Abstract

Most of the world's great earthquakes are inter-plate underthrusting events in the subduction zones of convergent margins. As the December 2004 Sumatra earthquake and Indian Ocean tsunami demonstrated, subduction zone earthquakes represent one of the greatest natural hazards on the planet, but little is known about rock properties deep within the region where earthquakes are generated — the seismogenic zone.

The Nankai Trough, the subduction zone south of Japan, has a 1300-year historical record of recurring and typically tsunamigenic great earthquakes about every hundred years, making it a prime candidate for scientific investigation. As part of a larger international program, to study earthquake genesis in convergent margins, the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) seeks to first image, and then drill into, the shallow portion of the region where earthquakes are generated.

During April-May, 2006, we collected an ~600 km2 3D seismic data volume over the Kumano forearc basin and Nankai Trough accretionary prism southeast of the Kii Peninsula of Honshu, Japan. Our data images the subducting Philippine Sea plate to a distance >60 km from the deformation front and to a depth of ~10 km under Kumano Basin. A large thrust fault ascends from near the top of the descending oceanic crust and reaches the surface near the seaward margin of Kumano Basin. Because the basin sediments above the thrust are strongly tilted landward, we infer that much of the plate motion is taken up by repeated movement along the thrust, probably during major earthquakes. If this thrust fault broke the surface during earthquakes, we would expect that huge tsunamis were generated.

The Integrated Ocean Drilling Program (IODP) is scheduled to begin drilling into the shallow portion of the Nankai subduction zone late in 2007 with the new Japanese deep drilling vessel, Chikyu, and another drill ship operated by the U.S. Beginning in 2008, a series of deep holes will be drilled with Chikyu, including one that is planned to penetrate all the way to the plate boundary in the region where earthquakes are believed to be generated. The ultimate goal of NanTroSEIZE is to sample and instrument the seismogenic portion of this subduction zone fault.

Presented at: 2007 South East Asia Petroleum Exploration Society (SEAPEX) Conference, Singapore, 2007