Collaborative Research: Unraveling the habitat and dynamics of slow slip events through integrated borehole observations in the northern Hikurangi subduction margin

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Subduction zone plate boundaries, where one tectonic plate dives or subducts beneath another tectonic plate produce the world’s largest earthquakes and tsunami, as starkly demonstrated by the M9.2 Indian Ocean earthquake in 2004, and the 2011 M9.0 Tohoku-Oki earthquake offshore northern Japan. Recently, scientists have recognized that subduction megathrust faults undergo slip in episodic slow-motion earthquakes, or slow slip events lasting days to months. In some cases, slow slip events have been observed to precede (and possibly trigger) major subduction earthquakes, increasing the need to understand them. Slow slip events occurring close to Earth's surface (5-15 km depth) exist at the Hikurangi subduction zone offshore New Zealand and are uniquely accessible to near-source investigations. In 2018, two observatories (penetrating up to 400 m below the seafloor) were installed during an International Ocean Discovery Program scientific drilling expedition at the Hikurangi subduction zone. Sensors in the observatories are now continuously measuring temporal changes in properties of Earth's crust near the source of slow slip events and will help reveal their causes, consequences, and relationship to destructive earthquakes. This project provides funding to analyze the data collected from the observatories, and to support another mission to the observatories to retrieve and replace the instrument strings at the observatories to enable their continued operation for another five years. The project supports the training of graduate and undergraduate students.Slow slip events (SSEs) involve transient aseismic slip on a fault (lasting weeks to months) at sliding velocities intermediate between plate boundary displacement rates and those required to generate seismic waves. The physical mechanisms leading to SSEs, their role in plate boundary strain accumulation and release, and their relationship to destructive seismic slip on subduction thrusts are poorly known, due in part to the fact that most well-studied subduction zone SSEs occur far too deep for near-field investigations. A notable exception to this is the northern Hikurangi subduction zone, New Zealand, where well-characterized SSEs occur within 5-15 km below the seafloor, and possibly propagate to the trench. In early 2018, as part of IODP Expedition 375, two borehole observatories were installed in the near field of northern Hikurangi SSEs to monitor deformation, thermal, hydrological, and geochemical responses through multiple SSE cycles. This project will: (1) recover and conduct analyses of data and samples recovered from the observatories; and (2) replace the geochemical sampling and temperature sensing instrument string in one of these boreholes, spanning an active thrust near the deformation front, during a proposed research cruise in early 2023. These data will be integrated with a network of seafloor sensors (absolute pressure gauges, seismometers, and fluid flowmeters) that were also recording along the drilling transect during a large slow slip event in 2019, offering an unprecedented opportunity to integrate multiple seafloor and subseafloor observations in the near-field of a large slow slip event.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.