Focal mechanism analyses for eastern Tennessee earthquakes (1981-1983)

Abstract

To investigate the neotectonic processes in the Southern Appalachians, 10 single-event focal mechanisms and 6 composite focal mechanisms were determined from 37 events that occurred in eastern Tennessee between September 1981 and July 1983. Both P -wave polarities and ( SV/P ) z amplitude ratios are input to a computer program that systematically searches the focal sphere for solutions acceptable within preset error limits. Hypocenter locations, azimuth, and departure angles are taken from locations obtained by the Tennessee Earthquake Information Center with a four-layer velocity model. A gradient velocity model, developed to improve focal depth estimates and to provide continuously varying (rather than discrete) departure angles, is used to relocate the seismic events for which single-event focal mechanisms and composite focal mechanisms solutions are obtained. The two different velocity models each produced focal mechanisms with similar nodal plane and P -axis orientations, indicating stable and reliable mechanism solutions; the differences between average strike, dip, and rake angles of the two data sets range from 2° to 11°. Both single-event focal mechanisms and composite focal mechanisms solutions exhibit predominantly strike-slip motion along nearly vertical northerly (right-lateral) or easterly (left-lateral) oriented nodal planes. Standard deviations about the average strike, dip, and rake angles are generally less than 20°. P -axis trends average about N50°E, with a nearly horizontal average plunge, and both trend and plunge estimates have standard deviations of 25° or less.