How are Seismic Site Classes Determined?
For commercial/industrial construction, and for structures that will generate large dynamic loads, knowledge of the seismic shear wave (S-wave) structure is critical. In order to comply with the International Building Code (IBC) or Uniform Building Code (UBC), and National Earthquake Hazards Reduction Program (NEHRP), it is necessary to know the “Soil Site Class”, which ranges from A (the best – hard rock), down to F (the worst – soft clay or swamp muck). Unknown sites default to D.

Proper site classification is critical because construction based on too lax a site class can lead to safety and liability problems, while an overly cautious site classification can lead to extraneous construction costs. Proper site classification early in the design stage can save thousands to millions of dollars, depending on the scope of the project.
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One of the parameters that can go into determining the site class is the weighted average S-wave velocity in the top 30 meters or 100 feet – also called V30.  This parameter can be determined completely non-intrusively using seismic surface wave methods that sample a large volume of the subsurface, and require no boreholes.  This contrasts favorably with downhole or crosshole methods that sample a very limited area around the borehole(s), and typically require specially constructed holes with post-construction wireline deviation logging data.  Surface wave methods are largely immune to interference from traffic, wind and construction noise – some passive methods even make use of the noise to enhance data quality at depth.  Active surface wave methods record multifrequency waves from a heavy weight drop or hammer blow, and provide accurate velocities at shallow depths. Passive methods record natural vibrations from nearby ocean waves, vehicular traffic, or wind-induced building or tree motion.