Abstract:

The Shanghai region is located south of the Yangtze River estuary, at the eastern edge of the Lower Yangtze Platform formed during the late Proterozoic Jinning cycle. The East China region is currently in the late stage of the second phase of seismic activity response, with heightened seismic activity expected to continue. The study area is characterized by numerous faults, including significant structures like the Xiaoshan-Qiuchuan fault zone(Shanghai segment)and many smaller NE- and NW-trending shallow fault zones, which are intertwined and cross each other horizontally and vertically. Background noise tomography technology relies on seismic background noise data to compute the corresponding Green's functions through cross-correlation, using the results to image phase velocities and surface waves. This method is widely applied due to its high resolution, low cost, and independence from seismic source data.

This study collected continuous vertical background noise data from 58 broadband mobile seismic stations in Shanghai and its neighboring areas over a year. Using background noise cross-correlation technology combined with a fast marching method(FMM)for imaging, we obtained the phase velocity structure of surface waves in the crust and upper mantle for 5s to 30s. Finally, using a least-squares linear inversion method, we derived a high-resolution 3D S-wave velocity structure from 5km to 30km below the study area.

The results indicate that the Shanghai area's S-wave velocity structure exhibits specific lateral and vertical heterogeneity. The distribution of velocity anomalies in the shallow layer and upper crust at depths of 5km to 10km shows a correlation with local topography and the distribution of major faults, notably influenced by the Huzhou-Suzhou Fault. In the middle to lower crust at depths of 15km to 20km, velocity anomalies are demarcated by the Fengjing-Chuansha area, where velocities gradually increase from northern Jiangsu to Hangzhou Bay, with significant structural uplift along the Fengjing-Chuansha fault in Shanghai. At depths of 25km to 30km, the distribution of velocity anomalies reflects a “west-deep, east-shallow” burial state in the lower crust and upper mantle.

From the analysis of three longitudinal profiles(AA', BB', CC'), we draw the following conclusions:

(1)The underground structure of Shanghai and its vicinity shows a gradual change, with overall crustal thickness decreasing from west to east, characterized by a “thick west, thin east” profile and a topography of “low west, high east with a central uplift.” The shallow S-wave velocity significantly correlates with local topography; the velocity values in the middle to upper crust are predominantly controlled by major faults, especially the Huzhou-Suzhou fault. The middle to lower crust beneath Shanghai exhibits an uplifted structural form, with a “low west, high east” fluctuation in the lower crust and upper mantle.

(2)The velocity anomalies and seismic activity are closely related to fault zones, with low-speed anomalies accompanying major faults. The distribution of earthquakes tends to cluster near low-speed bodies and fault zones, notably at the prominent uplift junctions beneath Taicang and Qingpu. The area's predominant occurrence of shallow-source earthquakes is likely due to stress imbalance between the fractured middle to the upper crust and the stable lower crust.

(3)Low-speed anomalies have been consistently observed at depths of 5km to 25km in the Changshu region. Based on related literature and imaging results from this study, we hypothesize the potential presence of thermal material upwelling in the Changshu area.

(4)The longitudinal profile results indicate that the thickness of the upper crust in the study area is approximately 12km, the middle crust around 10km, and the lower crust about 8km. Distinct layering within the crust is evident, with a gradual change in the middle to lower crust, while the upper crust shows significant heterogeneity. The influence of the faulting in the area controls the distribution of low-speed anomalies.

Key words: background noise imaging, cross-correlation, Shanghai and adjacent areas, S-wave velocity structure

摘要：

文中收集了上海及邻区58个宽频带流动地震台阵1a的连续背景噪声垂向数据, 基于背景噪声互相关技术结合快速行进FMM成像方法, 获得了上海及邻区5~30s周期范围内地壳及上地幔顶部的面波相速度结构。最后利用最小二乘线性迭代方法, 反演得到了研究区下方5~30km深度的三维S波速度结构。结果显示: 上海及邻区的S波速度结构呈现出一定的横向和纵向非均匀性特点。在5~10km深度的浅层及上地壳, 速度异常值的分布与域内地形地貌及大型断裂的展布存在一定相关性, 低速异常分布受湖-苏断裂的影响较大。在15~20km深度的中下地壳, 速度异常以枫泾-川沙断裂为界, 由苏北至杭州湾速度逐渐升高, 上海地区沿枫泾-川沙断裂表现出明显的构造抬升形态。在25~30km深度范围内, 速度异常分布反映下地壳及上地幔顶部呈现西深东浅的埋深状态。综合AA'、 BB'、 CC' 3条纵剖面的结果, 可得到以下结论: 1)上海及邻区地下结构变化平缓, 地壳整体西厚东薄, 起伏呈西低东高、 中间隆起的形态; 2)速度异常及地震活动性与断裂带关系密切, 低速异常与大型断裂伴生出现。地震分布具有依附低速体及在断裂带附近、 沿速度分界线排列的特点。结合相关结果分析认为, 域内以浅源地震为主可能是被断裂切割较破碎的中上地壳与稳定的下地壳之间的应力不均衡所致; 3)常熟地区5~25km深度内的低速异常一直存在, 推测常熟一带存在热物质上涌现象。

关键词: 背景噪声成像, 互相关, 上海及邻区, S波速度结构