GROUND SHAKING AND RESPONSE OF THE WASHINGTON MONUMENT RESULTING FROM THE 2011 MINERAL, VIRGINIA, EARTHQUAKE

The August 23, 2011, M_W 5.8 Mineral, Virginia, was centered about 130 km south-southwest of Washington, D.C. It is the strongest earthquake to occur in the eastern United States over the past century, and caused significant damage in the epicentral area and widespread minor damage across Virginia and the National Capitol Area. Notably, the National Cathedral and the Washington Monument in Washington D.C. were damaged. Damage to the Washington Monument was concentrated in the pyramidion and consisted primarily of cracking, spalling and shifted stone masonry units. The Monument was closed after the earthquake and will remain closed until the damage is repaired. The authors participated in post-earthquake assessments of the damage and the Monument’s vulnerability to future strong shaking.

We estimated ground motions that occurred at the Monument site during the Mineral earthquake; these estimates compared favorably to recordings of the event for a nearby site obtained after we perform our analyses. We also made site-specific estimates for historic earthquakes that occurred since completion of the Monument in 1884 (e.g., the 1886 Charleston earthquake) and performed site-specific probabilistic and deterministic analyses to characterize MCE ground motions for future earthquakes. Suites of time histories representing shaking during the Mineral earthquake and the MCE were developed. Dynamic soil properties at the Monument were characterized for site response and modeling of foundation behavior based on methodologies we developed. The ground shaking conditions, along with expected foundation behavior, were used in benchmarking studies of the Monument to help understand its response during the Mineral earthquake, to calibrate analytical models against actual damage, and to assess the Monument’s vulnerability to potentially more severe damage during future MCE-level ground shaking.

The assessments conducted by the project team demonstrated that the intensity of cracking and spalling that occurred in and around the pyramidion during the Mineral event is similar to that projected to occur during an MCE. The Monument is not projected to pose a collapse hazard during an MCE and structural strengthening measures are not deemed necessary to enable the Monument to withstand expected future MCE-level ground shaking.