Using Geophysics to Understand the Shallow Geologic Framework Beneath the National Mall and Memorial Parks of Washington, D.C., and Map the Trend of a Quaternary Fault

The Adams Mill fault, exposed near the original entrance to the Smithsonian National Zoo in Washington, D.C., thrusts Cambrian basement 2.4 m over an unconsolidated gravel. The fault is inferred to trend southeast on geological maps. A 451 ± 34 ka luminescence age for the faulted gravel, obtained by the U.S. Geological Survey (USGS) in 2016, indicates the Adams Mill fault is a Quaternary fault, and this has implications for seismic hazard analyses of the region. Electrical resistivity tomography (ERT), ground penetrating radar (GPR), and seismic refraction microtremor (ReMi) techniques were used to locate and identify the Adams Mill fault along its inferred southern trend and to test their efficacy in an urbanized setting. The fault is clearly identifiable in ERT and GPR data acquired at the National Zoo. The data also show there may be another fault ~ 30 m north of the Adams Mill fault. At the Washington Monument grounds, GPR data were ambiguous due to thick artificial fill, but ERT and ReMi techniques identified a fault ~80 m west of the Adams Mill fault\u27s position on geologic maps. This fault may not be the Adams Mill fault; however, 965 m due north of this fault lies a fault that was identified in 1976 by USGS drilling in Lafayette Park. If the faults at Lafayette Park and the Washington Monument are connected and continue further south, the fault trend would directly coincide with a ~20 km long, unusually straight channel section of the Potomac River. These results suggest the fault at the Washington Monument may be a previously unknown, north-south trending fault that appears to structurally control the Potomac River channel south of Washington, D.C., and the Adams Mill fault may be a stepover fault that connects this unknown fault to the north-south trending Rock Creek Shear Zone. A new luminescence age of 338 ± 38 ka for the faulted gravel verifies there was Quaternary slip on the Adams Mill fault