INVESTIGATING LAKE SEDIMENTARY DEPOSITS FOR EVIDENCE OF THE 2011 EARTHQUAKE IN THE CENTRAL VIRGINIA SEISMIC ZONE

Lake environments have potential to supplement and improve upon existing records of seismic activity derived from trenches, outcrops, cutbanks, and other terrestrial settings by preserving continuous deposition of sediment that can be constrained with radioisotope ages. Traditionally, lacustrine paleoseismic studies have relied on the identification of earthquake-induced slump deposits in large, steep-sided basins in mountainous regions. Previous work has shown some potential for shallow-sloping lakes, such as those common to the eastern U.S., to preserve seismically-induced sediment deformation features. In this study, we investigate whether small slump deposits, disturbed laminae, liquefaction protrusions, sediment homogenization, or abrupt changes in deposition resulting from the 5.8 magnitude (Mw) earthquake occurring near Mineral, VA in 2011 were preserved in any of three reservoirs proximal to the epicenter. We collected ground-penetrating radar (GPR) and multiple sediment cores from Northeast Creek Reservoir (7.7 km from epicenter), Bearden Pond (11.9 km from epicenter), and Lake Anna (25.4 km from epicenter and 5 km from the North Anna Nuclear Generating Station). Core-top ages were constrained with ²¹⁰Pb, ¹³⁷Cs, and ¹⁴C dates, and sediments were analyzed for bulk organic content, grain size, elemental abundances, bulk density, and magnetic susceptibility to assist paleoseismite identification. Vertical mixing of sediment is apparent at the 2011 CE (+/- 1 yr) horizon in Bearden Pond, and soft-sediment deformation is preserved up to 18 cm below 2011, where high water content (~83 %) allows for fluidity in the sediments. In Northeast Creek Reservoir, 2011 CE (+/- 7 yrs) coincides with a rise in organic matter that is unprecedented in the history of the reservoir. This organic matter is a mix of algal and vascular plant material in smear slides, which could derive from stimulated productivity and/or deliver of terrestrial organic matter related to earthquake-driven slope failure. Lake Anna sediments preserve evidence of disturbance at the 2011 CE horizon (+/- 2 yrs), including an abrupt increase in grain size. Lacustrine paleoseismology has potential for determining recurrence intervals and risk in the eastern U.S.