JOINT HYPOCENTER-VELOCITY INVERSION FOR THE EASTERN TENNESSEE SEISMICZONE

A joint hypocenter-velocity inversion for the eastern Tennessee seismi c zone (ETSZ) has resolved velocity features in basement rock below de tached Appalachian thrust sheets. P and S wave arrival times from 492 earthquakes have been inverted for one-(1-D) and three-dimensional (3- D) velocity models to midcrustal depths. The 3-D P and S wave velocity solutions are computed independly and are very similar. In relation t o the 1-D model, velocity anomalies range from 8% to +16% in the first layer (upper 5 km) and between +/-7% in deeper layers. Prominent velo city anomalies parallel the seismic zone and are consistent from layer to layer. The most persistent anomaly is a low-velocity region that b orders the seismic zone to the northwest and is flanked on either side by regions of anomalously high velocity. The New York-Alabama (NY-AL) magnetic lineament coincides with or lies close to the southeast boun dary of the prominent velocity low in both the P and S wave velocity i mages. The spatial coincidence between velocity, gravity, and magnetic gradients suggests that major discontinuities are present in the base ment. Relocation in the 3-D velocity model reduced the number of very deep earthquakes (below 20 km) and further accentuated differences in seismogenic properties on either side of the NY-AL lineament. After re location, most earthquakes occur in a vertically bounded region roughl y 30 km wide extending from 4 to 22 km in depth. Most earthquakes occu r in regions characterized by either average velocity or small velocit y anomalies.