Pn wave velocity and anisotropy in the middle-southern segment of the Tan-Lu fault zone and adjacent region

The Tancheng-Lujiang fault zone is the largest deep NNE-trending fault zone in the Eastern China, which has always been focused by many geoscientists because of its importance and complexity. The middle-southern segment of the Tan-Lu fault and surrounding areas (115 degrees E-122 degrees E, 29 degrees N-38 degrees N) is located in the crossing part of three Chinese geological blocks (North China block, Yangtze Craton, South China Folds). We carefully selected 6381 Pn travel time picks from the earthquake bulletins of the International Seismological Center (ISC), China national seismic network and regional seismic network to invert for Pn velocity variation and anisotropy of the uppermost mantle beneath the middle-southern segment of the Tan-Lu fault and its adjacent areas by Pn wave time-term tomography method. The result indicates that obvious lateral heterogeneities exist in the uppermost mantle beneath the study area and the Pn velocity varies from 7.68 similar to 8.24. km . s(-1) compared to the average velocity of 7.95 km . s(-1). The Pn velocity structure along Tan-Lu fault has segmentation. The middle-segment of the Tan-Lu fault zone shows obvious NE-trending low velocity zone. Low velocity may result from the strongly thinning of the lithosphere and the upwelling of the asthenosphere hot material. The southern part of the Tan-Lu fault zone has NNE-trending weak high-velocity anomaly zone, which is considered as a high low-velocity boundary of two different tectonic blocks, North China and Yangtze. We hold that Tan-Lu has the characteristics of tectonic block boundary. The boundary curve to the North China from SE to NW in Jiangsu domain. The intensity distribution of Pn velocity anisotropy is correlated with the distribution of velocity anomaly. On the whole, velocity anisotropy is strong in low-velocity anomalies like Luxi uplift and southern region or the transition zone from high to low velocities near the Maoshan fault zone. Anisotropy is relatively weak under stable block and basin like Subei basin and Hefei basin, which has high velocity anomalies. Pn velocity anisotropy is weak along Tan-Lu fault zone and cannot be detected mainly by Pn wave especially in the middle segment. Strong tectonic movement resulted from the upwelling of asthenosphere hot material along the weak tectonic belt (Tan-Lu) fault weakened the anisotropy trace, which left from early tectonic movement in the lower part of the lithosphere. The fast wave direction near southern segment of Tan-lu is consistent with the fracture extension direction. The Pn velocity lateral variation has a certain correlation with strong earthquake activity. Most of major earthquakes occured on the edges of low-velocity anomalies or the transition zone from high to low velocities. The epicenter of the Tancheng 8.5 earthquake locate on the transition zone from high to low velocities of the middle segment and southern segment, where the lateral variation of velocity is biggest and stress most concentrate easily and produce stress difference.