Most earthquake damage occurs due to ground amplifications which, for the same earthquake magnitude, change with ground stiffness at relatively shallow depths. Based on the premise established from a study that the top 30-m influences most and the shear-wave velocity (Vs) is the best indicator of stiffness, the average Vs in the top 30 m (denoted as Vs30m) is used as an important criterion in the design of building structures. In general, a lower Vs30m would be subject to a greater ground amplification (and suffer more damage from an earthquake).
The National Earthquake Hazard Reduction Program (NEHRP) established by the U.S. Congress in 1977 adopts this criterion and classifies a site into one of several different categories. The table below from NEHRP Provisions shows this classification. The International Building Code (IBC) published the same classification designations in 2000 as one of the parameters that should be accounted for in a structural design.
