地震地质 ›› 2024, Vol. 46 ›› Issue (1): 162-183.DOI: 10.3969/j.issn.0253-4967.2024.01.010

• 研究论文 • 上一篇    下一篇

云南洱源地区地壳三维精细速度结构成像

曹颖1)(), 钱佳威2,3), 黄江培1), 周青云1),*()   

  1. 1) 云南省地震局, 昆明 650224
    2) 河海大学, 海洋学院, 南京 210098
    3) 中国科学技术大学, 地球和空间科学学院, 合肥 230026
  • 收稿日期:2023-09-27 修回日期:2023-12-15 出版日期:2024-02-20 发布日期:2024-03-22
  • 通讯作者: *周青云, 男, 1986年生, 高级工程师, 主要从事活动构造、 地震学和震害防御研究, E-mail: zhouqingyun@pku.edu.cn
  • 作者简介:

    曹颖, 女, 1988年生, 高级工程师, 主要从事地震监测工作和地震层析成像研究, E-mail:

  • 基金资助:
    国家重点研发计划项目(2021YFC3000600); 云南省重点研发计划项目(202203AC100003); 国家自然科学基金(42304056)

CRUSTAL FINE VELOCITY STRUCTURE IN THE ERYUAN AREA, YUNNAN FROM DOUBLE-DIFFERENT TOMOGRAPHY

CAO Ying1)(), QIAN Jia-wei2,3), HUANG Jiang-pei1), ZHOU Qing-yun1),*()   

  1. 1) Yunnan Earthquake Agency, Kunming 650224, China
    2) College of Oceanography, Hohai University, Nanjing 210098, China
    3) School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
  • Received:2023-09-27 Revised:2023-12-15 Online:2024-02-20 Published:2024-03-22

摘要:

云南洱源地区地处青藏高原东南缘的滇西北地区中部, 区内地质构造复杂, 多条断裂交会穿过, 且地热活动活跃, 显示出很强的断裂构造特征。地区内地震活动频繁, 2013年以来在维西-乔后-巍山断裂西侧发生了多个5级以上地震。文中利用云南区域固定台网和滇西北密集台阵记录的2008年1月1日—2023年7月20日发生在洱源地区的地震走时数据, 采用波速比模型一致性约束的双差层析成像方法, 获得了云南洱源及其周边区域的地壳三维VPVSVP/VS模型及地震重定位结果。结果表明: 1)在龙蟠-乔后断裂以东, 维西-乔后-巍山断裂、 红河断裂和鹤庆-洱源断裂交会处聚集了大量小地震, 缺乏中强地震。从现有的层析成像结果分析认为浅层发生的部分小地震可能与地热流体无直接关系, 而从浅层向深层逐渐变高的VP/VS值可能暗示深部存在流体, 深部流体可能在循环流动过程中逐渐渗透到浅层岩石中, 并与部分密集小地震的发生有关。2)2013年以来发生的4个5级以上地震均发生在维西-乔后-巍山断裂西侧, 并呈NNW-SSE走向分布, 表明川滇块体西边界断裂系统的地震危险性有所增大。3)2013年3月3日和4月17日发生的洱源地震序列主要位于低VP、 低VS和低VP/VS值的异常体内。一般而言, 如存在流体, 则VSVP下降得更快, 从而导致高VP/VS值, 而低VP/VS则表明低VS并非由流体所致。因此, 现有的成像证据表明地震序列所处区域并不存在流体, 从而推断地震序列的发生与流体无直接关系。空间上与洱源地震序列接近的2017年3月27日 MS5.1 漾濞地震序列也具有相同的速度结构特征, 因此也可能与流体无直接联系。2016年5月18日发生的 MS5.1 云龙地震序列的主震和部分余震主要位于高VP、 高VSVP/VS相对高值区内, 高VS表明并非因存在流体而导致出现相对高的VP/VS值, 据此推测主震及周围的余震所处区域可能不存在流体, 流体并未直接参与到地震序列的发生过程中。

关键词: 洱源地区, 双差层析成像, 三维速度结构, VP/VS

Abstract:

The Eryuan area is located in the central part of the northwest Yunnan region on the southeastern edge of the Qinghai-Xizang Plateau. The geological structure in the area is complex, including the Weixi-Qiaohou-Weishan Fault and Honghe Fault, as well as the Longpan-Qiaohou Fault and Heqing-Eryuan Fault, which intersect in an “X” shape. The geothermal activity in the area is also active and exhibits strong fault structural characteristics. Earthquake activity is also frequent in the Eryuan area. Since 2013, multiple earthquakes with MS≥5.0 have occurred on the west side of the Weixi-Qiaohou-Weishan Fault. Given the complex geological and geothermal background and seismic activity in the Eryuan area, we use the seismic travel time data recorded by the Yunnan Regional Seismic Network and the Northwest Yunnan Dense Array from January 1, 2008, to July 20, 2023, VP/VS model consistency-constrained DD tomography method to obtain the 3D VP, VS, and VP/VS models and relocation results in the Eryuan and its surrounding areas. The research results indicate that: 1)based on the relocation results, the dense small seismic cluster develops at the intersection of the Weixi-Qiaohou-Weishan Fault, Honghe Fault, and Heqing-eryuan Fault deserves special attention. Four earthquakes with MS≥5.0 that have occurred since 2013 are mainly distributed on the west side of the Weixi-Qiaohou-Weishan Fault, with an NNW-SSE trend distribution. The fault system at the western boundary of the Sichuan-Yunnan block is very complex, and its seismic risk deserves our special attention. 2)Based on the tomography results, the widely distributed low-velocity anomalies in the upper crust of the study area may be related to the crustal material migration pathway caused by the escape of the Sichuan-Yunnan diamond block towards SSE. 3)Combining imaging results with geochemical research results for inference, the high VP/VS below 10km depth beneath the small seismic cluster at the intersection of the Weixi-Qiaohou-Weishan Fault, Honghe Fault, and Heqing-Eryuan Fault may correspond to hot spring geothermal fluids. Moreover, the circulation process of hot spring fluids in complex fault systems may have penetrated to a depth of 7~10km below the seismic cluster, accompanied by some small earthquakes. However, there is no imaging evidence of fluid within a depth of 5~7km where some earthquakes are densely distributed. The occurrence of these earthquakes may be related to the distribution of brittle rocks, but it cannot be ruled out that fluid may have a potential infiltration effect on them in the future. Based on the results of velocity structure, they are combined with pre-existing seismology research results. It is found that the Eryuan earthquake sequences on March 3 and April 17, 2013 were mainly located within low VP, low VS, and low VP/VS anomaly. Generally, if there is fluid present, VS decreases faster than VP, resulting in high VP/VS. So low VP/VS indicates that low VS is not caused by fluids, which may be due to lithology. Therefore, the imaging evidence indicates that there is no fluid in the region where the sequence is located, thus inferring that the occurrence of the sequence is not directly related to the fluid. The MS5.1 Yangbi earthquake sequence on March 27, 2017, is spatially close to the Eryuan sequence and also has the same velocity structure characteristics, so it may not be directly related to fluids. The mainshock and some aftershocks of the MS5.1 Yunlong earthquake sequence on May 18, 2016, were mainly located in high VP, high VS, and relatively high VP/VS regions. High VS indicates that it is not the fluid that causes relatively high VP/VS. It is speculated that there may not be fluid in the region where the mainshock is located, and the fluid did not directly participate in the occurrence process of the sequence.

Key words: Eryuan area, double-different tomography, 3D velocity structure, VP/VS