Abstract
We have developed improved seismic hazard maps with the effect of local
geology for the Washington, DC 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 map of Froelich (1976). 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 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.