Abstract:

The Tanlu fault zone(TLFZ)is an ancient intracontinental subduction zone and one of the largest active fault zones in eastern China. The 1668 Tancheng M_S8.5 earthquake occurred within the middle segment of the Tanlu fault zone(MTLFZ), making its deep geophysical structure and the deep seismogenic mechanism of large earthquakes a long-standing focus of research. In this study, high-precision Bouguer gravity data from the MTLFZ and adjacent regions were collected and inverted using the directional derivative method and the Parker-Oldenburg method to determine the distribution of major faults and the Moho depth. The actual crustal thickness was subsequently calculated by combining the Moho depth with digital elevation model(DEM)data, and the isostatic anomaly(Ⅲ)between the calculated crustal thickness and the Airy isostatic crustal thickness was obtained. Based on these results, the deep structural characteristics and deep seismogenic environment of the 1668 Tancheng earthquake were analyzed.

The results show that the Bouguer gravity field can be divided into one fault structural zone and five tectonic blocks, including the MTLFZ(Ⅰ), Luxi Uplift(Ⅱ), Xuhuai Block(Ⅲ), Jiaoliao Block(Ⅳ), Sulu Orogen(Ⅴ), and Lower Yangtze Block(Ⅵ). The MTLFZ is characterized by a NNE-trending high-gravity anomaly zone and a gentle gravity gradient zone, with gradient values generally≤0.6mGal/km. The east-west directional derivative of the gravity field produced the dx map, which reveals nearly NS-, NNE-, and NW-trending faults, including the Sunshidian Fault, Jiaxiang Fault, Tanlu fault zone, Tongzhidian-Sunzu Fault, and Xintai-Mengyin Fault. The north-south directional derivative generated the dy map, which identifies nearly EW-, NEE-, and NWW-trending faults, such as the Fushan Fault, Tiefogou Fault, Wulian-Rizhao Fault, Mengshan Piedmont Fault, and Cangni Fault. The vertical first derivative yielded the dz map, which highlights large-scale fault zones, including the Tanlu fault zone, Haisi Fault, Huaiyin-Xiangshuikou Fault, and Hongze-Goudun Fault. In total, 35 major faults and their intersection relationships were interpreted from the gravity directional derivative field in the study area. Numerous faults on the western side of the MTLFZ intersect the western graben, whereas very few faults intersect the eastern graben, indicating that the eastern graben possesses a relatively rigid structural framework.

The Moho depth gradually decreases from west to east, ranging from 30.6km to 36.0km. The MTLFZ forms a pronounced transition zone, with localized significant uplifts in the Weifang, Tancheng, and Suqian-Jiashan areas. By integrating the Moho depth with topographic elevation data, the actual crustal thickness was calculated and found to gradually increase from east to west. Its spatial distribution pattern is generally consistent with that of the Moho depth, although the amplitude of fluctuations is larger, ranging from 30.9km to 36.7km. The Airy isostatic crustal thickness was calculated to range from 33.99km to 37.08km. According to the gravity isostatic results, the regional crustal isostatic anomaly(Ⅲ) ranges from -1.33km to 3.11km, with relatively small overall variations. In most areas, the anomaly values are close to zero, indicating that the crust is either near isostatic equilibrium or evolving toward a stable state. Pronounced negative III values within the Luxi Block(I<-1.0km in the Tai'an and Jinan areas)suggest ongoing crustal uplift, whereas high positive III values in the eastern coastal region(I>1.1km in the Weifang area and I>1.75km in the Lianyungang area) indicate continued subsidence, driving the crust toward isostatic equilibrium. The isostatic states of the western and eastern grabens of the MTLFZ differ significantly: the western graben is close to isostatic balance, whereas the eastern graben remains in an unbalanced state.

The 1668 Tancheng earthquake occurred within the eastern graben of the MTLFZ, where the relatively rigid structural framework, caused by the scarcity of intersecting faults, favors stress and strain accumulation capable of generating major earthquakes. In addition, the crust beneath the Tancheng area within the eastern graben is characterized not only by a pronounced isostatic imbalance(I>1.1km), but also by significant Moho uplift(depth<33km). These conditions together constitute the deep seismogenic environment responsible for the occurrence of the 1668 Tancheng M_S8.5 earthquake.

Key words: Middle segment of the Tanlu fault zone, Bouguer gravity, gravity isostasy, deep seismogenic environment, 1668 Tancheng earthquake

摘要：

郯庐断裂带是中国大陆东部的一条古老的陆内俯冲带, 也是大型活动断裂带, 其中段曾发生过1668年郯城8.5级地震。文中利用郯庐断裂带中段及周边地区较高精度的布格重力场, 采用方向导数和Parker-Oldenburg界面方法, 得到区域主要断裂几何分布与莫霍面深度, 通过计算实际地壳厚度与Airy理论地壳厚度的均衡差异(I), 进一步分析深部孕震结构以及1668年郯城地震的深部环境。研究表明, 区域重力场可划分为1个构造带和5个块体, 其中郯庐断裂带中段作为走滑为主的构造边界, 表现为一条NNE方向的高异常条带或平缓的重力梯度带, 梯度值最高达0.6mGal/km。通过重力方向导数场解译了35条主要断裂的分布与交切。区域莫霍面深度由西向东逐渐变浅, 在30.6~36.0km之间, 其中郯庐断裂带形成了一条莫霍面陡变带, 且在潍坊、郯城和宿迁—嘉山地区莫霍面局部上隆。通过地壳均衡结果可知, 区域大部分地区处于稳定状态(I趋于0), 而鲁西块体及东部沿海的少部分地区处于地壳不均衡状态, 郯庐断裂带内部出现了东、西分异的现象, 西地堑地壳趋于均衡(I≈0), 而东地堑地壳处于不稳定状态。郯庐断裂带东地堑与之交切的断裂极少, 具备积累应力应变的刚性结构条件, 且东地堑郯城地区的莫霍面出现严重上隆(深度<33km), 加之郯城局部地区的地壳处于不均衡状态(I>1.1km), 1668年郯城地震是在这种深部孕震环境下发生的。

关键词: 郯庐断裂带中段, 布格重力, 重力均衡, 深部孕震环境, 1668年郯城地震