotearoa New Zealand, slow slip events (SSEs) recur every 6–24 months to 30 km depth. Although shallow SSEs (0–10 km) are well‐studied offshore, the deeper portion (10–30 km) remains poorly understood, limiting insight into SSE initiation. Here we investigate this deeper region and examine relationships between newly resolved SSEs and seismicity. Using time‐dependent inversions, we resolve two small SSEs ( 6.2 and 6.4), including one that extends 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 using a 3‐D velocity model and a new 1‐D model. Earthquake magnitudes range from −0.84 to 4.40, with a completeness magnitude of 1.7 and a b‐value of 1.06. Focal mechanisms reveal numerous normal‐faulting earthquakes, including some within the slab mantle. Vertically‐aligned seismicity and normal‐faulting earthquakes outline pathways linking the slab mantle to surface seeps of mantle‐derived fluids. We infer that normal faults form due to slab bending and localized uplift of subducting seamounts, which roughen the plate interface, 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. Plain Language Summary At the northern Hikurangi margin subduction zone, Aotearoa New Zealand, the Pacific Plate periodically slips slowly beneath the Australian Plate during slow‐slip events (SSEs). While shallow SSEs offshore are well‐studied, the deeper part (10–30 km) is harder to observe. In this study, we use a dense network of land‐based seismographs to examine two SSEs and the earthquakes that accompanied them. Our new earthquake catalog contains over 3,000 well‐located earthquakes. Some of these earthquakes show normal‐faulting mechanisms, some deep within the subducting slab. These normal faults and vertical clusters of earthquakes trace pathways that allow fluids to rise upwards from the slab mantle. These pathways connect to surface seep sites where mantle‐derived fluids are observed, demonstrating a link between deep slab processes and surface features. The seeps and the normal‐faulting earthquakes disappear farther inland, matching the downdip limit of shallow SSEs. We suggest that normal faults develop due to slab bending and localized uplift of subducting seamounts. These damage the upper plate and help fluids migrate upwards. Together, these findings show that the roughness of the subducting Pacific Plate plays a key role in controlling fluid flow and the depth range of slow slip behavior at the Hikurangi margin. Key Points Dense onshore seismic and geodetic data resolve two Hikurangi SSEs, including an unusually deep 15–30 km event Normal fault networks, developed due to a rough, seamount‐rich subduction plate interface, form fluid pathways from slab mantle to surface Down‐dip limit of SSEs is where surface seeps and normal‐faulting ends: rough plate interface controls forearc dewatering and SSE genesis