Abstract:

The Tan-Lu fault zone(TLFZ)is a major tectonic structure in eastern China, characterized by significant seismic activity and a complex geological setting. Understanding the spatial distribution of seismic activity and its correlation with crustal velocity structures is crucial for assessing seismic hazards and improving our knowledge of the underlying mechanisms. This study aims to provide a detailed analysis of the high-resolution crustal velocity structure and seismic activity in the middle and southern segments of the TLFZ, focusing on identifying the key factors controlling spatial variations in seismic activity.
We collected Pg and Sg arrival time data from 2009 to 2021, covering a broad region from 115°E to 122°E and from 30°N to 38°N. A total of 16,525 earthquakes were selected based on strict criteria, including a minimum of 3 recording stations per event and a maximum focal depth of 20km, to ensure high-quality data from 332 fixed seismic stations. We employed a two-dimensional tomographic imaging method with a grid size of 0.25°×0.25° to generate high-resolution velocity models for the upper crust. The method incorporates station and event corrections to minimize travel-time residuals and uses a damped least-squares inversion technique with Laplacian smoothing to stabilize the solution. The resulting velocity models are validated through resolution tests and residual analysis, demonstrating the robustness of the imaging results.
The tomographic results reveal distinct high- and low-velocity anomalies in the upper crust, which are closely correlated with regional geological structures. Based on the velocity structure and seismic activity, the TLFZ can be divided into four sub-segments: Bohai Bay-Wulian, Wulian-Tancheng, Tancheng-Jiashan, and Jiashan-Guangji. The Wulian-Tancheng segment exhibits significantly high-velocity anomalies and is associated with the highest seismic activity in the region. This segment is characterized by a small curvature and a near-vertical fault geometry, which facilitate the accumulation of elastic strain energy and promote seismic activity. Additionally, the high-velocity anomalies form a sharp velocity gradient with the underlying low-velocity zones, creating favorable conditions for strong earthquakes. The spatial distribution of seismic activity is closely related to the crustal velocity structure, with high-velocity anomalies favoring seismic events. The results suggest that seismic activity in the TLFZ is influenced by several factors, including the physical properties of the medium, external stress fields, fault geometry, and crustal fluid systems.
This study demonstrates that the spatial differences in seismic activity within the middle and southern segments of the TLFZ are controlled by the interplay of crustal velocity structure, fault geometry, and crustal fluid systems. High-resolution tomographic imaging provides valuable insights into the mechanisms underlying seismic activity in this region. The findings highlight the importance of considering multiple factors when studying seismogenic mechanisms and suggest that future research should integrate various methods and data to better understand the complex interactions between crustal structures and seismic activity. Besides, it not only contributes to understanding the TLFZ's seismic behavior but also has implications for seismic hazard assessment and mitigation in the region. The detailed analysis of the velocity structure and its correlation with seismic activity provides a foundation for further exploration of the deep processes that influence seismicity and can enhance understanding of seismogenic mechanisms in complex tectonic settings.

Key words: the middle and southern segments of the TLFZ, the upper crust, tomographic imaging, differences in seismic activity

摘要：

郯庐断裂带作为中国东部地震活动频繁的区域, 近年来受到了广泛关注。为深入理解郯庐断裂带中南段及邻区地震活动的空间分布特征, 文中收集了2009—2021年的Pg和Sg走时数据, 使用二维层析成像方法对该区域开展了0.25°×0.25° 的高分辨率层析成像, 得到研究区上地壳Pg、 Sg波的速度及波速比结构。研究发现: 郯庐断裂带中南段及邻区Pg波、 Sg波高、 低速异常与区域地质构造单元之间呈现出较强的相关性, 且速度结构的横向非均匀性反映出华东地区典型的盆山耦合构造特征。此外, 基于成像结果并结合地震活动空间分布特征, 可将郯庐断裂带中南段细分为渤海湾—五莲、 五莲—郯城、 郯城—嘉山及嘉山—广济4段。研究发现, 区域强震活动与高速异常存在明显的相关性, 其中五莲—郯城段呈现出显著的高速异常特征, 此区域同时也是郯庐断裂带中南段地震活动性最强的区间, 分析认为这种高速异常为地震孕育提供了有利的构造条件: 一方面, 五莲—郯城段具有小曲率、 近垂直的断层结构, 这可使少量的逆冲分量积累更多弹性应变能, 导致介质更容易处于闭锁状态; 另一方面, 高速介质在流体系统作用下更易发生破裂, 促进了中上地壳中小地震频发, 并与深部低速区构成速度梯度异常带, 导致深部易发生强震活动。综合而言, 郯庐断裂带中南段地震活动的空间差异性是受介质物理属性、 外部应力作用、 断层几何结构和地壳流体系统等因素共同作用的结果。

关键词: 郯庐断裂带中南段, 上地壳, 层析成像, 地震活动性差异