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Background
The USGS operates extensive networks of
seismic sensors for earthquake hazards studies
and for monitoring man-made explosions. The
size of a network, depending upon its purpose,
ranges from global to regional and to local.
Examples of the application of a local seismic
network include the study of aftershock
distribution after a major earthquake and the
evaluation of engineering site conditions in
sedimentary basins. USGS scientists are also
working in partnership with industry to develop
technologies for rapid earthquake notification and
response that will permit users of these data to
avert losses or to guide recovery efforts. One
technology recently developed is a three-component borehole seismometer.
The Three-Component Borehole Seismometer
Scientists at the U.S. Geological Survey (USGS)
have developed a three-component borehole
seismometer with leveling capability for use
during earthquake site response studies. There
are several reasons for using such a seismometer
in earthquake studies. For example, the low
velocity and high attenuation of weathered near-surface rocks
strongly influence the amplitude
and phase characteristics of seismic waves. It is
therefore important for earthquake-source and
seismic-shear-wave-polarization studies to place
seismometers at depths below the weathered
rocks. In another example, because surface-wave
amplitude decreases exponentially with
depth, the seismic noise generated by human
activities in a metropolitan area is reduced
considerably even at a shallow depth. Thus,
borehole seismometers can be used as elements of
a seismic network for determining earthquake
locations and magnitudes.
The USGS three-component borehole
seismometer uses 2-Hz electromagnetic moving-coil geophones as sensing elements. The system
features a compact internal device for leveling the
horizontal components after emplacement. The
USGS three-component borehole seismometer
has a low noise level, consumes no power, is
simple to build and is inexpensive. It is also
suitable for rapid deployment for after shock
studies and for low-noise operation at remote
sites.
Test Results
The USGS three-component borehole
seismometer has been installed at several sites in
California. Earthquake data recorded by the
borehole seismometers have been used in studies
on the effects of near-surface weathered rocks on
the corner frequencies of earthquake spectra near
Anza, CA; on the seismic-shear-wave-polarization characteristics of seismograms
recorded in the southern California batholith at
300 m depth; and on site-resonance amplification
caused by sedimentary deposits overlying
bedrock in the Marina District of San Francisco.
For More Information
The USGS is seeking partners to
commercialize this technology. For more
information, please contact the principal
researcher:
Hsi-Ping Liu
U.S. Geological Survey
Western Earthquake Hazards Team
345 Middlefield Road, MS 977
Menlo Park, CA 94025
tel: 650-329-5643
fax: 650-329-5163
E-mail: liu@usgs.gov
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