S-WAVE VELOCITY STRUCTURE IN SHANGHAI AND ADJACENT AREAS BASED ON AMBIENT NOISE TOMOGRAPHY

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 seismie activity expected to continue. The study area is eharaeterized 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 eompute the corresponding Green's functions through cross-correlation, using the results to image phase veloeities 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 veloeities 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, eharaeterized 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 loeal topography; the veloeity values in the middle to Upper crust are predominantly controlled by major faults, espeeially 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 influenae of the faulting in the area controls the distribution of low-speed anomalies. © 2024 State Seismology Administration. All rights reserved.