USING HISTORICAL MAPS AND DEMS TO REMAP EARTHQUAKE HAZARDS ON THE CHARLESTON PENINSULA

Our efforts to update earthquake hazards in the Charleston, South Carolina region have revealed that the current USGS surface geology map does not provide a good base upon which to make these estimates. Comparisons between recent seismic site amplification estimates and geotechnical borehole results and the surface geology map show significant discrepancies in several locations, particularly with respect to artificial fill overlying Holocene tidal marsh. This is likely because modern surface geology mapping efforts did not start until well after extensive human modification had occurred to the land surface. To combat this issue, we have digitized and georectified a number pre-existing historical maps of the Charleston Peninsula and compared them to the existing surface geology map plus a new LiDAR-derived digital elevation model (DEM). We found that the 1949 Halsey Map of the pre-colonial “high ground/marsh” boundary most accurately outlines existing drainages as seen in the DEM. Based upon this plus other map (e.g., the 1919 USGS topographic map of the Charleston quadrangle) and data comparisons, we are now creating new maps of the Charleston Peninsula that reflect not just the location of artificial fill but also whether or not it overlies Holocene tidal marsh versus Pleistocene near-shore deposits. This poster will present our process for producing the new maps. A companion abstract (Jaume, Cramer, Moulton and Levine, this meeting) will examine the impact of the new maps on earthquake hazard estimates.