SEISMIC AND LIQUEFACTION HAZARD MAPS FOR THE CHARLESTON, SOUTH CAROLINA, AREA

A previously developed community velocity model and liquefaction probability curves for the local geology in a three county area around Charleston, South Carolina, have been used to generate detailed seismic and liquefaction hazard maps for nine quadrangles. Hazard maps with the effects of local geology have been generated for a resolution of 500 m, expanding the pilot study done on the Charleston quadrangle in 2014. Finer resolution (100 m) maps for the Charleston quadrangle have also been generated. Additional products include a database of M7 time histories (series) for the engineering foundation geology of Cooper Marl at the surface and a comparison of 1886 M7 alternative source model scenario hazard maps with the intensities observed in the 1886 earthquake. The database of time histories is for use by the engineering community in site-specific evaluations in the Charleston area and has been generated at their request with their input. The 1886 intensity X observations of Bollinger (1977) have been compared with the varying source based scenario hazard maps. While differences between alternative sources for the 1886 earthquake cannot be resolved by the sparse intensity observations, the comparison suggests that the west-dipping fault model of Chapman et al. (2016) is a less likely source model for the 1886 earthquake. Obviously, the seismic hazard in the Charleston area is high due to its proximity to the 1886 earthquake source. PGA probabilistic hazard is reduced by about 50% from the USGS B/C boundary hazard maps and PGA M7.0 scenario hazard ranges from 0.4 g near the source to less than 0.2 g at greater distances. These results are similar or slightly higher than our 2014 pilot study results for the Charleston quadrangle due to improvements in the local geology model. Other possible M7.0 sources within 100 km to the NW and offshore to the SE show lower but still significant hazard. Liquefaction hazard varies with surface geology with younger sediments showing higher hazard than older sediments. The probability of moderate to severe liquefaction varies from greater than 90% to less than 20 %.