SEISMIC HAZARD MAPS WITH THE EFFECT OF LOCAL GEOLOGY FOR WASHINGTON, DC AND FAIRFAX COUNTY, VA

We have developed improved seismic hazard maps with the effect of local geology for the Washington, DC and Fairfax County, VA area. The input ground motion prediction equations, source model, and logic tree for the analysis is taken from the 2014 U.S. Geological Survey national seismic hazard model (Petersen et al., 2014). We have added an improved local geology model based on the overburden thickness maps of Froelich (1976) and Froelich and Heironimus (1972). As in our preliminary effort (Cramer et al., 2016) we use three shear-wave velocity profiles for Piedmont, Fall Line, and Coastal Plain regions (Olgun et al, 2015). We developed reference profiles from the three Olgun et al. profiles that extend to hard rock for site amplification relative to the rock conditions for the ground motion prediction equations. Our seismic hazard maps include both probabilistic (2% in 50 years) and scenario (M6.0 at Mineral, VA) maps. The local geology in the Washington DC area strongly amplifies higher frequency ground motions (peak ground acceleration, 0.2 s spectral acceleration) in keeping with the three site-specific profiles of Olgun et al. (2015) and the observations of Pratt et al. (2017). The soil response is driven by the 10 to 20 m thick low shear-wave velocity (200–300 m/s) top layers of the reference profiles. These low velocity layers are composed of residual soil and/or alluvium. The thicker Cretaceous Potomac Formation sediments, up to 600 m thick in the SE corner of the study area, have an effect on seismic hazard at 1.0 s and longer periods. The greatest effect on 1.0 s spectral acceleration seismic hazard is from the ~200 m thick sediments near the SE edge of Washington DC. Our maps have a resolution of 0.005 degree (500 m) and have some of the sub-km scale detailed geology variation in the Washington, DC and Fairfax County, VA area. These maps can serve as a guide to improving the understanding of seismic hazard and risk in the area and stimulate further work on a more detailed local geology model with higher resolution.