THE EASTERN TENNESSEE SEISMIC ZONE: SMALL EARTHQUAKES ILLUMINATING MAJOR BASEMENT FAULTS?

The southern Appalachians contain one of the most active seismic zones in eastern North America. Twenty years of monitoring has revealed a belt of seismicity in northeastern Alabama, northwestern Georgia and much of eastern Tennessee. The focal depths of most earthquakes range from 5 to 22 km, beneath the Paleozoic detachment. Focal mechanism solutions in the region indicate strike-slip faulting on steeply dipping planes and a uniform regional stress field with horizontal maximum compression trending N70E. The majority of mechanisms involve either right-lateral motion on NS planes or left-lateral slip on EW trending planes. A smaller population of strike-slip mechanisms shows right-lateral motion on northeasterly trending planes, parallel to the overall trend of the seismicity. The highest density of activity has occurred in a narrow northeasterly trending zone less than 100 km wide and approximately 300 km in length within the Valley and Ridge. The western margin of this more active zone is sharply defined and coincides with a prominent gradient in the total intensity magnetic field. Seismicity is not uniformly distributed within this zone of dense activity: the epicenters form northeasterly trending en-echelon segments. These linear segments, and the locations of their terminations, presumably reflect in some sense the basement fault structure that is being reactivated in the modern stress regime.

The Eastern Tennessee seismic zone presents many questions. Since 1982, 44 felt earthquakes have occurred in the denser zone of activity and 35 have had magnitudes exceeding 3.0. However, the largest historical shock was magnitude 4.6, and occurred in 1973. No evidence for larger prehistoric shocks has been discovered, yet the microearthquake data suggest coherent stress accumulation within a large volume. Physical processes for reactivation of basement faults in this region could involve a weak lower crust and/or increased fluid pressures within the upper to middle crust. Inversions of travel time data to date have not revealed anomalously low velocities within the seismic zone. However, there may be a marginal correlation between the seismicity and the major drainage pattern and general topography of the region. Perhaps a hydrological element links seismicity, uniform regional stress and basement structure.