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Geophysical Investigation of a Quaternary Fault Beneath the National Mall and Memorial Parks in Washington DC

Description: 

The Adams Mill fault (AMF) is a high-angle reverse fault that thrust crystalline bedrock over an unconsolidated Potomac River gravel near the original entrance of the Smithsonian National Zoological Park (SNZP) in Washington, D.C. Two luminescence ages, ~451 ka and ~338 ka, indicate the gravel, and therefore the fault, are Quaternary. USGS geologic maps published in 1994 and 2017 show the AMF trends ~165^o from the SNZP to a fault exposed at 18^th and California Streets (in a 1931 trench), and to Lafayette Park north of the White House, where USGS drilling identified a fault. The 1994 map terminates the AMF in Lafayette Park. The 2017 map shows the fault continues southward under the White House, across the Washington Monument (WM) grounds, and into the Dumfries Fault Zone near the Thomas Jefferson Memorial.

Geophysical profiles using electrical resistivity (ER) and refraction microtremor (ReMi) were acquired on the WM grounds to try to image and verify the fault. Two parallel ER profiles show the bedrock elevation abruptly drops ~150 m west of the WM, which is consistent with the presence of a fault. Similarly, the ReMi data showed a significant change in shear wave velocity across the same position ~150 m west of the WM. Unfortunately, the two ER profiles were not spaced far enough apart, and two possible fault orientations best fit the ER data. One possible orientation is ~055^o, which is roughly parallel to the Dumfries fault system on the 2017 map. This trend crosses through Arlington National Cemetery and lies ~500 m north of the Pentagon. The other possible fault orientation is 001^o. A southward projection of this trend aligns well with a very straight, 20 km long section of the Potomac River, which suggests this channel section may be fault controlled. Displaced reflectors in an unpublished seismic reflection profile from East Potomac Park tenuously supports this possibility. Whether the fault at the WM is kinematically linked to the AMF is uncertain, but the presence of a Quaternary fault in an aseismic region is notable and provides new insight into the long-term paleoseismic history of the D.C. region.

Session: The Landscape Record of Earthquakes and Faulting - I

Type: Oral

Date: 4/16/2025

Presentation Time: 09:00 AM (local time)

Presenting Author: Ron

Student Presenter: No

Invited Presentation: 

Poster Number: 5

Authors