HR: 0800h
AN: T21A-0443    [Abstracts]
TI: Borehole Array Observations of Non-Volcanic Tremor at SAFOD
AU: * Ellsworth, W L
EM: ellsworth@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, MS-977, Menlo Park, CA 94025
AU: Luetgert, J H
T21A-0443 AF: U. S. Geological Survey, 345 Middlefield Road, MS-977, Menlo Park, CA 94025
AU: Oppenheimer, D H
T21A-0443 AF: U. S. Geological Survey, 345 Middlefield Road, MS-977, Menlo Park, CA 94025
AB: We report on the observation of non-volcanic tremor made in the San Andreas Fault Observatory at Depth in May, 2005 during the deployment of a multi-level borehole seismic array in the SAFOD main hole. The seismic array consisted of 80 levels of hydraulically-clamped 3-component, 15 Hz omni-directional geophones spaced 15.24 m apart along a 1200 m section of the inclined borehole between 1538 and 2363 m below the ground surface. The array was provided by Paulsson Geophysical Services, Inc. (P/GSI), and recorded at a sample rate of 4000 sps on 24-bit Geode digital recorders provided by Geometrics, Inc. More than 2 TB of continuous data were recorded during the 2-week deployment. Selected local earthquakes and explosions recorded by the array are available at the Northern California Earthquake Data Center, and the entire unedited data set is available as assembled data at the IRIS Data Management Center. Both data sets are currently in the industry standard SEG2 format. Episodes of non-volcanic tremor are common along this reach of the San Andreas Fault according to Nadeau and Dolenc [2004, DOI: 10.1126/science.1107142], with many originating about 30 km southeast of SAFOD beneath the southern end of the Parkfield segment and northern end of the Simmler segment of the fault. We identified tremor episodes using spectrograms routinely produced by the Northern California Seismic Network (http://quake.usgs.gov/cgi-bin/sgrampark.pl) on which they appear as periods of elevated noise relative to the background. A particularly strong tremor episode occurred on May 10, 2005 between 19:39 and 20:00 UTC. In SAFOD, tremor spectral levels exceed the instrumental noise floor to at least 40 Hz. The spatially unaliased recording of the tremor wavefield on the P/GSI array reveal individual phases that can be tracked continuously across the array. The wavefield is composed of both up- and down-going shear waves that form quasi-stationary interference patterns in which areas of constructive interference recur at the same locations along the array. Such a pattern could arise from a spatially stationary source radiating an extended duration time function into a complex medium.
DE: 4430 Complex systems
DE: 7203 Body waves
DE: 7215 Earthquake source observations (1240)
SC: Tectonophysics [T]
MN: Fall Meeting 2005