A 3D SHEAR WAVE VELOCITY MODEL AND EARTHQUAKE SCENARIO GROUND MOTIONS FOR THE GREATER CHARLESTON, SOUTH CAROLINA REGION

We have integrated seismological, geological, hydrogeological, geophysical and geotechnical data to construct a 3-dimensional shear wave velocity model for the highly populated nine quadrangle (Summerville, Mount Holly, Stallsville, Ladson, North Charleston, Johns Island, James Island, Charleston and Fort Moultrie) area of the greater Charleston, South Carolina, region. We find the semi- and unconsolidated Atlantic Coastal Plain sediments range from less than 600 to nearly 1000 meters in thickness across our study area. Shear wave velocities of geological units range from up to 900 meters/second in a high velocity layer at depth we name the Gordon High Velocity Zone to less than 180 meters/second in near surface man-made and Holocene tidal marsh deposits. We estimate the depth of three significant impedance contrast surfaces (base of coastal plain, top of Gordon High Velocity Zone and the Quaternary/Tertiary contact) and reference the thickness of 7 pre-Quaternary sedimentary velocity units to these boundaries. Our shallowest layer (Quaternary and man-made sediments) is divided into estimates of shear wave velocity (both average and linearly increasing with depth) for 18 surficial units. This model was used to estimate deterministic and probabilistic ground motions for future earthquakes in this region. We find that the thick, low velocity sediments become nonlinear during strong shaking and limit the intensity of high frequency ground motion. We also find that strong ground motion is locally amplified at lower frequencies, mainly dependent upon the depth of the Gordon High Velocity Zone and shear wave structure above that unit. This is concerning in that, for larger magnitude events (M>6.5), long period motions in the downtown medical district of Charleston (which has many tall buildings) are expected to exceed those in the epicentral region of Summerville, South Carolina.