Abstract: In recent years, strong earthquakes of M_S8.0 Wenchuan and M_S7.0 Lushan occurred in the central-southern part of Longmenshan fault zone. The distance between the two earthquakes is less than 80 kilometers. So if we can obtain the inner structure of the crust and upper mantle, it will benefit us to understand the mechanism of the two earthquakes. Based on the high resolution dataset of Bouguer gravity anomaly data and the initial model constrained by three-dimensional tomography results of P-wave velocity in Sichuan-Yunnan region, with the help of the preconditioned conjugate gradient(PCG)inversion method, we established the three dimensional density structure model of the crust and upper mantle of the central-southern segment of Longmenshan, the spatial interval of which is 10 kilometers along the horizontal direction and 5 kilometers along the depth which is limited to 0~65km, respectively. This model also provides a new geophysical model for studying the crustal structure of western Sichuan plateau and Sichuan Basin. The results show obvious differences in the crustal density structure on both sides(Songpan-Ganzê block and Sichuan Basin)of Longmenshan fault zone which is a boundary fault and controls the inner crustal structure. In Sichuan Basin, the sedimentary layer is represented as low density structure which is about 10km thick. In contrast, the upper crust of Songpan-Ganzê block shows a thinner sedimentary layer and higher density structure where bedrock is exposed. Furthermore, there is a wide scale low density layer in the middle crust of the Songpan-Ganzê block. Based on this, we inferred that the medium intensity of the Songpan-Ganzê block is significantly lower than that of Sichuan Basin. As a result, the eastward movement of material of the Qinghai-Tibet plateau, blocked by the Sichuan Basin, is inevitably impacted, resulting in compressional deformation and uplift, forming the Longmenshan thrust-nappe tectonic belt at the same time. The result also presents that the crustal structure has a distinct segmental feature along the Longmenshan fault zone, which is characterized by obviously discontinuous changes in crustal density. Moreover, a lot of high- and low-density structures appear around the epicenters of Wenchuan and Lushan earthquakes. Combining with the projection of the precise locating earthquake results, it is found that Longmenshan fault zone in the upper crust shows obvious segmentation, both Wenchuan and Lushan earthquake occurred in the high density side of the density gradient zone. Wenchuan earthquake and its aftershocks are mainly distributed in the west of central Longmenshan fault zone. In the south of Maoxian-Beichuan, its aftershocks occurred in high density area and the majority of them are thrust earthquake. In the north of Maoxian-Beichuan, its aftershocks occurred in the low density area and the majority of them are strike-slip earthquake. The Lushan earthquake and its aftershocks are concentrated near the gradient zone of crustal density and tend to the side of the high density zone. The aftershocks of Lushan earthquake ended at the edge of low-density zone which is in EW direction in the north Baoxing. The leading edge of Sichuan Basin, which has high density in the lower crust, expands toward the Qinghai-Tibet Plateau with the increase of depth, and is close to the west of the Longmenshan fault zone at the top of upper mantle. Our results show that there are a lot of low density bodies in the middle and lower crust of Songpan-Ganzê Block. With the increase of the depth, the low density bodies are moving to the south and its direction changed. This phenomenon shows that the depth and surface structure of Songpan-Ganzê Block are not consistent, suggesting that the crust and upper mantle are decoupled. Although a certain scale of low-density bodies are distributed in the middle and lower crust of Songpan-Ganzê, their connectivity is poor. There are some low-density anomalies in the floor plan. It is hard to give clear evidence to prove whether the lower crust flow exists.

Key words: 3-D density structure, preconditioned conjugate gradient (PCG) method, central and southern part of Longmenshan, Wenchuan earthquake, Lushan earthquake

摘要： 基于高精度布格重力异常资料，以川滇地区P波速度三维层析成像结果为约束建立初始模型，采用预优共轭梯度（Preconditional Conjugate Gradiem，PCG）反演方法得到了龙门山断裂带中南段的地壳上地幔（深度范围0~65km）三维密度结构（网格间距为10km （横向）×10km （纵向）×5km （深度））。密度成像结果表明：龙门山断裂带中南段两侧地壳密度结构存在明显差异，四川盆地有约10km厚的低密度沉积层，松潘-甘孜块体因沉积层较薄，且部分地区有基岩出露，上地壳表现为高密度结构；松潘-甘孜块体中、下地壳有大范围低密度层分布，介质强度明显低于高密度的四川盆地，青藏高原东移物质受到四川盆地阻挡后更易于在低密度的一侧发生挤压形变及隆升，从而形成龙门山逆冲推覆构造带；龙门山断裂带内部在地壳结构上具有明显的分段特征，表现为沿着龙门山断裂带地壳密度变化不连续，以汶川地震和芦山地震震中为界，形成多个高、低密度异常区；同时，结合地震精定位结果分析，汶川地震及其余震多分布于壳内中央断裂带西侧高密度体内，芦山地震及其余震则集中在地壳密度变化梯级带附近并偏向高密度体一侧。四川盆地下地壳密度较高，其前缘随深度增加向青藏高原方向扩展，在上地幔顶部接近龙门山断裂带以西。松潘-甘孜块体中、下地壳虽然有一定规模的低密度体分布，但其连通性差，在平面上多形成局部低密度异常区，是否存在下地壳流仍无法给出明确的证据。

关键词: 三维密度结构, 预优共轭梯度, 龙门山中南段, 汶川地震, 芦山地震