Nankai Trough Seismogenic Zone Experiment
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| Nankai Trough Seismogenic Zone Experiment | |
|---|---|
| Name | Nankai Trough Seismogenic Zone Experiment |
| Location | Nankai Trough, Japan |
| Type | Scientific drilling and observatory program |
| Start | 2000s |
| Participants | Japan Agency for Marine-Earth Science and Technology; International Ocean Discovery Program; Ministry of Education, Culture, Sports, Science and Technology (Japan) |
Nankai Trough Seismogenic Zone Experiment The Nankai Trough Seismogenic Zone Experiment was an integrated offshore scientific drilling and observatory program focused on the Nankai Trough, the convergent margin off central Japan, linking marine geophysics, borehole science, and tsunami research. The program brought together institutions such as Japan Agency for Marine-Earth Science and Technology, the International Ocean Discovery Program, and national agencies to target the plate boundary megathrust where recurring great earthquakes have impacted Honshu and Shikoku. The project combined data from drilling vessels, submersibles, and seafloor observatories to resolve fault mechanics, fluid flow, and seismic coupling along a segment associated with historic events like the 1707 Hōei earthquake.
Overview
The experiment investigated the seismogenic portion of the plate interface between the Philippine Sea Plate and the Eurasian Plate beneath the Nankai Trough by deploying scientific platforms including the drillship Chikyū, the riser-equipped vessel operated by JAMSTEC, and research vessels associated with the Integrated Ocean Drilling Program lineage such as the JOIDES Resolution. It targeted sections of the megathrust historically implicated in episodes recorded in sources like the 1707 Hōei earthquake and seismic catalogs referenced by the Japan Meteorological Agency. The program integrated techniques from ocean drilling, seismic reflection profiling, and borehole observatories akin to designs from the Ocean Networks Canada and CORK installations.
Scientific Objectives
Primary scientific objectives included characterizing the physical state of the megathrust where slip generates earthquakes and tsunamis, constraining thermal and pore-fluid conditions that control rupture propagation, and measuring in situ stress and deformation across the plate boundary. Investigators from institutions such as Universidad Nacional Autónoma de México, Woods Hole Oceanographic Institution, and National Institute of Advanced Industrial Science and Technology contributed expertise in fault zone hydrogeology, rock mechanics, and paleoseismology. Complementary aims involved correlating drilled cores and borehole logs with seismic imagery from groups like USGS and Geological Survey of Japan to refine earthquake recurrence models used by agencies such as the Cabinet Office (Japan) for hazard assessment.
Drilling and Instrumentation Campaigns
Drilling campaigns used the riser-capable Chikyū and non-riser platforms coordinated with expeditions under the IODP and its predecessors, deploying borehole instruments including long-term temperature sondes, tiltmeters, and distributed acoustic sensing arrays. Cores were recovered for petrology and geochemistry analyses alongside downhole logging by teams from Lamont–Doherty Earth Observatory, Kochi Institute for Core Sample Research, and Earthquake Research Institute (University of Tokyo). Seafloor observatories and moorings deployed seismic and pore-pressure sensors were linked to real-time telemetry tested against networks like DONET and compared with tsunami observations archived by the Japan Meteorological Agency and international tsunami warning centers.
Key Findings and Results
Results demonstrated that the shallow megathrust harbors highly pressurized fluids and low effective stress conditions associated with smectite-rich fault gouge, corroborating laboratory friction experiments at facilities such as Rock Deformation Laboratory (Caltech) and OKC Rock Physics Laboratory. Borehole temperature anomalies constrained frictional heating during past slips, while microseismicity patterns imaged by dense arrays revealed slow-slip and transient events similar to those documented near Tokai and Boso regions. Integration of core petrology, geochemistry, and geophysical logs showed spatial variations in sediment compaction, permeability, and cementation that influence rupture nucleation and propagation, aligning with paleotsunami deposits studied by researchers at University of Tokyo and Kyoto University.
Seismic Hazard Implications
Findings informed probabilistic seismic hazard models for central Japan, impacting assessments used by the Cabinet Office (Japan) and infrastructure planning for cities like Osaka and Nagoya. The documentation of high pore pressure and weak fault materials at shallow depths implies potential for large tsunami-generating slips, echoing scenarios from the 1707 Hōei earthquake and the Ansei Tokai earthquake sequence. Results were incorporated into tsunami inundation modeling employed by municipal authorities and into national preparedness strategies coordinated with the Japan Meteorological Agency and international bodies monitoring subduction zone hazards.
International Collaboration and Funding
The experiment was funded and organized through partnerships among JAMSTEC, the Japan Agency for Medical Research and Development (for related health impacts studies), the International Ocean Discovery Program, national science agencies including NSF and NERC via cooperative agreements, and university consortia from United States, United Kingdom, Australia, and South Korea. Collaborative governance involved memorandum exchanges between ministries such as Ministry of Education, Culture, Sports, Science and Technology (Japan) and agencies including METI and research centers like IFREE. Peer-reviewed outputs were produced by multinational author teams from institutes including ETH Zurich, CNRS, and Geological Survey of Japan.
Future Research and Continuing Monitoring
Ongoing work emphasizes expanded borehole observatories, longer-term monitoring of slow-slip events, and high-resolution seismic imaging using deployments synchronized with programs like DONET and future IODP expeditions. Prospective studies involve multidisciplinary teams from Tohoku University, Seikei University, and international partners to refine rupture models, improve early warning systems managed by the Japan Meteorological Agency, and translate findings into resilience planning for coastal prefectures such as Kochi and Shizuoka. Continued integration with paleoseismology research and satellite geodesy from missions like GOSAT and platforms under JAXA will further constrain recurrence intervals and tsunami risk.