34-1 Finding Fault Orientations in Our Capitol

Session: Advancing Earthquake Geology and Surficial Deformation from Geologic Provinces to Political Entities through Multidisciplinary High-Resolution Data (Posters)

Presenting Author:

Authors:

Abstract:

The Adams Mill Fault (AMF) is a reverse fault that thrusts crystalline bedrock over an unconsolidated gravel in an exposure near the original entrance to the Smithsonian National Zoological Park (SNZP) in Washington, D.C. Luminescence dating indicates that the gravel is Quaternary, making the fault much younger than previously believed. 

U.S. Geological Survey (USGS) geologic maps from 1994 and 2017 indicate the AMF trends ~165°, extending from the exposure at the SNZP southward into Lafayette Park, but the 2017 update extends the fault beyond Lafayette Park, beneath the White House and the Washington Monument (WM) grounds, and connects it to the Dumfries Fault Zone near the Thomas Jefferson Memorial. Two parallel electrical resistivity (ER) profiles and a refraction microtremor (ReMi) passive seismic geophysical survey conducted on the WM grounds identified a sharp ~35 meter drop in the bedrock elevation, consistent with faulting. The geophysical data suggest the AMF may extend beyond Lafayette Park, but the fault identified in the geophysical profiles was west of the fault shown on the map. Consequently, whether the fault in the profiles is the AMF or a different fault is uncertain, and there are two possible fault trends consistent with the geophysical data. One trend is ~055°, which is parallel to the Dumfries Fault System as mapped in 2017, and the trend crosses Arlington National Cemetery ~500 meters north of the Pentagon. The second trend is ~001°, which is aligned with a known fault in Lafayette Park to the north and with a straight 20 km stretch of the Potomac River channel to the south, suggesting this river segment may be structurally controlled by faulting. Both of these possible orientations differ from published geologic maps and suggest that the geologic structure in the region is complex. Whether the fault near the WM is directly connected to the AMF or is a different fault is unclear, but the identification of a Quaternary fault in this historically aseismic region with no known Quaternary deformation is significant and illustrates the need for further investigations into the long-term paleoseismic history of the Washington, D.C. region.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6356

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