Gravity anomaly and fine crustal structure in the middle segment of the Tan-Lu fault zone, eastern Chinese mainland

Abstract The middle segment of the Tan-Lu fault zone (MTLFZ) is a large-scale active fault zone in the eastern Chinese mainland, and the 1668 Tancheng M8.5 earthquake occurred in its eastern graben. In this study, wavelet-based multi-scale analysis, Moho inversion, and gravity profile modeling are utilized to invert the high-precision Bouguer gravity anomaly of the MTLFZ and adjacent areas, and the fine crustal structure and deformation features are analyzed to understand the deep seismogenic mechanism. The results indicate that the regional gravity field can be subdivided into five blocks. The MTLFZ, as the tectonic boundary, is a NNE-trending gentle gravity gradient zone or a high anomaly zone with the highest gradient value of 0.6 mGal/km. In plane, the sedimentary layer to the upper crust of the Tan-Lu fault zone (TLFZ) shows a gravity pattern of “two grabens and one horst” on the north of Suqian, and the middle-lower crust is characterized by a long and narrow intermittent low anomaly zone, while the uppermost mantle is displays an accumulation of high anomaly materials. In profile, the TLFZ cuts through the Moho interface to the upper mantle and serves as a channel for upwelling of high density materials into the lower crust from mantle, where density value of lower crust is nearly 3.0 g/cm3, which is significantly higher than both the sides. The regional Moho depth increases from 30.8 km in the east to 36.0 km in the west, and an abrupt change is observed along the TLFZ, where the Moho interface is severely uplifted in the Juxian-Tancheng and Suqian-Jiashan areas. The gravity results reveal several NW or NWW active faults on the west side of TLFZ that intersect with it, which decreases the stress of the western graben. In contrast, the eastern graben is relatively rigid due to the lack of intersecting active faults. Moreover, the deep fault of the TLFZ displays a similar declination as the shallow branch faults of the eastern graben with little distortion, so the stress acts more easily and the hot mantle materials upwell more efficiently to induce major earthquakes in the eastern graben. It is interpreted that the 1668 Tancheng earthquake occurred through this mechanism: the Moho interface is strongly uplifted in the Tancheng area where the mantle material upwelled into the crust along the TLFZ, and the strain continued to accumulate due to crustal structure features of the eastern graben.