HR: 1330h
AN: S52A-0120    [PDF]
TI: Comparing Observed and Predicted Directivity-Amplified Ground Motion
AU: * Howard, J K
EM: jhoward@water.ca.gov
AF: California Department of Water Resources, Division of Safety of Dams, 2200 X St., Suite 200, Sacramento, CA 95818 United States
AU: Tracy, C A
AF: California Department of Water Resources, Division of Safety of Dams, 2200 X St., Suite 200, Sacramento, CA 95818 United States
AU: Burns, R G
AF: California Department of Water Resources, Division of Safety of Dams, 2200 X St., Suite 200, Sacramento, CA 95818 United States
AB: Near-source strong motion can have forward directivity amplification ("directivity") from constructive interference of body shear waves radiated ahead of the rupture front, and "fling" from co-seismic tectonic movement. In theory, directivity-amplified waves vibrate normal to fault strike (FN). "Fling", which is fault parallel (FP) motion in strike slip rupture and may add to FN motion in dip-slip rupture, is thought to be longer-period motion. Somerville et al. (1997) proposed empirical factors for spectral attenuation formulas to predict directivity-amplified spectral acceleration in FN and FP directions. Abrahamson (2000) revisions retained the implicit assumption that FN and FP represent directions of maximum and minimum response. We compared observed near-source motion to predicted directivity-amplified motion. We isolated the suspected velocity waveform(s) of 25 near-source time histories with strong forward directivity amplification, estimated the waveform polarity, rotated the record accordingly, and computed the rotated spectral response amplitude (i.e., the presumed maximum response amplitude and FN direction) for periods up to 2 seconds. Predictions were based on average results (median or +1 standard deviation) of three attenuation formulas modified by Somerville-Abrahamson factors. We compared velocity pulse-aligned spectra polarity and amplitude with other reported directions of maximum spectral response, actual FN direction and spectral amplitude, and predicted amplitude. Our initial findings are: 1) Records rotated to the velocity waveform polarity commonly yield the largest overall spectral response for periods between 0.5 and 2.0 seconds. 2) More than half the records so oriented have spectra that met or exceeded +1 s.d. SA predictions for some portion of this period range. 3) The velocity waveform rotation is generally close to FN for strike-slip faulting, but can deviate significantly from FN in dip-slip faulting. 4) The periods of maximum spectral amplitude do not seem predictable. A re-analysis of the velocity waveforms using refined procedures was performed and comparisons were made up to periods of 3 sec. Results will be presented at the Fall Meeting.
DE: 7212 Earthquake ground motions and engineering
DE: 7223 Seismic hazard assessment and prediction
SC: Seismology [S]
MN: 2003 Fall Meeting