The PI team will acquire, process, and interpret a high-quality 3-D seismic reflection data volume in the Kumano Basin region south of Kii Peninsula, Japan. The 3D seismic survey will be carried out in April 2006 by PGS ship R/V Ramform Victory (Norwegian registry), jointly supported by the NSF and JAMSTEC. These data will define the fundamental geometry, history and physical properties of the multiple faults that compose the up-dip end of the seismogenic zone, many of which ruptured in the 1944 M 8.1 Tonankai earthquake. The proposed work is a major component of the integrated Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), including the recently highly ranked shallow non-riser drilling and the deep riser drilling planned through the Integrated Ocean Drilling Program (IODP). With ground-truth from drilling and associated borehole geophysical experiments, this 3D seismic volume will provide unprecedented illumination of a plate boundary fault system to visualize its geometry, history, and associated rock properties, as it evolves down dip into a seismogenic zone. Furthermore, 3D imaging and seismic attribute mapping are essential components for refining drill site locations and for detailed planning for drilling and casing operations in support of the most ambitious scientific ocean drilling operation ever conceived. This project is a collaborative effort among four U.S. and two Japanese institutions to contract a seismic company to collect and provide preliminary processing of an ~20x80 km 3-D seismic reflection grid, along a transect from the Kumano Basin to the frontal thrust of the Nankai accretionary prism. The study encompasses a number of broader impacts, including the societal benefit of better understanding the origin of dangerous subduction zone thrust earthquakes.
The Nankai Trough subduction zone, south of Honshu, Japan, has experienced major earthquakes and tsunami on an ~150 year timescale for the past 2000 years of recorded history. To understand the internal structure of the region, we collected a 792 km2 3D seismic reflection data set. The original processing, through pre-stack time migration, was completed by a commercial contractor. As part of our grant, we worked with colleagues at the Japan Agency for Marine Earth Science and Technology (JAMSTEC) to produce a full 3D pre-stack depth migration, which is now archived at JAMSTEC. The data set was used to choose locations for Integrated Ocean Drilling Program (IODP) drilling. We have now drilled at 13 Sites, recovering cores and digital logging data (available from IODP). Our 3D data set images the plate boundary thrust fault and an overthrusting accretionary prism formed of sediment scraped from the descending Philippine Sea plate. The frontal region is a typical in-sequence thrust sequence. The frontal zone is separated from a deep forearc basin by a major out-of-sequence fault ("megasplay fault") that branches from the plate boundary fault and cuts through the older accretionary prism. Uplift caused by motion on the megasplay fault caused uplift, forming a barrier to sediment transport and forming the forearc basin about 1.5 million years before present. More than 4,000m of sediment that accumulated in the basin is progressively tilted landward due to continued movement along the megasplay fault. The 3D data set continues to guide IODP drilling, which is planned to continue in 2012-2014. The core and logging data allow us to place age and lithology constraints on our seismic interpretations, allowing us to determine the history of deformation in this active convergent margin. This project supported one post-doctoral researcher, one undergraduate student and two graduate students. It has also fostered international collaborative research with colleagues from Japan, Switzerland and the UK.
