A SEISMOTECTONIC MODEL FOR THE EASTERN TENNESSEE SEISMIC ZONE AND CLUSTER ANALYSIS OF RELOCATED HYPOCENTERS

A seismotectonic model for the active eastern Tennessee seismic zone (ETSZ) is developed using a combination of aeromagnetic, gravity and seismic data. The ETSZ trends NE-SW and is about 300 km long. Unlike other intraplate seismic zones, the ETSZ is not associated with an ancient rift structure but is remarkably correlated with the prominent New York–Alabama (NY-AL) magnetic lineament. The NE-SW trending NY-AL magnetic lineament probably represents a basement fault that played an important role as a continental transform between proto-Laurentia and Amazonia in the assembly of supercontinent Rodinia during the Grenville orogeny. High-resolution velocity models for the ETSZ reveal a sharp velocity contrast associated with the vertical projection of the NY-AL magnetic lineament that extends to depths of at least 24 km. A band of anomalously low velocity is NW of the lineament and anomalously high velocity SE. The low velocity band is interpreted to represent a major basement fault. The high velocity region on the SE hosts the earthquake activity and is characterized by very low magnetic anomalies and slightly elevated Bouguer gravity anomalies. Plausible rock types for the high velocity region include granodiorite and felsic granulite, suggesting that earthquake activity is occurring in basement rocks that are similar to Grenville basement exposures in the Appalachians. A clustering algorithm is used to analyze relocated earthquakes to distinguish clustered and diffuse seismicity. Clustered events align in near-vertical segments trending roughly parallel to the NY-AL magnetic lineament. Earthquakes in the most seismogenic portion of the ETSZ delineate another set of near-vertical faults trending roughly E-ESE. These apparent trends and steep dips are compatible with ETSZ focal mechanism solutions. The solutions are remarkably consistent and indicate strike-slip motion along the entire length of the seismic zone. We suggest that ETSZ earthquakes represent reactivation of a particularly well-preserved, distributed zone of deformation that involved transfer of the southern Appalachian basement to Laurentia along a transform fault during the Grenville orogeny.