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
"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
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