POSSIBLE EVIDENCE FOR QUATERNARY DEFORMATION IN THE EAST TENNESSEE SEISMIC ZONE

The East Tennessee seismic zone (ETSZ) is the second most active seismic zone in the eastern United States, behind the New Madrid seismic zone (NMSZ). The ETSZ is an approximately 75-km-wide, 350-km-long zone that extends from NE Alabama and NW Georgia into SE Kentucky. Modern seismicity in the ETSZ occurs at 7-25 km depths on faults that do not break the surface, consistent with intraplate seismicity. Both recent and current research have been aimed at identifying paleoseismic features produced by collateral damage from large (Mw >6.5) earthquakes in the ETSZ, and is now focused on determining recurrence intervals.

Our research is currently focused along Tellico Reservoir near Vonore, Tennessee, where the greatest concentration of earthquakes occurs in the ETSZ. Several techniques are being used to identify paleoseismic features in this area, with detailed geologic mapping a key research tool. River terraces and bedrock geology are both being mapped along with detailed mapping of artificial trench exposures within an industrial park on the northeast side of Tellico Lake. These trenches provide ideal exposures of Quaternary river terrace sediments in contact with Cambrian shales with faults containing red, sandy clay-and breccia-filled fractures that are similar to natural outcrops along the southern shoreline across the lake that cut Paleozoic structures. Several NE-striking, SE-dipping faults in the trenches truncate Paleozoic folds. The south-side outcrop consists of a NE-striking, SE-dipping red, sandy clay-filled normal fault with ≥ 2 m displacement that placed Quaternary river terrace sediment against Cambrian shale. Footwall shale contains numerous red, sandy clay-filled fractures and breccia zones. Cosmogenic beryllium-10 and aluminum-26 age dating of river terraces is another technique being used in conjunction with geologic mapping to determine potential recent fault displacement of river terraces and bedrock.

These faults are possibly part of a NE-trending, SE-dipping strike-slip system with normal and thrust fault stepovers that fit data produced from first-motion studies and modern stress field in the Eastern U.S..