Collaborative Research: Monitoring Fault Zone Fluid Flow and Earthquake Hazards with Subseafloor Observatories in the Japan Trench

Subduction zones produce Earth’s largest earthquakes and devastating tsunamis, including the 2011 Tohoku-oki earthquake along the Japan Trench. This project focuses on water contained in the pores and fractures within rocks beneath the seafloor. This water may influence when and how large shallow fault slip occurs. The pressure of the water affects how strongly faults are clamped together while the permeability of the surrounding rocks controls how that pressure is transmitted, drained, or maintained over time. To observe fluid pathways, the research team will use records from sensors installed in boreholes across the fault. These unique records will test whether fluid flows primarily within the damage zone near the plate boundary, along branching faults, or within the subducting tectonic plate. By understanding the distribution of fluid and how it changes over time, the project will increase understanding of earthquake and tsunami hazards that pose risk to coastal communities and critical infrastructure. Open data and reusable analysis tools will be shared to study fluid flow in other settings. The project will support a postdoctoral researcher and undergraduate researchers. A comic book for the public will share the story of how borehole observatories beneath the ocean measure fault zone fluids and temperature. The research will analyze data from International Ocean Discovery Program (IODP) Site C0019 at the Japan Trench, located in the region of extremely large shallow fault slip during the 2011 Tohoku-oki earthquake. At this site, NSF-supported IODP Expedition 405 installed dense temperature sensor strings in subseafloor borehole observatories in Holes C0019D and C0019Q in 2024. These observatories cross the shallow plate-boundary fault and span the overriding prism above it, and their records were recovered during a subsequent expedition in October 2025. The project also builds on legacy observations from IODP Expeditions 343/343T, which installed the original rapid-response observatory at the site in 2012. The primary data include roughly one year of temperature records from 185 sensors and complementary pressure records from the new observatories, together with 2012-2013 temperature records from 55 sensors. The team will construct background thermal models using measured thermal properties from cores and then interpret residual temperature and pressure signals from drilling disturbance, transient flow, tides, and a free-flow interval in Hole C0019D. These observations will be used to characterize fault zone hydrogeologic architecture in space and through time, including estimates of permeability, hydraulic diffusivity, pathway thickness, and bounds on fault zone fluid pressure. Results will be integrated with core descriptions, structural observations, logging data, and geochemical context to test where permeability is concentrated, whether permeable intervals continue between holes separated by about 40 meters, how flow pathways have changed since 2012-2013, and how this hydrogeologic architecture may influence pore pressure, effective stress, and shallow megathrust behavior. 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.