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.
The main objective of this project was to install pore pressure sensors in concert with a broadband seismometer, tiltmeter, strainmeter, and an array of geophones funded by collaborators at JAMSTEC in Japan. The installation comprises a co-located, multi-level hydrologic and geodynamic observatory, at a location directly above the subduction megathrust plate boundary near the updip edge of earthquake slip in the 1944 Mw 8.2 earthquake, and also directly above regularly occurring unusual very low frequency earthquake swarms that appear to localize along the plate interface. The observatory is the first of its kind, and marks one of the most complex observatory systems ever installed in IODP. From its installation in December, 2010 through late January, 2013, the observatory recorded pore pressure in three zones downhole and at a seafloor reference gauge at 1 minute intervals. The recorded data, including a clear response to the M 9.0 Tohoku earthquake and aftershock sequences, were downloaded for the first time on January 24, 2013. The observatory system was successfully connected to a real-time cabled network at that time. The cabled network provides power for the seismometer, tiltmeter, and strainmeter, and allows a sampling frequency of 1 Hz for the pore pressure sensors. The full suite of instruments is now transmitting data to shore in real time.
