Surveyors from NOAA's National Geodetic Survey take height measurements near the Washington Monument in spring 2012. Results of measurements taken in March and April indicate that the monument experienced no significant up or down movement as a result of a 2011 earthquake in the region. The monument rests on a 25-foot-deep fill foundation that is approximately 65 feet above bedrock—a foundation that can be vulnerable to soil liquefaction during earthquakes.
A survey conducted earlier this year by NOAA’s National Geodetic Survey (NGS) indicates that an August, 2011, earthquake in the region had no discernible impact on the settlement of the Washington Monument.
What the 2012 survey does show—based on five leveling surveys of relatively consistent points of measurement taken since 1926—is that the area of the National Mall built on landfill has settled at a rate of approximately 0.013 inches per year, or a total of approximately one inch. This region includes the Washington Monument and areas south to the Potomac River.
NGS further found that the rate of settlement for the Washington Monument itself, based on multiple measurements taken since 1901, is 0.02 inches per year. This indicates that the monument has settled approximately 2.2 inches since 1901.
The 2012 survey, which encompassed the entire National Mall, is the first of what NOAA and the National Park Service plan to be a series of periodic geodetic leveling observations of the Mall area to better track subsidence changes and ensure adequate preservation of the Mall.
The report recommends that NGS and the National Park Service take GPS observations of the damaged obelisk during upcoming repairs. Similar readings were last taken during a monument restoration project in 1999-2000. Comparing new observations with those from more than a decade ago is expected to result in more reliable data on the nature of subsidence in and around the District of Columbia.
The 2012 survey was conducted at the request of the National Park Service.