HR: 17:30h
AN: G32C-07    [PDF]
TI: Borehole Tensor Strain and Pore Pressure Noise from Mini-PBO Sites in the San Francisco Bay Area: Comparison with Parkfield Instruments
AU: * Johnston, M J
EM: mal@usgs.gov
AF: US Geol. Survey, 345 Middlefield Rd., MS977, Menlo Park, CA 94025 United States
AU: Myren, G D
EM: dmyren@usgs.gov
AF: US Geol. Survey, 345 Middlefield Rd., MS977, Menlo Park, CA 94025 United States
AU: Murray, M H
EM: mhmurray@seismo.berkeley.edu
AF: Berkeley Seismology Laboratory, University of California, Berkeley, CA 94720 United States
AU: Mueller, R J
EM: rmueller@usgs.gov
AF: US Geol. Survey, 345 Middlefield Rd., MS977, Menlo Park, CA 94025 United States
AB: Borehole strainmeters are an important component of the new Earthscope/PBO initiative. The first prototype cluster of five boreholes, usually termed Mini-PBO, was successfully installed along the San Andreas/Hayward fault system in the San Francisco Bay area from 2001-2003. Each 200m deep borehole was equiped with a tensor strainmeter, seismometer, pore pressure monitor, tiltmeter, GPS and temperature monitors. Records from the tensor strainmeters and pore pressure transducers are used to determine comparative noise spectra over nine decades of frequency (10E-7 to 100 Hz) at Mini-PBO locations with noise spectra from instruments at Parkfield. In particular, we compare the responses from the new Carnegie tensor strainmeters in the Mini-PBO sites with the Gladwin tensor strainmeters and Carnegie dilatometers at Parkfield. We also make the first measurements of pore pressure noise and determine the relations between pore pressure and strain, particularly during local and teleseismic earthquakes such as the November 3, 2002, M 7.9 Denali earthquake. The ten borehole strain instruments used are all installed at approximately 150-200 m depth. The general characteristics of power spectral density estimates obtained at all sites are similar with noise power decreasing with increasing frequency at roughly inverse frequency squared.Other than 10 dB microseismic noise peaks and 30 to 40 dB peaks at earth tidal frequencies, the spectra from the Parkfield instruments decrease from about -80 dB at a frequency of 10E-8 Hz to about -220 dB at a frequency of 100 Hz. Initial data from the Carnegie tensor strainmeters at the Mini-PBO sites appears to be about 10 dB noisier at frequencies between 10E-7 to 100 Hz. Pore pressure and compressional dilational strain are in phase and are highly coherent at short to intermediate periods.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 7223 Seismic hazard assessment and prediction
DE: 8123 Dynamics, seismotectonics
SC: Geodesy [G]
MN: 2003 Fall Meeting