HR: 11:15h
AN: S42A-04    [Abstracts]
TI: Nonlinear Soil Response Induced in Situ by an Active Source at Garner Valley
AU: * Pearce, F
EM: fpearce@lanl.gov
AF: Los Alamos National Laboratory, Geophysics Group, MS D443, Los Alamos, NM 87545 United States
AU: Bodin, P
EM: pbodin@memphis.edu
AF: University of Memphis, Center for Earthquake Research & Information, 3876 Central Ave., Memphis, TN 38152 United States
AU: Brackman, T
EM: tbrackmn@memphis.edu
AF: University of Memphis, Center for Earthquake Research & Information, 3876 Central Ave., Memphis, TN 38152 United States
AU: Lawrence, Z
EM: zlawrenc@memphis.edu
AF: University of Memphis, Center for Earthquake Research & Information, 3876 Central Ave., Memphis, TN 38152 United States
AU: Gomberg, J
EM: gomberg@usgs.gov
AF: United States Geological Survey, 3894 Central Ave., Memphis, TN 38152 United States
AU: Steidl, J
EM: steidl@crustal.ucsb.edu
AF: University of California at Santa Barbara, Institute for Crustal Studies, 1140 Girvetz Hall, Santa Barbara, CA 93106 United States
AU: Meng, F
EM: fymeng@mail.utexas.edu
AF: University of Texas at Austin, Department of Civil Engineering, 1 University Station C1700, Austin, TX 78712-0283 United States
AU: Guyer, R
EM: guyer@physics.umass.edu
AF: University of Massachusetts at Amherst, Department of Physics, Amherst, MA 01003-4525 United States
AU: Stokoe, K
EM: k.stokoe@mail.utexas.edu
AF: University of Texas at Austin, Department of Civil Engineering, 1 University Station C1700, Austin, TX 78712-0283 United States
AU: Johnson, P A
EM: paj@lanl.gov
AF: Los Alamos National Laboratory, Geophysics Group, MS D443, Los Alamos, NM 87545 United States
AB: As part of the nonlinear soil studies at Garner Valley, on August 18 we performed a sequence of exploratory experiments using the NEES's vibrator source T-Rex, in order to determine whether or not elastic nonlinear response could be induced and measured. The experiments described here are standing wave (resonance) measurements. This type of experimental approach, known as Nonlinear Resonant Ultrasound Spectroscopy, is commonly applied in the laboratory to characterize sample nonlinear response (Guyer and Johnson, 1999). The method relies on monitoring a resonance mode frequency change as a function of drive level. In the field experiment, the TRex source is step-swept in frequency encompassing the modal frequencies expected theoretically for the stratigraphy at the GVDA, measured independently 10 m distant from the experimental site (Bonilla et al., 2004). We focus on several interfaces determined from that study, and therefore selected a frequency band of 5-30 Hz for the step-sweep. An array of nine 3-component accelerometers located immediately adjacent to the vibration plate recorded motions generated by six progressively larger forces, ranging from 3000-21000 lbs in shear and 6000-54000 lbs in compression. We observed clear modes in both compression and shear. Measured accelerations reached 2G (the instrument clip-level) at full force, corresponding to strain amplitudes on the order of 4.5x10-3. The detected acceleration amplitudes span more than a decade, implying minimum strains are on the order of 10-4 to 10-5. Laboratory experiments on granular material suggest these strains should be sufficient to induce a strong nonlinear response (Guyer and Johnson, 1999). The raw data in both compression and shear show a clear, progressive shift in frequency downward as a function of driving force. References Guyer, R. and P. A. Johnson, The astonishing case of mesoscopic elastic nonlinearity, Physics Today, 52, 30-35, (1999). Bonilla, F., J. Steidl, J.-C. Gariel and R. Archuleta, Borehole response studies at the Garner Valley Downhole Array, southern California, Bull. Seism. Soc. Am. 92, 3165-3179, (2002).
DE: 7223 Seismic hazard assessment and prediction
DE: 6944 Nonlinear phenomena
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting