THREE DIMENSIONAL P
AND S WAVE VELOCITY STRUCTURE AND Vp/Vs RATIOS FOR THE EASTERN TENNESSEE
SEISMIC ZONE
DUNN, M., POWELL, C., and WITHERS, M., Center for Earthquake Research and Information, 3876 Central Ave., Suite 1, University of Memphis, Memphis, Tennessee 38152-3050, and VLAHOVIC, G., Department of Geography and Earth Sciences, North Carolina Central University, Durham, N.C., 27707, mmdunn@memphis.edu,
Three dimensional P and S wave velocity models have been constructed for the active eastern Tennessee seismic zone (ETSZ) using travel time tomography. The inversion utilized 5758 P wave and 4212 S wave arrival times, respectively, from 473 earthquakes recorded over the time period 1983 to 1994. Block size was 12 by 12 km horizontally by 4 km vertically. Model resolution was examined using chessboard and image reconstruction tests. Ray path coverage is similar in both models and both models contain similar velocity anomalies. The primary difference between the models is in the magnitude of the anomalies; P wave velocity anomalies are in the range +5 to -5% while S wave velocity anomalies are in the range +3.5 to -3.5%. The most significant velocity anomaly imaged is a region of low velocity that trends parallel to the seismicity and lies just to the SE of the prominent NY-AL magnetic lineament. The NY-AL magnetic lineament is associated with Grenville basement rock and coincides with the NW boundary of the ETSZ. Vp/Vs values for the low velocity region are either normal (1.72) or low, reaching 1.68. The imaged velocities and Vp/Vs ratios can be attributed to rock lithologies such as gneiss and do not require the region to be extensively fractured. Anomalously high P and S wave velocities are associated with a prominent mafic intrusion in eastern Kentucky and with portions of the crust in easternmost TN. The regions of high P and S wave velocities are also associated with high Vp/Vs ratios, suggesting the presence of mafic intrusions. Inversion results will also be shown for an expanded data set containing arrival times for the time period 1994 until 2002.