The moment magnitude (M_w) 5.8 Mineral, Virginia, earthquake of 23 August 2011, was centered ~130 km south-southwest of Washington, D.C. (USA), and caused minor damage across Virginia and the Washington metropolitan area. The Washington Monument sustained masonry damage; a post-earthquake survey of the monument performed for the National Park Service identified cracking and spalling of the pyramidion (the topmost piece of the obelisk), and cracking, spalling, and lesser damage over the entire length of the monument shaft. A seismic vulnerability assessment of the monument was then performed to evaluate the potential for damage to the monument from future earthquakes.

No ground-motion recordings of the Mineral earthquake were available for the monument site; therefore, deterministic and probabilistic seismic hazard analyses were performed to develop site-specific response spectra representing the Mineral earthquake and the maximum considered earthquake (MCE). These spectra were initially developed for a firm rock site condition, and each event was represented by a suite of seven three-component time histories. The rock motions were then modified through site-response analyses to develop time histories and response spectra representing ground motions in the clayey and gravelly soils that support the base of the monument. The results of the site-response analysis show significant amplification at short to intermediate response periods; this amplification is also observed in recordings of the 2011 Mineral earthquake obtained from another site in the general vicinity of the monument.

The ground shaking conditions, along with expected foundation load-deflection behavior, were used in detailed structural modeling of the monument to help understand the structure’s response and damage during the 2011 Mineral earthquake and to predict expected performance during future MCE-level ground shaking. The analyses indicate that the period range of the pyramidion’s modes of vibration corresponds closely with the characteristic period range of the monument’s subsurface profile, and is reflected by the peak response of the site-specific spectra analyzed for the Mineral scenario earthquake. This similarity caused amplification of motions experienced by the monument and increased damage to the pyramidion. The nonlinear analysis for the MCE ground motions indicates damage to the pyramidion similar to that from Mineral earthquake effects, but larger lateral displacements of the top of the shaft due largely to the greater soil-bearing stresses associated with a greater first-mode response. The monument was found to generally meet accepted seismic safety criteria without need for strengthening.