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Detecting Seismic
Activity
of
Man-Made Events |
Using
Seismic Data to Confirm, Assess, and Record Man-Made Events
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Understanding
Seismic Waves |
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When
you throw a pebble into a pond, it causes rippling
waves in all directions from the point of impact.
Earthquakes
generate seismic waves in much the same way. The
waves radiate out through the Earth and travel
great distances. Seismic waves
travel at high speeds losing much of their energy
as they travel. |
Sensitive
detectors, connected to a system for recording
information (seismographs or seismometers), gather
the wave data. The
permanent records of detected waves are called
seismograms. When
an earthquake occurs, vast quantities of energy
stored in the rock structures are released in a
very short time. The point at which the release
occurs is known as the focus. The epicenter is
the point on the surface immediately above the
focus. The focus may
be close to the surface or much deeper. *Note:
The terms Seismometer and Seismograph are generally
interchangeable
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Some of
the energy released by an earthquake is transferred
to heat, but the majority is transferred as wave
energy and transmitted for long distances. Earthquake
waves can be classified in three types - P or
push / primary waves, S or
shake/secondary waves and L or
surface waves. |
The P wave is
designated the primary preliminary wave because
it is the first to arrive at a seismic station
after an earthquake. It travels at a speed
usually less than 6 kilometers per second in the
Earth's crust and jumps to 13 kilometers per second
through the core.
The S wave is the secondary preliminary wave to
be recorded. It follows paths through the Earth
quite similar to those of the P-wave paths, except
that no consistent evidence has yet been found
that the S wave penetrates the Earth's core.
Source: USGS Read
more about Seismographs and how waves are recorded
at:
http://neic.usgs.gov/neis/seismology/keeping_track.html |
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Modern
Seismometer
Most seismometers
today are electronic, but the basic seismometer
is made of a drum with paper on it, a bar or
spring with a hinge at one or both ends, a
weight, and a pen. The one end of the bar or
spring is bolted to a pole or metal box that
is bolted to the ground. The weight is put
on the other end of the bar and the pen is
stuck to the weight. The drum with paper on
it presses against the pen and turns constantly.
When there is an earthquake, everything in
the seismometer moves except the weight with
the pen on it. As the drum and paper shake
next to the pen, the pen makes squiggly lines
on the paper, creating a record of the earthquake.
This record made by the seismometer is called
a seismogram.
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Strong
Motion Seismometer
Another type of seismometer
is a digital strong-motion seismometer, or
accelerograph.
The one pictured here is similar to those
used at the Pacific
Northwest Seismograph Network. Visit their
site for more information. http://www.pnsn.org/
A
strong-motion seismometer measures acceleration.
This can be mathematically integrated later
to give velocity and position. Strong-motion
seismometers are not as sensitive to ground
motions as teleseismic instruments but they
stay on scale during the strongest seismic
shaking. |
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Ancient
Chinese Seismometer
The
first seismometer was invented by
the Chinese astronomer and mathematician
Chang Heng about 2000 years ago. He called
it an "earthquake
weathercock." It was a special vase that
had several sculpted dragons mounted
around the sides. Held in the mouth of
each dragon was a small metal ball.
When the ground shook, one of the
balls would fall from the mouth of the
dragon into the waiting mouths of the
sculpted frogs making enough noise to
alert someone that an earthquake had
just occurred. Imperial watchmen could
tell which direction the earthquake came
from by seeing which dragon's mouth was
empty. Heng wrote Lingxian, a
summary of Chinese astronomical knowledge
and you can learn about him
at http://www.china.org.cn/english/scitech/131762.htm
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Events
that Cause Seismic Waves
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WORLD TRADE CENTER 9-11-01
On September 11,
2001 Columbia University’s Lamont-Doherty Earth
Observatory recorded seismic signals produced by the
impact of the two aircraft hitting the Twin Towers of
the World Trade Center.
