Abstract:

Spring water is strongly related to earthquake, and groundwater within fault zone carries a large amount of information about the water-rock response and tectonic activity. Meanwhile, hydrogeochemical monitoring in the area of strong seismic activity could well obtain the precursor information related to earthquake. Therefore, it is essential to analyze the sources and characteristics of hydrogeochemistry in areas of strong earthquakes. The Bayankara Block is a rectangular active block in the east-central part of the Tibetan plateau. In recent years, the perimeter of the block is undergoing a period of moderate to strong seismic activity and has become the major area of seismicity in mainland China. However, due to the tough geological conditions surrounding the Madoi area, little has been reported on water chemistry, and the geochemical background fields have yet to be established and identified.
On 22 May 2021, an earthquake of M_S7.0 struck Madoi County, Qinghai Province, the largest magnitude earthquake in China since the 2017 Jiuzhaigou M_S7.4 earthquake. After the earthquake, a near NWW-SEE surface rupture zone was formed, with a rupture area of about 70km, along which tension fissures, sand liquefaction, sand blasting and water bubbling can be seen, and there are cold springs upwelling near the surface rupture zone. One day after the earthquake, 21 water chemistry samples were taken. They are the water bubbling from the earthquake rupture zone and the hot springs near the East Kunlun fault zone, as well as 4 sandy soil samples from post-earthquake sandblasting and water bubbling sites. The ordinary and minor ionic components of spring water and stable isotopes of δD, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr were analyzed. Percentage of oxides in sand particles was also analyzed. The sources and characteristics of spring water and sandy soils were researched, and the differences between the groundwater in surface rupture zone and the geothermal water near the East Kunlun Fault are discussed. The results show that: 1)The range of TDS of the 21 springs is 113.2~1 264.6mg/L, pH values range from 7.6 to 8.3, conductivity ranges from 200.3 to 865.7μs/cm, and temperatures range from 3 to 49℃. The spring water samples near the surface rupture zone are all from cold springs(3 to 11℃). The degree of water-rock reaction is weak. The chemistry types of spring water are Ca·Mg-HCO₃, Ca·Mg·Na-HCO₃, Ca-HCO₃, Na·Ca·Mg-HCO₃·Cl, Ca·Na·Mg-HCO₃·SO₄, Ca·Na·Mg-HCO₃·SO₄ and Ca·Na-HCO₃. Calcium, magnesium and bicarbonate ions are the main ions of the spring. 2)The range of spring water average recharge elevation in the region is 0.8~2.8km. There is an abnormal hydrogen isotope value(δD=-59‰)in the spring water near the epicenter in the surface rupture zone, and Na⁺, Cl^-, $SO_{4}^{2-}$ and other ions have high values. 3)Overall, the springs do not contain high concentrations of elements such as Ca and Sr, and most elements have EF<1, which may be related to the weak degree of water-rock reaction in the springs. Lithium in springs near the East Kunlun fault zone(maximum value of 2 014μg/L)is much greater than in springs around the surface rupture zone(6.56~43.0μg/L); and metallic trace elements of Pb, Ba, Cu, and Zn are more enriched in springs around the surface rupture zone. 4)The source of the spring water is meteoric water, and the spring water near the surface rupture zone is mixed with the surrounding water, and the results of water temperature, γNa/γCl, and elements from mantle in the East Kunlun fault zone reveal that the hot spring water circulation is deeper in the East Kunlun fault zone, with faults cutting deeply and deeper elemental recharge. The Cl^- and(Na⁺+K⁺)concentrations in the spring near the surface rupture zone are significantly higher than those near the East Kunlun fault zone, where the springs are more enriched in δD and δ¹⁸O.
The hydrochemical characteristics and sources of the samples are discussed and the fluid geochemical differences between the two areas are compared, and the sources of the sand samples that emerged after the earthquake are analyzed. The paper concludes that it is of great significance for earthquake risk assessment of the East Kunlun Fault to carry out hydro-geochemical monitoring and further study of hot springs in the East Kunlun Fault in the future. The paper fills the gap of background groundwater data in the region, meanwhile, discusses the response of water chemistry after the earthquake and the characteristics and sources of water chemistry in the middle Bayan Kara block.

Key words: hydrogeochemistry, hydrogen and oxygen isotopes, trace elements, Eastern Kunlun Fault, Madoi M_S7.4 earthquake

摘要：

泉水与地震活动密切相关, 断裂带内的泉水地下水可反映许多断裂内部的水-岩反应、 构造活动等信息。2021年5月22日玛多县发生M_S7.4地震后1d, 从本次地震形成的地表破裂带内仍然在喷砂冒水的点和东昆仑断裂内的温泉采集了21个水化学样品, 以及4个震后喷砂冒水点中的砂土样品。文中分析了泉水及砂土的来源与特征, 讨论了地表破裂带与东昆仑断裂附近泉水的差异性。结果表明: 1)21个泉水的TDS范围为113.2~1 264.6mg/L, 水化学类型为Ca·Mg-HCO₃、 Ca·Mg·Na-HCO₃、 Ca-HCO₃、 Na·Ca·Mg-HCO₃·Cl、 Ca·Na·Mg-HCO₃·SO₄、 Ca·Na·Mg-HCO₃·SO₄和Ca·Na-HCO₃, 水-岩反应程度弱。2)地表破裂带内靠近震中的泉水存在异常氢同位素值(δD=-59‰), 且Na⁺、 Cl^-、$SO^{2-}_{4}$等离子出现高值。3)东昆仑断裂带附近泉水中的Li含量(最大值为2 014μg/L)远大于地表破裂带周围的泉水中的含量(6.56~43.0μg/L); 而地表破裂带周围泉水中的Pb、 Ba、 Cu、 Zn等金属微量元素更富集。4)泉水的来源为大气降水, 地表破裂带附近的泉水有周围水体混入, 东昆仑断裂带内的温泉水循环深度大, 断裂切割更深, 有更多深部元素的补给。未来对东昆仑断裂内温泉水文地球化学开展监测与深入研究, 对判断东昆仑断裂的地震危险性具有重要意义。文中在讨论震后水化学的响应以及巴颜喀拉中段水化学特征与来源的同时, 也填补了区域内地下水背景场的空缺。

关键词: 水文地球化学, 氢氧同位素, 微量元素, 东昆仑断裂, 玛多M_S7.4地震