PALEOLIQUEFACTION RECONNAISSANCE IN EASTERN NORTH CAROLINA: IS THERE EVIDENCE FOR LARGE MAGNITUDE EARTHQUAKES BETWEEN THE CENTRAL VIRGINIA SEISMIC ZONE AND CHARLESTON SEISMIC ZONE?

Approximately 90 km of riverbank were examined along the Tar and Neuse rivers near Tarboro and Kinston, NC for evidence of liquefaction-forming earthquakes in the vicinity of the Grainger’s wrench zone in eastern North Carolina in June 2016. The Grainger’s wrench zone is a fault zone in the Coastal Plain of eastern North Carolina that has well-documented Paleogene and younger deformation of Cretaceous to Eocene strata. Low-magnitude earthquakes in the vicinity of the fault zone (e.g., M2.1 in 2013, 13 km SSW of Kinston) suggest it may have produced larger earthquakes in the past. This region is about equidistant from Holocene paleoliquefaction sites in the Central Virginia seismic zone (CVSZ) and liquefaction sites formed during the 1886 M 7.1 Charleston, SC earthquake. The northernmost Holocene paleoliquefaction features associated with the Charleston seismic zone (CSZ) are at Southport, NC.

Conditions suitable for liquefaction were identified at many sites on both rivers. Stratigraphy consists of gneiss, Cretaceous sandstone/shale, Paleocene mudstone/claystone to Eocene fossiliferous limestone, Quaternary unconsolidated cross-bedded sand and gravel, and Holocene alluvium. Two stratigraphic conditions for liquefaction – unconsolidated sand beneath capping strata – were identified in detailed examinations at 35 sites: 1) weathered Cretaceous (K) sandstone or Quaternary (Q) sand capped by Holocene alluvium; and 2) weathered K sandstone or Q sand capped by clay-rich Bt soil horizons. Many soft-sediment deformation features, mostly load casts, were observed, but none could be conclusively established as seismogenic. A few examples of pseudo-sand dikes were also identified: sand filled cypress root casts, and pedogenic weathering fronts creating the appearance of sand dikes and sills.

A comparable survey in 2015 of 110 km of riverbank exposures in the CVSZ yielded 19 paleoliquefaction sites (Tuttle et al. 2015); geochronology suggests that these CVSZ features formed from at least two M6+ earthquakes in the last 5 Ka. However, this survey in eastern NC revealed no paleoliquefaction features, suggesting liquefaction-forming earthquakes may not have affected this region during the Holocene.