Detailed Soil Column Descriptions
Soils and amplification data are used to determine the local influences on the shaking. For example, soft soils or deep sedementary basins can amplify the shaking caused by an earthquake.
Scientists collect geologic and geophysical descriptions of typical soil columns between site and bedrock. The most applicable sites are those beneath strong-motion recorders and those underlying most cities. For the western US, these might be alluvium with considerable distance to bedrock, the lower portion of which may be old enough to be quite consolidated and, and hence “stiff,” with relatively high seismic velocities. Other sites may have recent, weakly consolidated materials, and hence “soft.” The bedrock itself may be highly weathered. These two effects decrease the respective seismic velocities.
For the eastern US there may be deep alluvium, which has accumulated too quickly to be stiff. In many locations the bedrock is very high velocity. When the bedrock underlies soils, the velocity contrast may produce higher site amplifications than in the west. Where bedrock lies at the surface the effect may be to have lower ground motion than average bedrock or stiff soil sites in the western US.