MID TO UPPER CRUSTAL VELOCITY STRUCTURE ASSOCIATED WITH THE EASTERN TENNESSEE SEISMIC ZONE

Detailed P wave velocity (Vp) and S wave velocity (Vs) models and Vp/Vs ratios are determined for the active eastern Tennessee seismic zone (ETSZ) based upon arrival times from more than 1000 local earthquakes. We perform a simultaneous inversion for Vp, Vs, and hypocentral locations, yielding a detailed tomographic image to mid-crustal depths. The ETSZ is associated with a pronounced, northeast-southwest trending, linear magnetic anomaly called the New York-Alabama (NY-AL) magnetic lineament. Most earthquakes occur to the southeast of the lineament below the decollement in Grenville basement rocks. A distinct change in crustal velocity occurs across the lineament; high Vp and Vs occur to the southeast and low Vp and Vs occur to the northwest. The velocity contrast is imaged over the depth range 4 to 20 km and the strongest contrast corresponds with the steepest portion of the magnetic gradient defining the NY-AL lineament. Resolution for the Vp and Vs models is comparable in the central portion of the inversion solution; here, low Vp/Vs values are associated with the most seismogenic portion of the ETSZ and suggest the presence of compositional variations (i.e. increased quartz content) rather than increased fracturing and fluid pressure. Prominent negative Vp anomalies correspond with areas of high elevation in the Blue Ridge and Valley and Ridge provinces, supporting the presence of crustal roots. Synthetic velocity models, generated using the same source-receiver geometry and smoothing parameters used in the inversion of the real data, suggest the presence of granite gneiss northwest of the NY-AL magnetic lineament. Rock composition southeast of the lineament is enigmatic; synthetic modeling suggests felsic granulite or diorite. Synthetic modeling also suggests the presence of anorthosite along the Alabama- Tennessee border at depths of 8 to 20 km. Velocity results will be interpreted in terms of past tectonic events in order to better understand the basement feature associate with the NY-AL magnetic lineament.