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The Tan-Lu fault zone (TLFZ) is one of the most important active faults in the eastern Chinese mainland, and the seismic activity of the Su-Lu segment is most prominent. The fault structures of the Su-Lu section are very complex and intersect with many other NW-striking and NE-striking fault zones, resulting in the extreme complexity of the crustal structure in this area. In this study, Bouguer gravity and aeromagnetic data are used to invert the crustal structures surrounding the Su-Lu segment and to construct a three-dimensional model. Our results indicate that the TLFZ presents obvious high anomalies in gravity and aeromagnetic fields, cuts through the Moho interface and acts as an upwelling passage for high-density materials from the uppermost mantle and asthenosphere. The gravity and aeromagnetic anomalies are greatly influenced by the crustal structures. Both the Moho and Curie interfaces are characterized by substantial undulations. The NW-striking and NE-striking faults intersect the TLFZ with different relations—for example, the Wuxi-Suqian fault and Cangshan-Nishan fault cut off the TLFZ with distinct gravity and aeromagnetic anomalies, and the Xintai-Mengyin fault and Mengshan piedmont fault cut off the western part of the TLFZ. In addition, most faults cut to the middle crust, and the lower crust is ductilely deformed because of magmatic underplating, reforming, remelting and thinning of the eastern North China block. Most earthquakes occur in the northern part of the Su-Lu segment, where magmatic activity is intense. [ABSTRACT FROM AUTHOR]

Areas with sharp spatial changes in crustal structure are usually considered prone to strong earthquakes, especially late Cenozoic structural zones. However, quantitative relationships have not been established between the occurrence of earthquakes and the extent to which the crustal structure changes. The crustal structure can be reflected by gravity anomalies. In this study, we investigate the crustal structure variation in the eastern Tibetan Plateau based on the gravity data acquired by the Gravity field and steady‐state Ocean Circulation Explorer satellite. The multiscales wavelet decomposition and power spectrum methods are used to analyze the spatial variation of crustal structure at different depths. Based on a fourth‐order wavelet map, four variable factors are constructed to represent the spatial variations of the crustal structure, and their statistical relationship with earthquakes is studied. The results show that the changing rate of the gravity anomalies is significantly positively correlated with the occurrence of earthquakes. Areas of high earthquake hazards are places where the gravity field changes dramatically, usually with a value of 11°–18° for the SLP (slope) index. The assessment efficiency of the SLP increases with the earthquake magnitude. According to the SLP index, several regions are assessed to have a high level of risk for strong earthquakes in the eastern Tibetan Plateau, including the Longmenshan Fault belt, where a Ms8.0 earthquake occurred in 2008, thus demonstrating the promising implications of our method in the earthquake susceptibility assessment. Plain Language Summary: The distribution of earthquake epicenters is uneven in space. At present, it is generally believed that locations with crustal material changes are prone to earthquakes. However, not all earthquakes are distributed in these locations. To answer the questions of how many earthquakes are distributed in these locations and whether these characteristics can be used to predict the locations of earthquakes, we construct some factors based on gravity data and active faults and conduct statistical analyses in this paper. The results show that the greater is the variation in the gravity field, the higher is the frequency of earthquakes. The assessment efficiency of these characteristics increases with the earthquake magnitude. Key Points: Based on the fourth‐order wavelet detail map of gravity, we constructed four quantitative indexes to express the spatial changes in the crustEarthquakes are concentrated in the high‐value region of the slope (SLP) factor, approximately 11°‐18°The earthquake risk assessment efficiency of the SLP increases with the earthquake magnitude [ABSTRACT FROM AUTHOR]

[Display omitted] • Bouguer gravity data is used for multi-scale field inversion and profile modeling. • The lower crust underneath the Tan-Lu fault zone has relatively high density. • The Moho depth underneath the fault zone shows an abrupt change and local uplift. • The crustal structures in the West and East graben of the fault zone are different. • The 1668 Tancheng earthquake was caused by Moho uplift and crustal fault structure. 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 M 8.5 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. [ABSTRACT FROM AUTHOR]

• Potential field inversion and 3-D visualization are revealled from Bouguer gravity data. • There is a low-density body in the lower crust of the Longmen Shan fault zone. • The density structure of the fault zone is complex and the Moho depth is an abrupt change. • A counter-clockwise rotation of the Sichuan Basin is shown by source edge detection. • The rotation is one of the important dynamic backgrounds for the 2008 Wenchuan earthquake. In this paper, we adopt the methods of Euler deconvolution, source parameter imaging (SPI), source edge detection (SED), inversion of the Moho interface, and three-dimensional (3-D) visualization of the crust for the inversion of the Bouguer gravity anomaly of the Longmen Shan and adjacent regions, analyse the 3-D structure, deep structure, and deformation characteristics of the crust, and investigate the cause of the deep seismogenic environment and the Wenchuan earthquake. The research indicates that the Longmen Shan fault zone is characterized by a NE-trending large gravity gradient zone with a gradient value that exceeds 3.7 mGal/km. The Euler diagram reveals that there is a low-density anomaly body in the lower crust of the Longmen Shan fault zone that is a NE-trending narrow strip approximately 100 km long, 65 km wide and 35–50 km deep. The Songpan-Garze block on the northwest side does not have the material source and pathway of the lower crustal flow in the Euler diagram. Therefore, the low-density anomaly body is related to the upwelling of the mantle heat flow below it. In comparison with the Songpan-Garze and Chuan-Dian blocks, the strength of the field source of the Sichuan Basin is high (−175 to −96 mGal), indicating that the average density of the crust is high, and the depth of the field source is stable at 15–25 km. The SED map reveals that there has been counter-clockwise rotation of the Sichuan Basin since the Cenozoic. The SPI shows that the Longmen Shan fault zone is narrowly located in the beaded linear "shallow source anomaly" belt and that its two sides are stable areas of deep source anomalies, indicating characteristics of crustal hardness on both sides, so that it is prone to accumulate stress and strain. The Moho depth across the Longmen Shan descends from 41.8 km in the east to 61.6 km in the west. The complex intersection of high-density and low-density anomaly bodies appears in the deep part of the Longmen Shan tectonic belt, and the crustal structure is fragmented. Moreover, there is a strong abrupt change in the Moho topography, and therefore, this region has a deep environment for the genesis of strong earthquakes. The dextral strike-slip movement of the Longmen Shan fault zone is caused by the movement of the Songpan-Garze block and the counter-clockwise rotation of the Sichuan Basin; therefore, the counter-clockwise rotation of the Sichuan Basin is one of the important dynamic backgrounds for the 2008 Wenchuan earthquake. [ABSTRACT FROM AUTHOR]