The ground shaking was consistent with the energy
released by small earthquakes, however, it was not sufficient
to cause the collapse and damage to the surrounding buildings. The
buildings around the Twin Towers were damaged by the kinetic
energy of the falling debris and the pressure exerted on
them by the debris and particle laden blast produced during
the collapse of the two towers. Learn
More at the Earth Institute at Columbia University. |
Seismographic
recordings of the tower collapses were recorded in five
states, as far away as 428 kilometers [266 miles] in
Lisbon, New Hampshire. Lamont’s home station, in
Palisades, New York, is located above the Hudson River,
34 kilometers [21 miles] from downtown Manhattan, where
the towers stood. The aircraft impacts registered local
magnitude (ML) 0.9 and 0.7, indicating minimal earth
shaking as a result. The subsequent collapse of the towers,
on the contrary, registered magnitudes of 2.1 and 2.3,
comparable to the small earthquake that occurred beneath
the east side of Manhattan on January 17, 2001. Source:
November 20, 2001, issue of Eos, published by
the American Geophysical Union, seismologists from Columbia’s
Lamont-Doherty Earth Observatory |
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MINNING EXPLOSIONS
Explosions can generate seismic waves
similar to those produced by earthquakes and other natural causes. Seismograms
at a given station for explosions at the same mine tend to be
similar from event-to-event, both in the relative times and amplitudes
of different seismic phases within each seismogram and in the absolute
amplitudes of the seismic phases. Source: USGS
The
seismic and acoustic discrimination of large surface and
underground mine blasts, including mine collapses and rock
bursts, continues to be a difficult scientific problem. Source:
Report of a Working Group from Government, Industry
and National Laboratories, 1999 Image: Kyanite Mining Corp. |
Under
a Comprehensive Nuclear-Test-Ban Treaty monitoring regime, the
discrimination of signals from such large mine blasts
may be ambiguous. The difficulty is due, in part, to the lack of complete understanding
of the source models for the seismic and acoustic
signals. Mines operate in both soft and hard rock environments,
use different blasting techniques, and signals from such activities will most
likely be recorded regionally, and are difficult to interpret
Longwall roof-collapses
and non-planned mining-associated rockbursts and mine collapses
have usually have been reported in regular USGS/NEIC earthquake
catalogs if their magnitudes were of a size that they might affect
estimates of seismic hazard in the regions in which they occur.
Some large and otherwise atypical mining explosions might also
be occasionally listed in the regular USGS/NEIC earthquake catalogs. Source: MINING-INDUCED
EVENTS IN THE EARTHQUAKE CATALOGS OF THE USGS/NEIC |
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Can Seismology
Be Used To Determine If An Event Is Man-Made ? |
According to Peter D
Marshall, O.B.E. in his article titled: Synergy
and the International Monitoring System; "To
detect and locate underground nuclear explosions, the seismic
network of primary and auxiliary stations is fundamental. However,
for source identification purposes, seismology is only a complementary,
not a definitive technique. It is not possible through seismological
means to identify a source as being a nuclear or conventional
explosion; for this task the detection of radio nuclides is
essential. Radio nuclides from an underground nuclear explosion
may leak to the surface through fissures or fractures surrounding
the cavity created by the explosion." Read more
of Peter Marshall's report by clicking on the link.
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Related
Links for Research |
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CTBTO - Preparatory Commission
for the Comprehensive Nuclear Test Ban Treaty Organization
The International Monitoring
System (IMS) network consists of 337 monitoring facilities
all over the globe, comprising seismic, infra sound, hydro
acoustic and radio nuclide monitoring stations as well as radio
nuclide laboratories. |
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USGS - Earthquake Hazard Program - Identifying
Routine Mining Seismicity
Information on mining related seismic events. Listings
of routine explosions and planned roof collapses at mines and quarries
in the United States |
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SMU - Department of Geological Sciences
Combining video, seismographic, and
infra sound data of mining blasts. This paper is also
available (without movies) as a 16 MB Adobe
PDF document and in Adobe PDF format with movies included
in mpeg format as a 113 MB document. |
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SMU - Abstract: Synchronization
of Video with Seismic and Acoustic Data using GPS Time
Signals
Seismic and acoustic monitoring of large
industrial mining explosions provide detailed data sets that
are central to understanding the source mechanisms by which
these events couple seismic energy into the ground. MS
WORD Document 1.5MB |
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SCIENCE.GOV -
Abstract:
Seismic detection of
small, evasively tested underground nuclear explosions
remain as a major challenge to effective verification of any
eventual CTBT. Most seismic detection research
reported to date has focused on analyses of regional seismic signals
recorded from explosions at the few known nuclear weapons
test sites and, consequently, represent only limited ranges of
the source and propagation path conditions of potential monitoring
interest. In this study, they analyze regional seismic data
recorded at the Borovoye station in Central Asia . [PDF] 2MB
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