DETAILED GEOLOGIC STUDIES OF PALEOSEISMIC FEATURES EXPOSED AT SITES IN THE EAST TENNESSEE SEISMIC ZONE: EVIDENCE FOR LARGE, PREHISTORIC EARTHQUAKES

The East Tennessee seismic zone (ETSZ) is the second most active in the eastern United States, but recorded earthquakes do not exceed M_w [moment magnitude] = 4.6. Earthquake epicenters are located 5-26 kilometers deep in autochthonous basement, and faults producing these earthquakes do not break the surface. Detailed paleoseismic investigations at sites within the ETSZ include: detailed geological mapping, trenching, aerial photograph reconnaissance, X-ray diffraction (XRD), grain-size analysis, and optically stimulated luminescence dating of alluvium.

Site DL-6 near Dandridge, Tennessee, reveals a complex array of features providing evidence that at least 4‒6 M_w > 6 earthquakes affected the area. A thrust fault with ~1 meter displacement is traceable for ~50 meters in the mapped area from Quaternary terrace sediments into Ordovician Sevier Shale saprolite where it develops multiple splays. This fault truncates a 30 centimeter-wide Quaternary sediment-filled fissure; the fissure is also offset ~10 centimeters by a small strike-slip fault that truncates against a splay of the main thrust fault. The fissure may be traceable for over 2 kilometers across Douglas Reservoir. Fault surface and slickenline orientations indicate west-vergent slip on many thrust splay surfaces. Fracture orientations measured at site DL-6 are close to those in Sevier Shale here; reactivation of near-vertical fractures is mostly strike-slip. Additionally, shale chips in Quaternary sediment at a nearby site at Dandridge, Tennessee, contain the same mineralogy as underlying Sevier Shale, indicating that these chips may have been liquefied and brought up through Quaternary terrace sediments during an earthquake.

Sites along the Chattooga River near Lyerly, Georgia, provide evidence for at least one M_w > 6 earthquake. Sand dikes emanating from a source bed in a cut bank contain liquefiable sediments that intruded overlying cap sediments; grain-size and XRD analyses suggest cap sediments are more clay-rich than source or dike sediments. Aerial photographs of the area contain elliptical discontinuities that may represent sand blow deposits. Field reconnaissance of the area identified possible feeder dikes to sand blows; further investigation is required.

Collectively, these data provide substantial evidence for the occurrence of M_w > 6 earthquakes in the ETSZ, and motivate further paleoseismic studies here.