滇东南弧形构造区三维速度结构特征

Characteristics of 3D Velocity Structure in the Arcuate Tectonic Belt of Southeastern Yunnan

利用2008—2021年云南地震台网记录到的M≤5.1地震走时数据,采用V_(P)/V_(S)模型一致性约束的双差层析成像方法,获得了滇东南弧形构造区的地壳三维V_(P)、V_(S)和V_(P)/V_(S)模型及地震重定位结果。结果表明,研究区内发生的2018年8月13日通海M_(S)5.0地震和2021年6月10日双柏M_(S)5.1地震的震源深度主要位于V_(P),V_(S)的高、低速异常的交界处或低速异常体的边缘,及低V_(P)/V_(S)异常体内。这可能意味着这2个地震所处深度范围内的流体或熔融对地震的发生没有直接作用,而是因为震源位置介于脆、韧性岩体之间,当能量积累到一定程度后该位置更易发生脆性破裂。小江断裂、曲江断裂和石屏—建水断裂下方深处存在高V_(P)/V_(S)异常体,结合区内温泉地球化学研究,推测高V_(P)/V_(S)异常体可能是地热流体而非熔融物质,地热流体的存在可能会削弱已表现为强闭锁的小江断裂带与曲江断裂和石屏—建水断裂相交处的闭锁体,从而降低断层破坏的构造应力阈值并加速地震孕育过程,应重点关注地热流体的影响。

This paper used the seismic travel-time data from January 2008 to December 2021 recorded by the Yunnan Seismic Network,and adopted a new TomoDDMC method that can improve the resolution and reliability of the V_(P)/V_(S)model to obtain the three-dimensional crustal V_(P),V_(S)and V_(P)/V_(S)models,as well as the seismic relocation results in southeastern Yunnan.The results show that the 2018 Tonghai M_(S)5.0 earthquake and the 2021 Shuangbai M_(S)5.1 earthquake occurring in the study area are mainly located at the junction of the areas with high-and low-velocity anomalies,or at the edge of the area with low-velocity but low V_(P)/V_(S)anomaly.It is possible that the fluid or melting material within the depth range of these two earthquakes has no direct effect on the occurrence of them,but because they are between brittle and ductile rock mass,brittle fracture easily occurs when the energy is accumulated to a certain level.We also find that there are high V_(P)/V_(S)anomalies under the Xiaojiang fault,the Qujiang fault and the Shiping-Jianshui fault.By the help of the geochemical study of hot springs,we speculate that the high V_(P)/V_(S)anomaly bodies are geothermal fluid rather than molten material.The existence of geothermal fluid may weaken the locking body at the intersection of the Xiaojiang fault,the Qujiang fault and the Shiping-Jianshui fault,which has shown strong locking,so as to reduce the tectonic stress threshold of fault failure.Therefore,the influence of geothermal fluid on the Xiaojiang fault,the Qujiang fault and the Shiping-Jianshui fault deserves special attention.