PALEOSEISMIC EVIDENCE FOR LARGE, PREHISTORIC EARTHQUAKES IN THE EAST TENNESSEE SEISMIC ZONE

The East Tennessee seismic zone (ETSZ) is the second most active in the central and eastern US, but it has no recorded history of M > 4.8 earthquakes. Faults producing these earthquakes are sourced in the autochthonous basement and are thought to have no direct surface expression. Paleoseismic features have been mapped and trenched at multiple sites in the ETSZ. Additional sites of possible paleoseismic features, some of which lie beneath reservoirs today, have been identified in 1930s- to 1950s-vintage aerial photographs. Work to date has found evidence for several M > 6.5 earthquakes in the French Broad River valley ~50 km E of Knoxville.

Several thrust and strike-slip faults that cut Quaternary alluvium with 25 cm to ~1 m of displacement have been identified along Douglas Lake near Dandridge, TN. On the north side of Douglas Lake, a 30 cm-wide N33E Quaternary sediment-filled fissure in Ordovician Sevier Shale is offset ~1 m by a fault that thrust the shale over Quaternary alluvium. Areas of shale clasts in similar river terrace sediment have also been identified; these clasts may have boiled up from underlying weathered Ordovician Sevier Shale during paleoseismic events. X-ray diffraction (XRD) analyses matched the mineralogy of the shale chips and underlying Ordovician shale.

Vintage aerial photos of flood plains along the Tennessee River in eastern TN and northeastern AL, and along the Chattooga River in northwestern GA, reveal circular to elliptical features in river terrace sediment that may be related to liquefaction. Reconnaissance along a segment of the Chattooga River has discovered possible paleoseismic sand dikes. Grain-size analyses of sand dikes, source bed, and host bed sediments confirm that the sediments fall in the size range of liquefiable sediments, and that the host is finer grained than the sand dikes or source bed. XRD analyses reveal that the sand dikes and source bed have nearly identical mineralogy, but the host contains more clay minerals. These analyses indicate the source bed may be the parent material of the sand dikes.

Together, these data support the capability of the ETSZ to produce M > 6.5 earthquakes, which increases the earthquake hazard potential in the southeastern U.S.