Collaborative Research: Development of a Long-term Hydrologic Observatory Above the Seismogenic Zone Offshore the Kii Peninsula, Japan

This project is the U.S. component of a borehole observatory to be installed at a drillsite in the Kumano Basin, offshore Honshu Island, Japan. Integrated Ocean Drilling Program (IODP) drilling in this area is part of 'NanTroSEIZE' (the Nankai Trough Seismogenic Zone Experiment), a multi-stage, international collaboration to understand the seismogenic zone of subduction megathrusts. Establishment of a distributed long-term observatory network spanning the up-dip limit of the seismogenic zone is one fundamental goal of the overall drilling program. At this site, pore pressure, strain, and temperature will be monitored in forearc basin sediments and the underlying accretionary wedge. The specific objectives of monitoring are to document (1) temporal patterns of strain, (2) formation pressure response to known tidal loading to constrain formation compressibility and hydraulic diffusivity, and (3) formation pore pressure, both to determine ambient values and to record hydrologic transients. Additionally, from an operational standpoint, the observatory is critical as a pilot hole and testbed for planning operations for deeper riser drilling at this location.

Collocated long-term monitoring of seismicity, pore pressure, and strain are needed to test hypotheses for strain localization and locking along faults, and for plate boundary weakness that invoke suprahydrostatic fluid pressures within wall rocks and along faults. For the former application, pore pressure provides a sensitive indication of volumetric strain that complements strainmeter observations. The selected site is especially critical for monitoring, because it is the landward-most site in the planned network, and lies immediately above a zone of significant slip during the last great earthquake (1944 Tonankai M 8.2 event), and also above a cluster of very low frequency (VLF) thrust earthquakes on the plate boundary system in 2004.