ASDSO Dam Safety Toolbox

Velocity Structure

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The velocity structure is a generalized model of the earth’s crust using layers having different assumed seismic velocities. It is important in a SHA to have a well-defined velocity structure. Several parameters (VS30, Z1.0, Z2.5) are used in most attenuation relationships. The most important parameter is VS30, which is defined as the shear wave velocity in the top 30 meters. This can be estimated based on surface geology or geotechnical data, or directly measured in the field. The preferred approach for detailed design would be directly measured in the field, with more preliminary analyses using estimates from the literature.

Field measurements of shear wave velocity can be obtained by several different methods:

  • Destructive methods:
    • Down-hole (and Up-hole)
    • Cross hole
    • Seismic Piezo cone (SCPT)
  • Non Destructive methods:
    • Seismic Refraction
    • Spectral Analysis of Surface Waves (SASW)
    • Multichannel Analysis of Surface Waves (MASW)


Revision ID: 6117
Revision Date: 12/20/2022