Crustal thickness and Poisson's ratio beneath the middle-southern segment of Tan-Lu fault zone

The middle-southern segment of the Tan-Lu fault zone (TLFZ) is related to the subduction of the west Pacific plate and the destruction of the North China Craton. The study of deep structure in the TLFZ is of great significance for understanding the tectonic evolution of the TLFZ and the seismogenic mechanism of strong earthquakes. In this study, we use data from dense seismic arrays to estimate the crustal thickness and Poisson's ratio beneath the middle-southern segment of the TLFZ by H-kappa stacking method. The results show that the distribution of crustal thickness can be divided into three segments with Lujiang and Xinyi as the subsection points. The segmentation characteristics in the north-south direction are consistent with that of the geological structure. In the southern segment, the crustal thickness on the west side of the TLFZ is about 5 km greater than that on the east side. In the middle segment, the crustal thickness difference between the two sides of the TLFZ decreases to about 3 km. In the northern segment, the crustal thickness is characterized by "thick -thin-thick" and the Moho interface is uplifted beneath the TLFZ. The high Poisson's ratio beneath the TLFZ is presumed to be the result of thermal and chemical erosion caused by mantle material. According to the Airy isostasy theory, we concluded that: (1) In the southern segment, the crustal thickness is related to the elevation, which largely conforms to the Airy isostasy model. (2) In the middle segment, the elevations on both sides of the TLFZ are close but the crustal thicknesses are quite different, which indicates that the deep structure does not conform to the Airy isostasy model, which may be related to the higher density lithosphere of the North China Craton on the west side of the TLFZ than that of the Yangtze Craton on the east side, for high -density lithosphere can produce a downward "drag force" which leads to a deeper Moho interface than that by thoery. (3) In the northern segment, the Moho interface is deeper than that based on the Airy isostasy model. The deep structure of this area is similar to that of the west side of TLFZ in the middle segment, which belongs to the North China Craton. Within the rupture zone of the M8. 5 Tancheng earthquake, there are three types of structural features: uneven transition of Moho interface, the boundary of active blocks, bifurcation and intersection of faults. These structural features are beneficial for stress concentration, thus leading to the occurrence of the M8. 5 Tancheng earthquake in 1668.