GEOLOGY/GEOTECHNICAL-RELATED VARIABILITY OF GROUND MOTION CHARACTERISTICS AND OBSERVED EFFECTS IN WASHINGTON, D.C., DURING THE 2011 MINERAL, VIRGINIA, EARTHQUAKE

The 2011 M_W 5.8 Mineral, Virginia, earthquake is the strongest seismic event to occur in the eastern United States in more than a century. The earthquake, centered about 130 km south-southwest of Washington, D.C., caused significant damage (Modified Mercalli Intensity [MMI] VIII)in the epicentral region and widespread minor damage across Virginia and the National Capitol Area. Reported MMI effects in Washington, D.C., and its immediate vicinity were typically V to VI, although a few locations experienced lesser (IV), as well as greater (VII), effects. Local geologic/geotechnical conditions at some locations produced significant amplification of ground shaking within relatively narrow period ranges and this ground shaking appears to have contributed to the observed damage of notable structures such as the National Cathedral, the Washington Monument, and the Smithsonian Castle, in Washington, D.C., as well as to other buildings in the District and its immediate vicinity. Site response analyses conducted for these sites, using site-specific subsurface conditions (i.e., measured shear wave velocity profiles), suggest that there was amplification of the earthquake’s bedrock ground motions at the sites (estimated based on recent CEUS rock attenuation relationships) “tuned” to the characteristic site period of the subsurface profile. Ground shaking characteristics estimated from the site response analyses compared favorably to ground motions recordings of the earthquake obtained at Reston, Virginia (the nearest USGS strong-motion station to Washington, D.C.), and to recordings we received for instruments at a building site near the National Mall. The results of these analyses also highlight the importance of looking beyond simple code-based response spectra site coefficients based on Site Class to develop an understanding of the potential amplification effects that a subsurface profile can have on ground shaking characteristics. The damage experienced by structures along and in the general vicinity of the National Mall in Washington, D.C., is an example of where code-based response spectral characteristics may not adequately capture the response effects of local geologic/geotechnical conditions and may have an adverse affect on the design and/or performance of a structure.