National earthquake hazard risk prevention (NEHRP) Vs30 site classification for the east bay area of California.  Velocity amplfiication increases earthquake hazards.
Local amplification of strong ground motion by shallow soils is considered a significant seismic hazard.   Amplification is measured according to the Vs30 scale, which are average velocities measured to a depth of 30 meters from stimulation at the surface.  Increased velocities in the upper 30 - 60 meters of soil can greatly increase earthquake hazard and endurance.
The fulle scale includes class “E” (Vs30 below180 m/s), class ”D” (Vs30 between180 and 360 m/s), class “C” (Vs30 between 360 and 760 m/s), and class “B”(Vs30 greater than 760 m/s)
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