DETAILED GEOLOGIC MAPS OF TWO SITES SOUTH OF DANDRIDGE, TENNESSEE, RECORD EVIDENCE OF POLYPHASE PALEOSEISMIC ACTIVITY IN THE EAST TENNESSEE SEISMIC ZONE

The East Tennessee seismic zone (ETSZ) in the southern Appalachians is the second most active intraplate region east of the U.S. Rocky Mountains. This ~50 km-wide, 300 km-long zone of seismicity extends from NE Alabama and NW Georgia to just NE of Knoxville, Tennessee, producing earthquakes at depths of 5-26 km within the basement below the major Paleozoic thrust sheets. Although the ETSZ has not produced historical earthquakes of M>5, others have suggested that it may be capable of generating an “infrequent” M~7.5 event. Small faults and numerous bleached-clay-filled fractures discovered in late Quaternary French Broad River terraces (optically stimulated luminescence ages of terraces range from several tens to >100 ka) along Douglas Reservoir led to a need to grade and map (at 1 in = 5 ft) two small areas E and W of the TN 92 bridge S of Dandridge, Tennessee. The site W of the bridge yielded less information, but still revealed at least three sets of crosscutting fractures (dominant orientations 045, 065, 085, 305; 553 measurements from map) that terminate up-slope against the base of an overlying colluvium. The E site revealed numerous fractures (dominant orientations 055, 305, 335; 701 measurements) and a fault with ~20 cm displacement. Moreover, several bedrock shale “boils” are present that are cut by younger, red clay-filled fractures. Few fracture sets in Quaternary sediments parallel those measured in Tennessee Valley and Ridge and Blue Ridge bedrock, but these fractures do not relate well to the present-day 070 orientation of maximum principal stress. The small faults discovered here and elsewhere around Douglas Reservoir all have approximately the same displacement; comparison with estimates of earthquake magnitude from active faults in bedrock suggests the earthquakes that produced these faults were M>6. They have a recurrence history spanning thousands of years.