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

doi:10.3969/j.issn.0253-4967.2024.01.010

Abstract

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 M_S≥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, V_P/V_S model consistency-constrained DD tomography method to obtain the 3D V_P, V_S, and V_P/V_S 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 M_S≥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 V_P/V_S 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 V_P, low V_S, and low V_P/V_S anomaly. Generally, if there is fluid present, V_S decreases faster than V_P, resulting in high V_P/V_S. So low V_P/V_S indicates that low V_S 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 M_S5.1 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 M_S5.1 Yunlong earthquake sequence on May 18, 2016, were mainly located in high V_P, high V_S, and relatively high V_P/V_S regions. High V_S indicates that it is not the fluid that causes relatively high V_P/V_S. 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, V_P/V_S

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V_P、 V_S和V_P/V_S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V_P/V_S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V_P、低V_S和低V_P/V_S值的异常体内。一般而言, 如存在流体, 则V_S比V_P下降得更快, 从而导致高V_P/V_S值, 而低V_P/V_S则表明低V_S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 M_S5.1 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 M_S5.1 云龙地震序列的主震和部分余震主要位于高V_P、高V_S和V_P/V_S相对高值区内, 高V_S表明并非因存在流体而导致出现相对高的V_P/V_S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, V_P/V_S

CAO Ying, QIAN Jia-wei, HUANG Jiang-pei, ZHOU Qing-yun. CRUSTAL FINE VELOCITY STRUCTURE IN THE ERYUAN AREA, YUNNAN FROM DOUBLE-DIFFERENT TOMOGRAPHY[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(1): 162-183.

曹颖, 钱佳威, 黄江培, 周青云. 云南洱源地区地壳三维精细速度结构成像[J]. 地震地质, 2024, 46(1): 162-183.

Figures/Tables 9

Fig. 1 Tectonic background in the study area.

Fig. 2 Distribution of P-wave seismic ray paths(a), distribution of S-wave seismic ray paths(b), time-distance curves of P- and S-wave phases(c), distribution of the grid nodes(d).

Fig. 3 Selection of smoothing factor(a)and optimal damping factor(b)and comparison of travel time residuals before(white histogram)and after(gray histogram)inversion for P-wave(c)and S-wave(d).

Fig. 4 Uncertainty histograms of earthquake relocations in the X (a), Y (b) and Z (c) directions.

Fig. 5 Recovered results of the checkerboard test at various depths.

Fig. 6 Distribution of earthquake relocated events.

Table1 A comparison of relocation results from this study and other’s

5级以上地震	本文重定位结果	其他研究的重定位结果
2013-03-03洱源M_S5.5地震	25.952°N, 99.780°E, 7.9km	25.952°N, 99.779°E, 7.5km(赵小艳等, 2014)
2013-04-17洱源M_S5.0地震	25.902°N, 99.798°E, 9.9km	25.900°N, 99.800°E, 8.5km(赵小艳等, 2014)
2016-05-18云龙M_S5.1地震	26.095°N, 99.586°E, 13.8km	无具体位置,深度约15km(李姣等, 2017)
2017-03-27漾濞M_S5.1地震	25.891°N, 99.790°E, 10.8km	无具体位置,深度约15km(李姣等, 2020)

Fig. 7 Horizontal slices of the VP(a), VS(b)and VP/VS(c)images at various depths.

Fig. 8 The distribution of VP, VS and VP /VS on the vertical profile AA'(a)and BB'(b).

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