NEW DATA TO HELP UNDERSTAND THE PALEOSEISMIC HISTORY OF THE EAST TENNESSEE SEISMIC ZONE

The paleoseismic history of the East Tennessee seismic zone (ETSZ) has had little detailed investigation despite being the second most active seismic zone in the eastern U.S. Current seismic activity extends from northeastern AL and northwestern GA through TN, and likely into southeastern KY. Our recent work has indicated seismic events of Mw > 6.5 have occurred in the ETSZ during the last 15 ka. We have found paleoseismic features in Quaternary river sediments, including faults with displacements up to 2 m (several involving bedrock), intensely fractured zones, and paleoliquefaction features, e.g., sand dikes. Characteristic ETSZ fault zones and associated fractures contain red sandy clay that sharply contrasts with the enclosing brown Quaternary sediments or bedrock saprolite. These faults occur in the ETSZ over some 90 km from near Vonore, TN, ~60 km south of Knoxville, to Dandridge, TN, ~40 km east of Knoxville. The red sandy material occurs along main faults, subsidiary faults, in breccia, and fractures in both the hanging walls and footwalls of faults. Motion sense on faults is determined from slickenlines on fault surfaces, drag folds, and displacement sense observed in the field. Largest displacement faults observed to date are a thrust with 1 m of separation and a normal fault with at least 2 m displacement. Both involve shale bedrock and Quaternary alluvium from river terrace materials. We also have observed folded terrace gravels on carbonate saprolite along Tellico Lake with limbs dipping >40 degrees. The normal fault on Tellico Lake may link with a larger strike-slip fault system, indicated by parallel and non-parallel fault segments containing compressional structures, with the normal fault representing part of a stepover rhomb graben.

Based on our data indicating prehistoric earthquakes of at least Mw = 6.5, it would be prudent to review building codes in the region, which may not be adequate for events of this magnitude.