Researchers fresh from an eight-week scientific drilling expedition off the Pacific coast of Japan today reported their discovery of strong variation in the tectonic stresses in a region notorious for generating devastating earthquakes and tsunamis, the Nankai Trough. The scientists conducted their expedition aboard the new scientific drilling vessel Chikyu, drilling deep into the zone responsible for past and likely future tsunamis, and collecting physical measurements and images made using advanced borehole logging technology. Their achievement marks the launch phase of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a major research initiative into the triggers and mechanisms of earthquakes and tsunamis supported by the Integrated Ocean Drilling Program (IODP). NanTroSEIZE is expected to continue until 2012, with the ultimate objectives of drilling across the plate boundary fault responsible for magnitude 8 earthquakes to sample the rocks and fluids in the fault, and to place instruments within it to monitor activity and conditions leading up to the next great earthquake.
IODP NanTroSEIZE Expedition 314 researchers (16 onboard scientists representing six countries) conducted logging-while-drilling operations at five sites in the Nankai Trough accretionary wedge (the undersea mountain range formed where the plates meet) to measure geological formation properties. Data recovered informed researchers about rock and sediment density, porosity, the velocity of sound speed, natural gamma ray, and resistivity. In waters more than 2,000 meters (1.2 miles) deep, they drilled holes ranging from 400 to 1,400 meters below the sea bed to study conditions in the boundary plate region.
“The rock caught up in the tectonic plate boundary is literally falling apart as a result of the intense stresses of tectonic plate convergence,” said co-chief scientist Harold Tobin, associate professor of geology and geophysics at the University of Wisconsin-Madison. “By drilling a transect spanning the area of tsunami generation, we found that the region that lies above the earthquake-producing zone exhibits very different stress conditions than other parts of the plate boundary.”
Co-chief scientist Masataka Kinoshita elaborated, “Caught in the vise between two converging rigid tectonic plates, the wedge was found to be undergoing strain in preparation for the next earthquake.” Kinoshita leads a group of deep-sea researchers at the Institute for Research on Earth Evolution (IFREE) at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). JAMSTEC’s Center for Deep Earth Exploration operated and managed Expedition 314 on behalf of IODP.
In addition to the tectonic stress findings, the research team identified a methane hydrate-rich zone 220-400 meters below the seafloor at one drill site, using resistivity imaging and other advanced logging techniques. Gas hydrates are concentrated in the sandy layers of numerous turbidites in this zone. The hydrate layer was imaged in a three-dimensional seismic reflection survey and the recorded logs now allow researchers to pinpoint the location and volume of this substantial gas hydrate deposit with unprecedented precision. This new information is expected to augment past research related to gas hydrate deposits and their relationship to climate change.
Drilling expeditions following Expedition 314 will complete Stage 1 of this four-stage drilling project. IODP Expeditions 315 and 316 will revisit the drilled sites to take continuous core samples from the upper section of the active accretionary prism and across the plate boundary faults down to 1,000 meters below the sea floor. A pilot hole for riser drilling also will be drilled in preparation for future deeper scientific ocean drilling. Future stages of NanTroSEIZE target the seismogenic portion of the plate boundary fault at as much as six kilometers (3.7 miles) below the sea bed. Cores will be collected and conditions within and around the fault zone will be measured to test stresses detected on Expedition 314 and observations that extend to depth and relate to earthquake generation.
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