HR: 1340h
AN: S13C-1074    [Abstracts]
TI: Frequency Content of Ambient Seismic Noise in North-Central Illinois
AU: * Higuera-Diaz, I C
EM: higuera@geol.niu.edu
AF: Northern Illinois University, Department of Geology and Environmental Geosciences, Northern Illinois University, Department of Geology and Environmental Geosciences, DeKalb, IL 60115 United States
AU: Carpenter, P J
EM: phil@geol.niu.edu
AF: Northern Illinois University, Department of Geology and Environmental Geosciences, Northern Illinois University, Department of Geology and Environmental Geosciences, DeKalb, IL 60115 United States
AB: High and low-frequency ground motion vibration measurements were made in DeKalb and Kane Counties, Illinois, to characterize different sources of seismic noise and determine directions and magnitude of motion produced by each source. The Seismic Analysis Code 2000 (SAC2000) was used to process seismic noise data recorded with a low-frequency digital system,, earthquake records downloaded from the WILBER Web site, and noise data recorded by an engineering seismograph with high-frequency geophones. Power-spectral density estimates were computed from an autocorrelation series in most cases. Using the power density spectra routine of SAC2000, selecting autocorrelation windows of 20 s for the low-frequency data and 2 s for the high-frequency data, we have found distinctive seismic noise peaks among the different sites, regardless of the background noise level. The earthquake low-frequency data showed a microseism peak at a frequency of 0.2 Hz for stations located in the Midwestern U.S. Microseism peak frequency did not decrease with increasing distance from Lake Michigan, suggesting the lake is not the primary source of the microseisms, which may be generated in ocean basins. Ambient ground motion recorded by the Northern Illinois University seismic station with a 2 Hz natural period seismometer exhibited peaks around 0.8 and 2.2 Hz. Ground motion from trains, traffic, air-conditioning units and water pumping equipment was recorded with an engineering seismograph. Vertical geophones of natural frequency 2, 8, and 50 Hz were used, as well as a 4.5 Hz horizontal geophone. Train noise exhibits strong peaks in the 5-10 Hz range, both for the vertical and horizontal geophones. Noise peaks at 25, 60, 90 and 115 Hz are probably related to traffic, electrical transformers, and air-conditioning units. Measurements made near a sewage treatment plant in DeKalb showed peaks at 10, 30, 93, and 109 Hz, probably related to pumping and mechanical equipment. Seismic noise collected about 100 m from municipal swimming pool with vertical geophones showed spectral peaks at about 10-30 Hz and 180 Hz. Horizontal geophones at the same location exhibited peaks at 5-15 Hz and 140 Hz. Pool activities, pumping equipment and traffic may contribute to this noise. Delineating particle motion at the recording sites will determine whether dominant motion is due to Rayleigh, Love or body waves. Geophone arrays may also be deployed to determine the origin direction and velocity of each seismic noise component.
DE: 3025 Marine seismics (0935)
DE: 0935 Seismic methods (3025)
DE: 0999 General or miscellaneous
SC: Seismology [S]
MN: 2004 AGU Fall Meeting