HR: 13:40h
AN: U23C-01    [Abstracts]
TI: High Frequency Signal and Noise for Assessing Earthquake Hazards
AU: * Williams, R A
EM: rawilliams@usgs.gov
AF: US Geological Survey, 1711 Illinois St, Golden, CO 80401 United States
AU: Stephenson, W J
EM: wstephens@usgs.gov
AF: US Geological Survey, 1711 Illinois St, Golden, CO 80401 United States
AU: Odum, J K
EM: odum@usgs.gov
AF: US Geological Survey, 1711 Illinois St, Golden, CO 80401 United States
AB: In earthquake hazard assessment applications we use high-resolution P- and S-wave seismic-reflection/refraction data to image faults and constrain sedimentary basin geologic structure and seismic velocity in the 10 to 1000 m depth range. NEHRP site classifications are also determined from the average velocity to 30-m depth (Vs30). These data sets help explain earthquake damage patterns, the variation in ground motion due to differences in S-wave velocity, and the presence of earthquake site resonances caused by near-surface impedance boundaries. Inversion of surface wave dispersion curves from ambient noise recordings supplement these data for S-wave velocity (Vs) site characterization, especially when urban traffic noise overwhelms more traditional seismic methods. Direct comparisons of our results with downhole seismic velocities suggest that seismic reflection/refraction and surface-wave dispersion data are a fast, non-invasive, and inexpensive alternative for determining NEHRP site classifications. Variations in site classification mapped by these non-invasive methods are correlated with strong ground motions recorded at these sites during the 2001 Nisqually earthquake in Seattle. In San Jose areas of amplified earthquake ground motion correlate with lower Vs and position relative to buried faults. We also find a high correlation between Vs30 and the travel times of S-wave first-arrival phases at 100 m offset from the seismic source. This correlation suggests that a much simpler interpretation process of the refraction data may be just as valuable as Vs30 for determining NEHRP classifications. Potential earthquake resonance frequencies, usually in the 1 to 10 Hz range, are directly determined from the reflection data based on a quarter-wavelength conversion of the reflection two-way travel time. Resonances observed in earthquake seismograms recorded in Seattle and Memphis are correlated with the site's near-surface seismic reflections. In all of these cases the resonance accounts for the highest amplitude shaking at the site above 1 Hz. Some resonances observed during the Nisqually earthquake may be generated by impedance boundaries in the upper 60 m.
DE: 7212 Earthquake ground motions and engineering seismology
SC: Union [U]
MN: Fall Meeting 2005