A SEISMICITY/QUATERNARY FAULTING CORRIDOR IN THE EASTERN TENNESSEE SEISMIC ZONE: AN UPDATE

The N45E-trending eastern Tennessee seismic zone (ETSZ) is one of the principal areas of historic earthquake (EQ) activity in eastern North America. Although the rate of seismicity is high for an intraplate setting, historic EQs have been less than M5. Thus, the crucial question from an EQ hazard perspective: how strong have late Quaternary prehistoric EQs been in the ETSZ? Our field reconnaissance found three localities in eastern Tennessee (Dandridge, Alcoa, and Vonore) with faults offsetting late Quaternary sediments 1 to 2 m. These faults have collinear 050-060 strikes defining an 80-km long corridor that overlaps a 060 seismicity alignment to the southwest for 30 km. The 060 seismicity alignment also extends 30 km farther SW of the fault corridor (as currently mapped), giving the seismicity/Quaternary fault corridor a total length of 110 km. With the exception of one of the Quaternary faults at Vonore with normal offset, faults at all localities have strong contractional slip components and are up-on-the-SE. This 060 corridor is coincident with a steep gradient in S-wave velocities at mid-crustal depths (20-24 km) from high on the SE to low on the NW. The corridor is also coincident with the foot of the NW Blue Ridge topographic escarpment.

Deformation of French Broad River alluvium near Dandridge by thrust faulting and fissuring records at least two significant post 25 ka paleo-EQs. The later event is estimated to have been M6.5 to 7 based on 1 m of dip slip. Thrusting of Little River alluvium near Alcoa records two significant post 16 ka paleo-EQs that are estimated to have been M6 to 6.5 based on 0.5 m of dip slip per event. At Vonore, Little Tennessee River alluvium is dropped at least 2 m along a normal fault, suggesting a M6.5 to 7 EQ. Also at Vonore, colluvium containing alluvial cobbles is thrust at least 1 m by a steeply-dipping fault that may also have a significant strike-slip component. A M7 would be expected for an event that ruptured most or all of the 80 to 110 km corridor length, consistent with M7 EQs estimated from faulting.