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July 9, 2001

Researchers seek clues to submarine quakes at plate collision zone

By Tim Stephens

Seeking new clues to the cause of some of the Earth's most powerful earthquakes, an international group of scientists aboard the Ocean Drilling Program vessel JOIDES Resolution is using special technology to measure and monitor physical properties at a convergent plate margin off the shore of Japan. The researchers hope their findings improve risk assessment of catastrophic quakes, which also can trigger tsunamis (sometimes called tidal waves.)

The Ocean Drilling Program (ODP) is an international partnership of scientists and research institutions organized to explore the evolution and structure of the Earth. ODP Leg 196 is a two-part program of drilling, logging, and installing long-term observatories to monitor geologic activity in the area known as the Nankai Trough. Scientists are seeking information about the connection between fluid-flow in the sediments and deformation processes and when tectonic plates collide, and how this ultimately affects earthquake events.

"This knowledge is not only important scientifically, but also has great societal relevance," said Casey Moore, a professor of Earth sciences and one of three chief scientists heading the research project. "The great forces associated with convergent margin subduction and deformation can produce devastating earthquakes, trigger tsunamis, cause rapid subsidence and uplift in coastal areas, and create lines of active volcanoes both on land and on the seafloor."

The Nankai Trough is a geologically active area where two of Earth's tectonic plates collide, producing major quakes. Referred to as a convergent margin, Japan's Nankai Trough is typical of such settings worldwide. It forms as the more buoyant (or lighter) continental crust of Japan overrides a denser, heavier oceanic plate sliding under the Japanese islands. At the same time, the continental crust pushes up a thick wedge of sediments called an accretionary prism.

"Imagine that Japan is the bulldozer scraping sediments off the seafloor of the Philippine Sea plate as it collides and dives beneath Japan at the Nankai Trough," explained Nathan Bangs, a scientist from the University of Texas at Austin who sailed on ODP Leg 196.

The earthquake research on the first part of Leg 196 (May 3 to June 1) was conducted with special logging-while-drilling (LWD) technology developed by the petroleum drilling industry. To gather data, or logs, using traditional methods, tools are lowered through boreholes drilled in the Earth after the core has been removed. With LWD, however, scientists simultaneously collect data from monitoring devices, or logging tools, located directly behind the drill bit as the hole is being drilled. The special tools used on Leg 196 were developed by the Schlumberger Technology Corporation, which is involved in executing ODP's logging program at sea.

"The dynamic deformation processes and high fluid pressures in the sediments of an accretionary prism make it extremely difficult to collect these data using conventional methods, because the holes collapse before the tools can be deployed," said David Goldberg, director of the Borehole Research Group at Columbia University's Lamont-Doherty Earth Observatory.

One of the Schlumberger LWD tools deployed on Leg 196, the Resistivity-at-the-Bit (RAB) tool, collects 360-degree images of the borehole during drilling, allowing scientists to identify borehole structures such as fractures, faults, and stratigraphic contacts.

"During this cruise, the RAB tool provided us with the clearest and most dramatic images of borehole 'breakouts' that have ever been recorded in the Ocean Drilling Program," said Goldberg. "Breakouts are hole enlargements that indicate regional stress direction and magnitude, and are key to understanding the tectonics in this area."

Scientists also detected the invasion of borehole fluids into the borehole formations and collected information on porosity that helps define zones under high pressure, which may contribute to fluid flow along fault lines.

During the second half of Leg 196 (June 3 to July 3), two drill holes were sealed with special monitors called "advanced circulation obviation retrofit kits," or ACORKs. The monitors will track fluid flow and tectonic processes for a period of three to five years.

Keir Becker of the University of Miami's Rosenstiel School of Marine and Atmospheric Science developed the ACORKs. "Instrumenting the two holes with ACORKs will begin a long-term program of observation of seismicity, fluid-flow parameters, and fluid geochemistry at the Nankai Trough, a program that will involve future revisits by manned and unmanned submersibles," he said.

The Ocean Drilling Program is funded principally by the U.S. National Science Foundation, with substantial contributions from its international partners. These include the Federal Republic of Germany, Japan, the United Kingdom, the Australia/Canada/Chinese Taipei/Korea Consortium for Ocean Drilling, the European Science Foundation Consortium for Ocean Drilling (Belgium, Denmark, Finland, Iceland, Ireland, Italy, The Netherlands, Norway, Portugal, Spain, Sweden, and Switzerland), France, and the People's Republic of China. The program is managed by Joint Oceanographic Institutions, a consortium of 14 U.S. institutions. Texas A&M University is responsible for science operations, and Lamont-Doherty Earth Observatory of Columbia University is responsible for logging services.

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