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Topic Name: Researchers Distinguish Waves from Mine Collapses from Other Seismic Activities
Category: Earthquake Engineering
Research persons: Bill Walter
Location: Lawrence Livermore National Laboratory, United States
Details
Researchers have devised a technology that can distinguish mine collapses
from other seismic activity. Using the large seismic disturbance associated with
the Crandall Canyon mine collapse last August,
Lawrence Livermore National
Laboratory scientists and colleagues from the
Berkeley Seismological
Laboratory at UC Berkeley applied a method developed to detect underground
nuclear weapons tests to quickly examine the seismic recordings of the event and
determine whether that source was most likely from a collapse. They also found
an additional string of secondary surface seismic waves that occurred when the
mine collapsed, which are like no other mine collapse events in recent history.
The tragic collapse of a Utah coal mine on Aug. 6 resulted in the deaths of
six miners. Ten days later, another collapse killed three rescue workers.
The event was recorded on the local network of seismic stations operated by
the U.S. Geological Survey as well as the
National Science Foundation
Earthscope USArray stations. The collapse registered as a 3.9 magnitude event.
"Our group had already been working on a full seismic waveform matching
technique as a means to distinguish between nuclear explosions, earthquakes and
collapse events by their seismic signals," said Bill Walter, one of the LLNL
researchers.
The new study could help researchers better differentiate underground nuclear
tests from earthquakes, mine collapses, mine blasts and other events that
generate seismic waves.
UC Berkeley graduate student and LLNL Lawrence Scholar Sean Ford was able to
quickly collect the data from the Crandall Canyon seismograms at the time and
plug it into the Laboratory algorithm that pointed to a collapse rather than an
earthquake.
"These results were posted within a few days after the event and were helpful
in resolving the source of the magnitude 3.9 seismic signal," Ford said.
The new technique compares model seismograms to the observed seismograms at
local to regional distances (0-1,500 kilometers) at intermediate periods (five
to 50 seconds).
Another notable fact about the collapse: The team detected Love waves (also
named Q waves - surface seismic waves that cause horizontal shifting of the
earth). Typically small in instances such as large mine collapses or hole
collapses that sometimes follow nuclear tests, Walter said the Love waves from
the Crandall Canyon collapse are "larger than expected for a pure vertical
collapse due to gravity."
Though the cause of the Love waves is not fully known, there are several
theories, according to Walter.
"One speculative explanation consistent with the data is that the collapse
was uneven, with one side closing more than the other," he said. But he said
further studies are necessary.
Ford said the Crandall Canyon event was relatively small, magnitude wise.
"The fact that we could identify the Crandall Canyon event from its seismic
signature gives us confidence that it would be possible to identify even
relatively small nuclear explosions using this technique."
"We are excited about the potential of this regional seismic full waveform
matching technique and are continuing to develop and test it on other events in
others parts of the world to fully understand it," Walter said.
Douglas Dreger of the Berkeley Seismological Laboratory also contributed to
the research.
About Seismic Wave
Seismic waves are waves that travel through the Earth, most often as the result
of a tectonic earthquake, sometimes from an explosion. Seismic waves are also
continually excited by the pounding of ocean waves and the wind. Seismic waves
are studied by seismologists, and measured by a seismograph, which records the
output of a seismometer, or geophone. For seismic studies of oil reservoirs,
hydrophones may give additional information.
There are two types of seismic waves, body waves and surface waves. Other modes
of wave propagation exist than those described in this article, but they are of
comparatively minor importance. An excellent audience demonstration for seismic
waves is shown in slinky seismology.
Surface waves are analogous to water waves and travel just under the Earth's
surface. They travel more slowly than body waves. Because of their low
frequency, long duration, and large amplitude, they can be the most destructive
type of seismic wave. There are two types of surface waves: Rayleigh waves and
Love waves.
About Crandall Canyon Mine
The Crandall Canyon Mine, formerly Genwal Mine, was an underground bituminous
coal mine in northwestern Emery County, Utah.
The mine made headline news when six miners were trapped by a collapse in August
2007. Ten days later, three rescue workers were killed by a subsequent collapse.
The Crandall Canyon Mine is located at 39°27′36″N, 111°10′03.5″W (39.460000°,
-111.167639° with its entrance at an elevation of 7,385 feet (2,251 m). It
resides in Emery County, Utah, about 15 miles (24 km) west north-west of
Huntington. Located just off State Route 31, the mine is about 34 miles (55 km)
southeast of Fairview, and 140 miles (225 km) south of Salt Lake City. The mine
is within Crandall Canyon in the Wasatch Plateau Coal Field. The permit area for
the mine covers an area of more than 5,000 acres (20 kmē) utilizing fee land,
federal, and state leases. The Manti-La Sal National Forest surrounds the mine.
The mine conducts surface operations on 10 acres (40,000 mē) of disturbed land
within the forest.
The mine is co-owned by UtahAmerican Energy, Inc. (formerly Andalex Resources),
a company with approximately US$65.1 million in annual sales, headquartered in
Sandy, Utah. UtahAmerican is a subsidiary of Cleveland, Ohio based Murray Energy
Corporation, owned by Robert E. Murray. The Crandall Canyon mine is operated by
Genwal Resources Inc., an operating division of UtahAmerican. The other co-owner
is the Intermountain Power Agency (IPA) of South Jordan, Utah.
Founded in 1952, Lawrence
Livermore National Laboratory is a national security laboratory, with a
mission to ensure national security and apply science and technology to the
important issues of our time. Lawrence Livermore National Laboratory is managed
by Lawrence Livermore National Security, LLC for the U.S. Department of Energy's
National Nuclear Security Administration.
In figure, (A) Locations of the 6 August 2007 Crandall Canyon seismic event
and six seismic stations used to analyze it. (B) On a source-type plot the three
types of seismic events separate cleanly into explosion (red), earthquake (blue)
and collapse (green) populations. The Crandall Canyon (yellow star) event plots
with the collapses. (C) The source type plot comes from fitting model
seismograms (red) to the observed data (black).
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