LIQUEFACTION EVIDENCE FOR A STRONG EARTHQUAKE IN THE LOWER OHIO RIVER VALLEY DURING THE MID TO LATE HOLOCENE

A clastic dike recently discovered in the lower Ohio River Valley indicates that seismically induced liquefaction occurred in the region during the middle or late Holocene. Although the Ohio River traverses regions affected by the Wabash Valley and New Madrid Seismic Zones, this is the first prominent paleoliquefaction feature found east of the Wabash River in the Ohio River Valley.

The dike is in a cut bank of the lower Green River, 21 river miles from its confluence with the Ohio, where the Green River is inset within Ohio River outwash. It is weakly cemented, 4 to 7 cm wide, and composed of sand and gravel injected into silty Green River floodplain deposits. The dike measures 3.3 m in height, but its base was underwater so its maximum height is unknown. The lower part of the dike is dominated by gravel up to 1 cm in diameter, and the upper section by fine sandy gravel. Whether this represents a fining-upward sequence from a single event or is the result of multiple seismic events could not be initially determined. The dike pinched together approximately 1.5 m below the modern floodplain surface and appears pedogenically altered by a well-developed fragipan soil, so its age is estimated to be on the order of thousands and not hundreds of years old.

The dike's estimated age of thousands of years is comparable to a 2,000-year-old liquefaction event in the New Madrid zone or possibly a 5,000-year-old liquefaction event in the Wabash Valley. Yet liquefaction features from the New Madrid event 2,000 ka are sparse and constrained to the meizoseismal region of 1811-12. In addition, no dikes of comparable size have been found between this site and those source zones, suggesting a local seismic source created the dike. Minor soft-sediment deformation features scattered in the immediate area of the dike may also be associated with a local seismic event.

Data from historical earthquakes indicate that the moment magnitude required to produce clastic dikes in this environment is at least M 5.5, but the presence of gravel in the dike and its large height suggest that it could be larger, especially if it originated in the New Madrid or Wabash Valley zones. Future work will constrain the timing of the dike emplacement with luminescence and radiocarbon dating, and search for other dikes in the region.