GETTING UNDER THE SURFACE OF THE CHARLESTON PENINSULA TO PREDICT EARTHQUAKE GROUND MOTION

Beginning in 2014, the Charleston Area Earthquake Hazards Mapping Project (CAEHMP) has been working to define the geophysical, geological and geotechnical characteristics of the Atlantic Coastal Plain sediments beneath the greater Charleston, South Carolina region, in order to better predict ground motion site response during future earthquakes. Research to date has revealed that the Charleston Peninsula largely overlies an apparent paleochannel of the Wando River, which results in thicknesses of Quaternary and younger sediments >20 meters across most of the peninsula. In addition, when looking more closely at the surface geology of the Charleston Peninsula, we found that “artificial fill” was not a good seismic site class category, since this material overlies a variety of different substrates. Based on these observations, we more recently (2017) began to compile both modern and historical geological and geotechnical information to better characterize the subsurface of the Charleston Peninsula. On the edges of the peninsula, we find that artificial fill overlies a variety of pre-existing materials, including both the Quaternary Wando Formation, Quaternary Silver Bluff Beds plus Holocene tidal marsh. In many places the fill is relatively thin (<2 meters) but can reach up 5 meters where it overlies tidal marsh. Tidal marsh deposits at the edge of the peninsula can themselves reach up to 10 meters in thickness. The geotechnical borehole data also suggest that some of the areas currently mapped as Quaternary Silver Bluff are actually artificial fill above Holocene marsh, particularly on the east (Cooper River) side of the peninsula. On the west (Ashley River) side of the peninsula, geotechnical data suggests another, younger paleochannel may exist, in this case eroded into the top of the Wando Formation. This last feature may be particularly relevant to earthquake hazards, since the downtown medical district of the Charleston Peninsula overlies this feature. These results help make sense of previous microtremor site amplification observations, where the amplitude of a predominant 1-2 Hz amplification correlates more with the material underlying artificial fill more than the presence of fill.