EARTHQUAKE HAZARDS ON THE CHARLESTON PENINSULA, SOUTH CAROLINA BASED UPON “HISTORY-INFORMED” GEOLOGIC MAPPING

Seismic site amplification estimates and geotechnical borehole results do not match expected results based upon the current USGS surface geology map in many parts of the Charleston Peninsula. This is likely because modern surface geology mapping efforts did not start until well after extensive human modification had occurred to the land surface. In a companion poster (Dedrick, Jaume and Levine) we describe our process of using historical maps (1949 Halsey Map and 1919 USGS topographic map) and a LiDAR-derived digital elevation model (DEM) to “remap” the Charleston Peninsula with respect to earthquake hazards. In this presentation we will make a side-by-side comparison of ground motion and liquefaction hazards using both the current USGS surface geology map and our newly produced maps. In particular we will point out where consideration of pre-existing surface geology beneath artificial fill changes expected ground motions and previously unmapped artificially-filled tidal creeks changes expected liquefaction susceptibility. We will also present evidence that parts of the west side of the current Charleston Peninsula, which in both the USGS surface geology and our new maps are shown as artificial fill overlying Holocene tidal marsh deposits, may actually consist of islands of either Pleistocene near-shore deposits (Harris, pers. comm.) or oyster-shell mounds (Doar, pers. comm.) within the tidal marsh. The 19^th century residents of Charleston built a series of mills on the Ashley River in this area, which survived the 1886 earthquake well and two of which are still in use today.