Characterizing the signature of recent earthquakes preserved in central Virginia reservoir sediments

Abstract:

In 2011, a 5.8 magnitude (Mw) earthquake occurred at ~6 km depth near Mineral, VA within the central Virginia seismic zone (CVSZ) and was the largest recorded earthquake in the CVSZ since at least the late 18^th century. The tremors exceeded the seismic design specifications of the North Anna nuclear power plant, 20 km northeast of the epicenter, resulting in an unprecedented complete safe shutdown of the facility. Unlike typical earthquakes in the western U.S., shaking during east coast earthquakes tends to extend over exceptionally broad areas, threatening population centers that are not constructed to withstand large earthquakes. Developing long histories of earthquake activity in active seismic zones such as the CVSZ is crucial for determining recurrence intervals and assessing the risk for future large events. Paleoseismic studies in lacustrine environments have shown that lake sediments record evidence of past intense earthquakes, including slump deposits, disturbed laminae, and liquefaction protrusions, and can extend the historical record thousands of years into the past. In this study, we identify deformation features of possible seismic origin in reservoir sediments from the CVSZ. The goals of our research are to: 1) determine whether the historic 2011 earthquake left a fingerprint in the sedimentary record and 2) characterize these deformation features for improved identification of paleoearthquake features preserved in shallow-sloping subaqueous environments in other central/eastern U.S. earthquake zones. We used ground-penetrating radar to map bathymetry and sub-bottom stratigraphy in reservoirs proximal to the 2011 epicenter to identify sites that preserved sedimentary strata at least 50 cm thick. We collected sediment cores from 3 of these sites: Northeast Creek Reservoir (7.7 km from epicenter), Bearsden Pond (11.9 km from epicenter), and Lake Anna (25.4 km from epicenter). We present evidence for cm-scale paleoseismites within the upper 40 cm at each site identified from macroscopic and microscopic description of the sediment cores and loss-on-ignition, grain size, and elemental abundance analyses. Preliminary chronologies based on ¹⁴C, ¹³⁷Cs, and ²¹⁰Pb ages indicate these deformation features occurred in sediments that had been deposited after 1880 and prior to the 2011 earthquake.