Shallow ShearVelocity Transects of Urban Areas, and SeismicHazard Mapping

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Shallow Shear-Velocity Transects of Urban Areas,
and Seismic-Hazard Mapping
Louie, J. N. Scott, J. B. Rasmussen, T. Thelen,
W. A. Pancha, A. Clark, M. Park, H. Lopez, C. T.
www.seismo.unr.edu/ hazsurv
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Transect Results Partly Explain PGA
Supported by USGS-NEHRP and IRIS-PASSCAL
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Three Transects Show Geologic Variations
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Statistics of Geologic Variation

• Linear slope in log-log spatial spectrum
• Fractal variations are probably geological
• Levels out at end
• Some variance across lt0.6 km is experiment error

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V30 vs Soil Type

• Large standard deviations with good averages
• Units 3 and 4 well-sampled over long distances

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V30 vs Soil Type

• Standard deviations increase with sample size
• Average velocities of units change along transect

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Las Vegas Transect
Supported by DOE-LLNL and IRIS-PASSCAL
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Fields (2001) Amplification Mapping
Fields (2001) Amplification Mapping

• Needs only basin depth and Vs30
• Trial maps made using coefficients from LA

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Extrapolating Vs30 Las Vegas

• Vs30 assigned to soil-map units using transect
• Maps dont predict new measurements- B. Luke, UNLV

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Summary

• 300 sites on 3 urban transects measured for Vs30.
• Vs30 may partly explain PGA, hazard variations.
• Long transects show fractal, geologic variations
  in Vs30.
• Geologic, soil maps do not have sufficient Vs30
  predictive value for engineering.