AUSTIN, Texas—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 offshore 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 earth.
The research team is exploring the Nankai Trough, 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 Dr. Nathan Bangs, a scientist at The University of Texas at Austin participating on the current research voyage. The voyage is a two-part program of drilling, logging and installing long-term observatories to monitor geologic activity in the area.
Scientists are seeking information about the connection between fluid-flow in the sediments and deformation processes and when tectonic plates collide, as well as information on how this ultimately affects earthquakes.
"This knowledge is not only important scientifically, but also has great societal relevance," said Dr. Casey Moore, a geoscientist from the University of California at Santa Cruz and one of the 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."
Earthquake research that took place on the vessel from May 3 through 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 the 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 in this research effort were developed by the Schlumberger Technology Corporation, which is involved in executing Ocean Drilling Program’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 Dr. David Goldberg, director of the Borehole Research Group at Columbia University’s Lamont-Doherty Earth Observatory.
Scientists deployed three Schlumberger state-of-the-art LWD tools — the Azimuthal Density Neutron (ADN), Isonic and Resistivity-at-the-Bit (RAB) tools. The RAB allows scientists to collect 360-degree images of the geologic formation while the hole is being drilled, making it possible to identify borehole structures such as fractures, faults and stratigraphic contacts. Scientists can use the RAB, which has a resolution of 15-to-30 centimeters, to peer at structural information within a fault zone or an active tectonic area.
"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."
During the research effort currently in progress, two drill holes are being 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.
Funded by the U.S. National Science Foundation, with substantial contributions from international partners, the program is managed by Joint Oceanographic Institutions, a consortium of 14 U.S. institutions. The University of Texas Institute for Geophysics (UTIG) is known internationally as a leading academic research group in geology and geophysics.
For more information, contact Dr. Nathan Bangs at (512) 471-0424 or Dr. Katherine Ellins at (512) 232-3251. Visit the UTIG Website at <http://www.ig.utexas.edu/>