LATE HOLOCENE LIQUEFACTION FEATURES IN THE CENTRAL VIRGINIA SEISMIC ZONE (Invited Presentation)

There are at least two generations of earthquake-induced liquefaction features in the Central Virginia seismic zone (CVSZ) that pre-date the 2011 moment magnitude, M, 5.7 Mineral earthquake. Forty-one sand dikes, sand sills, and soft-sediment deformation features were studied at twenty-four sites exposed in cutbanks along portions of several rivers: (1) the South Anna River, where paleoliquefaction features were found within 5 km of the Mineral earthquake; (2) the Mattaponi, North Anna, and Pamunkey Rivers east of the Fall Line, where liquefiable sediments are more common than in the epicentral area of the Mineral earthquake; and (3) the James and Rivanna Rivers, where sand dikes were found during a previous study. Liquefaction features are grouped into two categories based on weathering characteristics and dating of host sediments. Younger liquefaction features exhibit slight iron staining or mottling and likely formed during the past 350 years. They are small (dikes 1-3 cm wide), few in number, and appear to be limited to the James and Pamunkey Rivers. The 1875 M 4.8 ± 0.2 earthquake, thought to be located northwest of Goochland and the largest historical event to strike the area, might have caused these features. Older liquefaction features exhibit mottling and bioturbation and likely formed between 350 and 2800 years ago. These features are larger, more numerous, and more broadly distributed than the younger features. There is no evidence to suggest that they formed as a result of multiple earthquakes. The larger features (dikes ≤ 20 cm wide) occur on the Pamunkey and Mattaponi Rivers, suggesting that the paleoearthquake source may be located nearby, perhaps in the Ashland area where small (M < 5) earthquakes have been recorded since 1974. According to the relation between earthquake magnitude and greatest distance to liquefaction, a M ≥ 6.5 paleoearthquake centered near Ashland could explain the larger features along the Mattaponi and Pamunkey Rivers as well as the other features along the South Anna and Rivanna Rivers to the west. A similar conclusion is reached using liquefaction potential analysis. Additional mapping and dating of liquefaction features and analysis of scenario earthquakes are needed to better understand the geologic record of paleoearthquakes and the earthquake potential of the CVSZ.