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.
In this project, we used extremely high-resolution three-dimensional imaging of the Earth’s crust beneath the Pacific Ocean to shed light on the offshore tectonic faults that form the boundary between two tectonic plates. These offshore faults, known as subduction zones, are the source of the largest earthquakes experienced on the planet (those of magnitude 8 or above), and also of tsunamis when they cause the seabed to be uplifted by fault motion during the earthquake. Understanding what governs the likelihood, timing, location and magnitude of fault slip for such plate boundary faults is a major research problem in marine geophysics in light of the destructive tsunamis of the past decade. The project involved seismic reflection (sound wave) imaging of the plate boundary at the Nankai Trough offshore Japan. This location has been intensively studied in recent years by seismologists and geologists from all over the world as a "natural laboratory" that typifies the offshore subduction faults that present tsunami hazards globally. It has the world’s longest historical record of magnitude 8 or greater earthquakes recurring every 120 years on average, and it has repeatedly produced tsunamis that have killed thousands of people. Because of this long history, geophysical studies here are used to understand not only the considerable hazard this particular subduction zone represents, but also the universal processes that govern such faults and the tsunamis they can produce. In an international collaboration that began this project, a 55 by 11 kilometer area was imaged with reflected sound waves by a dedicated seismic imaging ship to more than 12 kilometers depth. After extensive sophisticated image processing, the resulting three-dimensional dataset reveals the architecture and characteristics of the rocks and sediments and the many complex faults they host in unprecedented detail. In this NSF-funded project, we analyzed that 3D image volume specifically to map out the major active faults of the plate boundary zone and unravel its geologic history. Our work has shown that certain faults were active at different times over the past several hundred thousand to a few million years (and therefore over many earthquake recurrence cycles). This plate boundary fault zone history and detailed geometry provides key evidence for the subsurface environment in which earthquake slip has occurred and will again in the future. This work allows our team and others to draw inferences about the values of rock strength, pore water pressure, stress, and other parameters of the zone rocks, all of which are used in models of earthquake strain buildup and slip. Our work also has defined which branches of the fault system have been activated during past earthquakes and fault slip, shedding light on the question of what conditions favor larger or smaller tsunamis for a given size earthquake. This seismic imaging study also provides the basis for scientific drilling targets in the still-ongoing Nankai Trough Seismogenic Zone experiment, or "NanTroSEIZE," a major international research project involving drilling into, sampling, and instrumenting the Nankai subduction fault zone beneath the deep ocean. The drilling at Nankai had been done as part of the Integrated Ocean Drilling Program (IODP), using the scientific drilling vessel Chikyu, operated by Japan as part of this international collaboration involving scientists from more than 20 countries. Our work on the 3D seismic images allowed us to select and define the drilling targets at depths of up to 5 or more kilometers (about 3 miles) beneath the sea floor, pinpointing the locations on the plate boundary fault zones where core samples and measurements can best be used to determine properties of the rocks and sediments in their "native habitat" deep beneath the sea bed. Our work on the detailed reflection properties of those faults zones, calibrated and "ground-truthed" with the drilling samples, has allowed us to determine properties of the fault zones and how they vary far from the boreholes themselves. Taken together with the work of our many collaborators on this over-arching NanTroSEIZE project, this study has produced new understanding of the conditions inside plate boundary faults. Those advances are rapidly expanding our understanding of the causes of destructive tsunamis and earthquakes.
