SEISMOLOGY AND GEOLOGY ›› 2023, Vol. 45 ›› Issue (3): 772-794.DOI: 10.3969/j.issn.0253-4967.2023.03.010

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FAULT GAS OBSERVATION AND SURFACE RUPTURE FEATURE INTERPRETATION OF THE MS7.4 MADOI EARTHQUAKE

WANG Bo1,2)(), CUI Feng-zhen3), LIU-ZENG Jing1,3),*(), ZHOU Yong-sheng1), XU Sheng3), SHAO Yan-xiu3)   

  1. 1) State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China
    2) China Earthquake Networks Center, Beijing 100045, China
    3) School of Earth System Science, Tianjin University, Tianjin 300072, China
  • Received:2022-05-25 Revised:2022-12-15 Online:2023-06-20 Published:2023-07-18

玛多 MS7.4地震断层土壤气特征与地表破裂的相关性

王博1,2)(), 崔凤珍3), 刘静1,3),*(), 周永胜1), 徐胜3), 邵延秀3)   

  1. 1) 中国地震局地质研究所, 地震动力学国家重点实验室, 北京 100029
    2) 中国地震台网中心, 北京 100045
    3) 天津大学, 地球系统科学学院, 天津 300072
  • 通讯作者: * 刘静, 女, 1969年生, 教授, 主要从事活动构造、 地震地质、 古地震和地貌学等方面的研究, E-mail: liu_zeng@tju.edu.cn
  • 作者简介:

    王博, 男, 1984年生, 2023年于中国地震局地质研究所获构造地质专业博士学位, 主要研究方向为构造物理实验及流体动力学, E-mail:

  • 基金资助:
    国家自然科学基金(42030305); 国家自然科学基金(42104061); 地震动力学国家重点实验室开放基金(LED2020B03); 中国地震局地震科技星火计划(XH19055); 国家重点研发计划(2019YFC1509203)

Abstract:

An MS7.4 earthquake occurred in Madoi County, Guoluo Tibetan Autonomous Prefecture, Qinghai Province of China at 02:04 (Beijing Time) on May 22, 2021. A total of seven 800~3 000m trans-fault survey lines were targeted laid along different parts of the seismic surface rupture zone(the west, mid-west, mid-east, and the east), one month after the earthquake when the detailed field investigation of the coseismic displacement and the spread of the seismic surface rupture zone had been carried out. The soil gases were collected and the concentrations of Rn, H2, Hg, and CO2 were measured in situ.
The results show that the maximum value of Rn, H2, Hg and CO2 concentrations in different fracture sections of the surface rupture was 2.10~39.17kBq/m3(mean value: 14.15kBq/m3), 0.4×10-6~720.4×10-6(mean value: 24.93×10-6), 4~169ng/m3(mean value: 30.72ng/m3)and 0.73%~4.04%(mean value: 0.59%), respectively. In general, the concentration of radon is low in the study area, which may be related to the thick overburden and the lithology dominated by sandstone. The concentration characteristics of hydrogen and mercury released from soil have good consistency, and the concentrations are higher at the east and west ends of the surface rupture zones but were lower in the middle of the rupture zone. This is consistent with the field investigation showing that the earthquake-induced surface rupture zone and deformation are more concentrated in the western section, while the eastern section has a large amount of seismic displacement.
The fault strikes at the east and west ends of the Madoi MS7.4 earthquake surface rupture have deviated from the NW direction to a certain extent, and there also exits two branching faults and rupture complexities at the east end of the main fault of the Madoi earthquake. In the west end of the surface rupture, i.e., the south of Eling Lake, the fault strike turns to EW direction. We laid two survey lines(line 2 and line 3)at the west end of the rupture, the concentration of Rn, H2 and Hg escaped from line 3 is the lowest one among all lines while the gas concentration of line 2 is significantly higher. In the vicinity of line 3, the field geological survey did not find the cracked and exposed surface rupture, and only a small number of liquefaction points were distributed near the Eling Lake. The soil gas concentrations and morphological characteristics were consistent with the field phenomena. At the east end of the rupture zone, the soil gas morphological characteristics of the south and north fault branches were inconsistent: the soil gas of the south branch showed a single-peak type which was more similar to that at the west end, but the gas concentration pattern of the north fault branch showed a multiple-peaks type. This phenomenon is consistent with the characteristic shown in the surface fracture mapping, that is, the deformation zone of the rupture where is wider.
To find out the source of soil gas and the possible influencing factors of soil gas concentrations in the study area, the carbon isotope and helium isotope of the collected gas samples were analyzed. The value of 3He/4He shows that the noble gas in the study area is mainly an atmospheric source, but the results of δ13C and CO2/3He show that the soil gas along the surface rupture of the Madoi earthquake has the mixed characteristics of atmospheric components and crustal components, which to a certain extent reflects the cutting depth of main fault-Jiangcuo fault may be shallow, and it is speculated that the surface rupture caused by Madoi MS7.4 earthquake may be confined to the shallow crust.

Key words: Madoi MS7.4 earthquake, fault soil gas, carbon isotope, 3He/4He, Bayan Har block

摘要:

2021年5月22日2时4分, 青海省果洛藏族自治州玛多县发生 MS7.4 地震。震后一个月, 在对地震地表破裂带的展布和同震位移进行详细勘察后, 有针对性地沿地表破裂带不同部位(西段、 中西段、 中东段、 东段)布设了7条800~3 000m的跨断裂测线, 对土壤气Rn、 H2、 Hg和CO2进行浓度测量和气体采集, 并对采集样品进行了碳同位素和氦同位素分析。测量结果表明, 地表各破裂段土壤气浓度的最大值差别较大, 破裂带东、 西两端的气体浓度较高而中段气体浓度较低, 可能与断层不同分段的破裂方式和应力分量不一致有关。土壤气中H2和Hg的浓度特征具有较好的一致性, 在地表破裂带内或紧邻处浓度较高。玛多 MS7.4 地震的发震断层东端出现多条分支, 破裂具有复杂性。从土壤气浓度测量结果来看, 南支和北支断层的活动都较强, 但北支断裂土壤气逸出浓度的曲线形态特征和断层产状不一致, 可能与北支断裂地表破裂范围大且存在多条次级断裂有关。3He/4He测定结果表明, 研究区土壤气中的稀有气体主要为大气来源, 但δ13C测值和CO2/3He计算结果显示玛多地震断层土壤气具有大气组分与地壳组分的混合特征。

关键词: 玛多MS7.4地震, 断层土壤气, 碳同位素, 氦同位素, 巴颜喀拉块体