USING GEOPHYSICS AND LUMINESCENCE GEOCHRONOLOGY TO DETERMINE THE AGE AND EXTENT OF THE ADAMS MILL FAULT AT THE SMITHSONIAN NATIONAL ZOOLOGICAL PARK, WASHINGTON, DC

Washington D.C. was built where the Piedmont Plateau and Atlantic coastal plain meet at the fall zone. The region is structurally complex, consisting of deformed crystalline Piedmont rocks and Cretaceous-Pliocene Atlantic Coastal Plain sediments overlain by unconsolidated gravel deposits of the ancestral Anacostia and Potomac Rivers. On the Piedmont at the original entrance of the Smithsonian National Zoological Park, the Adams Mill fault thrusts Cambrian basement over a Potomac River gravel deposit in a roadcut exposure. This exposure, along with faults observed at 18^th and California NW and at Lafayette Square are along the same trend, and two published USGS maps infer they are all on a ~5 km long transfer fault between two, NW striking, parallel features: the Rock Creek Shear Zone in the northwestern half of the city on the Piedmont, and the Stafford Fault System in the southern half of the city on the Atlantic Coastal Plain.

Although the region is aseismic, luminescence dating of feldspars in the faulted gravel indicates faulting is younger than ~451 ± 34 ka. This is significant because USGS drilling at Lafayette Square suggests fault slip could be episodic, which would classify the fault as a capable fault according to Nuclear Regulatory Commission guidelines. Additionally, depending on the USGS map, the fault either stops at Pennsylvania Avenue or continues south under the White House and Washington Monument and joins the Stafford Fault near the Thomas Jefferson Memorial. Geophysical techniques of Electrical Resistivity imaging (ER), ground penetrating radar (GPR), and refraction microtremor seismic (ReMi) were used test the geologic maps and see if the fault could be imaged and located, and to test how these techniques would work in a heavily urbanized environment. The GPR and ERT surveys at the National Zoo and Washington Monument clearly show the fault in multiple east-to west profiles. GPR and ER at the zoo also show the fault, which is visible in a roadcut. ReMi data are still being processed.

Mapping the extent of the Adams Mill fault is significant for seismic hazard and risk assessments. The 2011 M_w 5.8 Mineral, Virginia earthquake demonstrated how well seismic energy can be transmitted along faults at depth and how seismic energy is amplified through the unconsolidated sediments of the Atlantic coastal plain.