SITE AMPLIFICATION IN THE WASHINGTON DC AREA DURING THE MINERAL, VIRGINIA EARTHQUAKE

The August 23, 2011 M5.8 Mineral, Virginia Earthquake presented an unprecedented opportunity to study issues related to seismic site response in the Washington DC region. Observations and follow-up studies have underscored the importance of the geological conditions Central and Eastern United States (CEUS) in relation to seismic site amplification in this region. These studies following the Mineral, Virginia Earthquake has increased our understanding of site response in the CEUS and provided a chance to assess the adequacy of the building code provisions for the seismic design of structures.

The geologic conditions in the Washington DC region are unique and they are significantly different from the conditions encountered in tectonically active regions from which most of the site amplification factors are derived from. The Fall Line runs through the Washington DC region separating the Coastal Plain and Piedmont geologic provinces. Sites near the Fall Line typically consist of profiles with shallow hard rock with shear wave velocities up to 2,500 m/s overlain by 20-50 meter in thick soil deposits. The resulting shear wave velocity profiles include sharp impedance contrasts, which can be critical for site amplification. Furthermore, the weathered transition zone between the hard rock and near surface soils can be highly variable over short distances creating uncertainty in prediction of the ground surface response.

Site response analyses from thirty DC region sites with measured shear wave velocity are presented. Equivalent linear analyses using synthetically generated ground motions are used to investigate seismic site response in the DC region for different levels of ground shaking and a range of transition zones to capture the effect of this variability. Particular emphasis is given to the evaluation of site amplification factors from the comparison of ground surface and rock motions. The results are discussed and compared with observations of shaking/damage during the 2011 Virginia Earthquake and seismic design provisions in the building code.