2021年11月17日江苏大丰海域M5.0地震震源区三维P波速度结构

The Three-dimensional P-wave Velocity Structure across the Source Region of Dafeng Sea Area M5.0 Earthquake,Jiangsu,on 17 November 2021

使用近20年固定台站记录到的初至P波到时数据,利用双差地震层析成像方法反演了2021年11月17日江苏大丰海域M5.0地震震源区及邻区三维P波速度结构和震源位置参数。结合该地区的构造背景,研究了该地震孕育与发生的深部构造背景及速度结构与地震活动性之间的关系。结果表明:研究区P波速度结构存在明显的横向非均匀性,浅层的P波速度结构与地表地质构造及地形有较好的对应性,在造山带和隆起区表现为相对高速,盆地地区表现为相对低速。研究区地震活动性与速度结构存在明显的相关性,大丰海域地震和经过重定位后的中小地震均发生在P波速度梯度带上或者偏向高速一侧。综合速度结构水平和垂直剖面,震源机制解,震源的空间位置分析认为此次地震可能为右旋走滑型地震,发震断裂可能为苏北-滨海断裂,也可能与小海-陈家堡断裂及震源区其他的隐伏断裂有关。此次地震发生可能与震源区附近断裂交汇处应力积累,遇到高速的阻碍体,阻碍体发生破裂有关。研究区西部新生代印度-欧亚板块的碰撞挤压和东部菲律宾海板块和太平洋板块对欧亚大陆的俯冲可能是这一地震的外部地球动力学来源,而江苏陆区及近海大陆架因地形高程差异引起的重力势则可能是此次地震的内部驱动力。

Based on the P-wave travel time data over more than twenty years in the Jiangsu-South Yellow Sea area,the double-difference seismic tomography method was applied to obtain the three-dimensional P-wave velocity structure and source location parameters.The deep seismogenic environment of the Dafeng M5.0 earthquake,which occurred on 17 November 2021,was investigated along with the relationship between the velocity structure and seismicity,taking into account the geological tectonic settings.The results show that there is significant lateral heterogeneity of P-wave velocity in the study area.The near-surface P-wave velocity structure correlates well with the surface geological structure,and the orogenic belt and uplift zone exhibit comparatively high velocity,while the basin area shows comparatively low velocity.There is a significant correlation between seismicity and velocity structure in the study area.The Dafeng M5.0 earthquake and the relocated small earthquakes mainly occurred in the P-wave velocity gradient zone or on the high VP side.By combining the horizontal and vertical velocity profiles,the seismic source mechanism,and the spatial location of the earthquake,it is suggested that the Dafeng earthquake may be a dextral strike-slip earthquake,and the Subei-Binhai fault may be the seismogenic fault.However,it cannot be ruled out that the Xiaohai-Chenjiabao fault and other hidden faults in the source area are seismogenic faults.The occurrence of the Dafeng earthquake may be related to the broken of the high-velocity obstacle caused by long-time stress accumulation.The collision of the India-Eurasia plate during the Cenozoic era on the Western side of the study area and the subduction of the Philippine Sea Plate and the Pacific Plate over the Eurasia plate on the eastern side may be the external geodynamics source of this earthquake.Meanwhile,the gravity potential caused by the terrain elevation difference in the continental area and offshore continental shelves in Jiangsu province may be a non-negligible internal driving force of the earthquake.