PALEOLIQUEFACTION STUDY OF THE EARTHQUAKE POTENTIAL OF THE CENTRAL VIRGINIA SEISMIC ZONE (CVSZ)

Sand dikes, sills and soft-sediment deformation features, interpreted to be earthquake-induced liquefaction features, have been documented at 19 sites within 55 km of the epicenter of the 2011 moment magnitude, M, 5.8 Virginia, earthquake. These features were found during systematic surveys of cutbank exposures along 119 km of six rivers: (1) the South Anna River, where likely paleoliquefaction features were found within 5 km of the epicenter following the 2011 earthquake; (2) the North Anna, Pamunkey, and Mattaponi Rivers east of the Fall Line, where liquefiable sediments are more common than in the epicentral area; and (3) the James and Rivanna Rivers, where sand dikes were found during an earlier paleoliquefaction study in the 1990s.

The sand dikes (0.6 to 6 cm wide) and the sills (1 to 4 cm thick) are composed of silty fine sand, are tabular, and have lateral continuity. The dikes branch and pinch upward and intrude pre-existing dessication cracks and root casts. Several sand dikes were traced downward to their source beds. Cross-cutting relationships and the degree of weathering of the sand dikes suggest at least two generations of features. At two sites, soft-sediment deformation features, including load casts and pseudonodules, had formed in an interbedded sand and silt deposit, very similar to the source of sand dikes at other nearby sites. The upper 10-40 cm of most dikes are bioturbated, iron-stained, and mottled, suggesting that they did not form during the 2011 event and are prehistoric in age. Radiocarbon dating of organic samples in host sediment at two sites on the Pamunkey (5475-5595 calibrated yr B.P.) and the South Anna (4290-4435 calibrated yr B.P.) Rivers provides maximum age constraints of the features.

During this study, paleoliquefaction features were documented in a 3100 km² area within the northeastern CVSZ. The ages of the liquefaction features are as yet poorly constrained; but most of the features probably formed during the past 5,600 years. To produce the observed liquefaction field, a paleoearthquake centered within this area would have to be at least M 6. With the existing data, however, it is premature to estimate timing, source areas and magnitudes of the earthquakes responsible for these features. Additional study over a broader area is needed to better define the earthquake potential of the CVSZ.