HR: 11:30h
AN: S42A-05 INVITED     [Abstracts]
TI: Shear-Wave Velocities to 100 m Depth From Seismic Reflection/Refraction, ReMi, and MASW Techniques Compared With Borehole Velocities: Implications for Earthquake Ground Motion Assessment
AU: * Stephenson, W J
EM: wstephens@usgs.gov
AF: U. S. Geological Survey, Box 25046 MS 966, Denver, CO 80225 United States
AU: Williams, R A
EM: rawilliams@usgs.gov
AF: U. S. Geological Survey, Box 25046 MS 966, Denver, CO 80225 United States
AU: Odum, J K
EM: odum@usgs.gov
AF: U. S. Geological Survey, Box 25046 MS 966, Denver, CO 80225 United States
AU: Worley, D M
EM: worley@usgs.gov
AF: U. S. Geological Survey, Box 25046 MS 966, Denver, CO 80225 United States
AB: Shallow shear-wave velocity (V$_{s}$) has long been recognized as a key factor in variable ground motion amplification and site response in sedimentary basins. Seismic hazard maps traditionally have not dealt with local geologic site effects in detail; however, hazards-mapping methodology is advancing to more accurately incorporate local V$_{s}$ information into the hazards calculation, particularly in urbanized areas. This trend is expected to accelerate with future expansion of these efforts. Incorporation of scenario earthquakes into future hazards characterization will also depend on reliable V$_{s}$ in the upper few hundred meters. As such, the need to rapidly and inexpensively determine shallow V$_{s}$ over large urban sedimentary basins will become critical to accurately represent site response in future urban hazard maps. Borehole logging is generally considered the standard for obtaining V$_{s}$ data, but drilling and logging to the depths generally required for earthquake ground motion investigations is very expensive, and it is becoming increasingly problematic in heavily urbanized settings. Traditional active-source seismic reflection/refraction has been used extensively for V$_{s}$ characterization. MASW (Multi-channel analysis of surface waves) and ReMi (refraction microtremor) are two of the most recently-developed surface techniques for determining shallow V$_{s}$. We conducted a blind experiment to determine V$_{s}$ depth profiles at four boreholes in the Santa Clara Valley, CA, with these three methods. Average V$_{s}$ estimates at 30, 50 and 100 m depth demonstrate that the surface methods as implemented in this study can generally match borehole results to within 15% at these depths. Spectral amplifications predicted from the respective borehole velocity profiles similarly compare to within 15% or better from 1 to 10 Hz with the surface-method velocity profiles. Overall, no one method was consistently better at matching the borehole velocity profiles or amplifications. These results suggest reflection/refraction, MASW, and ReMi surface acquisition methods are all appropriate for estimating V$_{s}$ in urban settings for hazards assessment.
DE: 7223 Seismic hazard assessment and prediction
DE: 7294 Instruments and techniques
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting