Preliminary estimate of the amplification of possible earthquake ground motion at a site in Charleston County, South Carolina

We estimate site amplification at the location of a proposed bridge near Charleston, South
Carolina. Model calculations indicate that amplification at periods of 1 s and longer is likely to be
strongly influenced by the effects of a large contrast in shear-wave velocity at a depth of
approximately 1 km (3,000 ft). On-site borehole data, regional geological and geophysical
information, and data from a geologically similar setting near Memphis, Tennessee allowed us to
estimate profiles of shear-wave velocity, shear-wave attenuation, and density from ground level
down to metamorphic and igneous rocks that are approximately 3 km (9,500 ft) beneath the site.
We modeled amplifications that would be produced at the surface and at the top and bottom of
the Cooper Marl. Amplification estimates that are based only on the shallow shear-wave
structure, for example in the upper 100 m (300 ft), can severely underestimate long-period
amplification at the site. Additional modeling could help determine whether new data should be
collected, to resolve remaining uncertainties about likely amplification.