UTIG RESEARCH PROJECTS ARCHIVE
Principal Investigators:
Nathan L. Bangs (UTIG)
Tom Shipley (UTIG)
J. Casey Moore (University of California, Santa Cruz)
Gregory F. Moore (University of Hawaii)
Julia K. Morgan (University of Washington)
Asahiko Taira (Ocean Research Institute, The University of Tokyo)
Shuichi Kodaira (Deep Sea Research Department, Marine Science & Technology Center)
Shozo Kimura (Earthquake Observatory, Kochi University)
Toshinori Sato (Earthquake Research Institute, University of Tokyo)
Ryota Hino (Observation Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University)
Kiyoshi Suyehiro (Ocean Research Institute, The University of Tokyo)
Funding agency: National Science Foundation - Award #9730637
Project Summary:
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We propose a three-dimensional (3-D) seismic reflection, active refraction and passive ocean bottom seismicity experiment in the Nankai Trough where great earthquakes have repeatedly struck and historical and archaeological evidence document 1300 years of interplate seismicity. This is a study of the subduction thrust from the aseismic zone at the trench to where the thrust lies 12 km subseafloor, is well within the seaward portion of the seismogenic zone, and fault strength was sufficient to accumulate stress that contributed to the Nankaido earthquake of 1946 (M=8.1). Our goal in this proposal is to examine the transition from the dominantly consolidating aseismic front of a subduction thrust into the realm of brittle deformation and ultimately into the seismogenic zone. Distinct differences in the seismic reflection waveforms are expected from the subduction megathrust as sediments subduct through significant sediment-to-rock transitions. Existing seismic cross sections illustrate the presence of a fault zone reflection extending from the aseismic zone at the trench to segments of the thrust that are potentially seismogenic. But, a properly imaged, higher signal-to-noise data set is necessary for quantitative modeling of physical properties across the major transitions along the thrust. The passive seismicity experiment combined with a Japanese cable-OBS program will improve the location of the region of microseismicity and will allow us to related changes in fault properties to seismicity.
The Nankai subduction thrust is arguably the best place to examine fault processes because of the high heat flow and hence shallow sediment-to-rock transitions. A U.S. Margins program workshop on the Seismogenic Zone Experiment (SEIZE) in 1997 identified Japan (Nankai Trough and Japan Trench) and Central America (Costa Rica and Nicaragua) as preferred sites for field programs for studying the subduction thrust earthquakes, and this proposal could become an element of a SEIZE program for the Nankai Trough. Furthermore, we have not discovered any other place along the Nankai margin, or elsewhere, that has these clear seismic imaging advantages relative to the location of the seismogenic zone, seafloor multiple and location of the décollement within the sedimentary section.
The proposed program provides valuable additions to the previous as well as potential future drilling programs. The 3-D survey will place Ocean Drilling Program Site 808 into a spatial perspective and its relationship to adjacent active fluid conduits. This work will provide additional information for shallow subbottom drilling in the toe region for studying the interrelationship between fluid flow and deformation at three sites proposed along this transect (JOIDES proposal 445). Identification of the relatively shallow (2-3 km subbottom) up-dip extent of active out-of-sequence thrusts soling into the décollement at depths of 10-15 km could be important JOIDES Resolution targets for sampling the fluid chemistry as a proxy for processes occurring at great depth. Our proposed survey should also reveal potential targets for riser drilling with the OD21 project in less than 2500 m water depth.