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Three-Component Borehole Seismometer for Earthquake Seismology

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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