Subsurface fault structures are generally difficult to delineate, particularly in areas covered by thick sediments. Soil-gas geochemical surveys can provide useful constraints on concealed structures after major earthquakes and complement conventional geophysical methods in sediment-covered basins. This study investigates the 2023 Jishishan M6.2 earthquake as a representative case. Previous studies have suggested that the nearby Lajishan fault alone cannot fully account for the seismogenic structure of this earthquake, while no obvious surface rupture has been identified within the epicentral basin, complicating post-earthquake reconstruction planning and fault-avoidance measures. The spatial distribution of coseismic damage suggests the possible presence of previously unrecognized concealed structures. To investigate post-seismic soil-gas degassing and its spatial relationship with these structures, we measured Rn, Hg, H2, and CO2 along two profiles across the severely damaged area. Two prominent gas-anomaly zones were identified and showed spatial correspondence with the inferred concealed structures. Integration of the soil-gas anomalies with aftershock relocation, shallow seismic reflection, and ambient-noise surface-wave tomography provides preliminary constraints on the location and geometry of a concealed active structure spatially associated with the broader seismogenic fault system. Hg and H2 anomalies were more closely associated with the inferred major structural zone than those of Rn and CO2, suggesting that their distributions may be preferentially influenced by permeable fault-related pathways. These findings provide new constraints on concealed active structures associated with the Jishishan earthquake sequence and highlight the potential value of integrating soil-gas geochemistry with geophysical observations as a complementary approach for post-earthquake concealed-fault investigation in sediment-covered basins.

Geochemical variations in thermal spring compositions are sensitive to changes in regional stress, temperature, and space conditions, which are indicators of seismic and tectonic activities. However, the temporal and spatial variations in water chemistry components and their inducing mechanisms are insufficiently understood. This study aims to evaluate the effective temporal coincidence/lag of seismic events based on the long-term continuous water chemistry monitoring of seismic areas and draw attention to water chemistry changes in thermal waters during earthquake hazard monitoring. We investigated the major elements, trace elements, and hydrogen and oxygen isotopes of 21 thermal springs along the Lancang-Gengma fault (LGF) zone, which is in a highly deformed, seismically active area of the Southeastern margin of the Tibetan Plateau. In the LGF zone, the temperatures of the studied thermal springs range from 44.7 degrees C to 96.2 degrees C. The reservoir temperatures range from 91 degrees C to 195 degrees C and the reservoir depth range from 4-9 km. According to the spatiotemporal chemical variations from two-year regular monitoring of Na+, Cl-, F-, SO42-, Li, B, delta D, and delta O-18 in five thermal springs, the thermal springs in these fault-controlled areas are sensitive and responsive to seismic activity. Different tectonic regions exhibit significant and diverse short-term precursory anomalies in water chemistry before earthquakes with M >= 5.0. Their characteristics and mechanisms are region-specific, and the study area is divided into two monitoring capacity range areas. Area 1: The MG, MM and XF monitoring points located on the Baoshan block are highly sensitive to seismic responses in the SW direction of the study area. For example, continuous Na+, Cl-, SO(4)(2-)and F- anomalies were observed in MG, MM and XF before the M(L)5.0 and M(L)5.9 Myanmar earthquakes (southwest of the study area). Area 2: The NKL and EL monitoring points located on the Simao block are highly sensitive to seismic responses in the SE direction. For instance, the Na+ concentrations in EL and NKL sharply increased above the normal threshold before the M(L)6.2 Laos earthquake and M(L)5.0 Honghe earthquake. Notably, significant spatial directional differences in fluid chemical responses to earthquakes were observed. This directional difference may be related to the complexity of the regional stress field in the area and the local characteristics of fault activities. The chemical composition changes of these thermal springs during fluid circulation in the LGF can be a good tracer of seismic activity.

对西秦岭北缘断裂带定点台站(马窑、毛集、原家庄)断层土壤气氡、二氧化碳、痕量氢、甲烷观测资料使用正常动态分析方法,从潮汐、气温、气压、降雨方面进行对比分析.分析结果表明:断层土壤气体背景值稳定,与气温呈正相关,与气压呈负相关;断层土壤气能够反应固体潮潮汐变化;气氡、痕量氢和甲烷在地震前表现出明显的临震特点.

介绍西秦岭北缘断裂带土壤气观测台站的设计思路、建设内容以及运行情况,并通过一年来观测数据的分析认为,各观测台站断层土壤气体背景值稳定,观测数据合理可靠,且3个观测站同一测项的变化形态具有相似性、同步性,并在榆中MS3.6、玛多Ms7.4和门源Ms6.9地震前出现同步异常响应,表明西秦岭北缘断裂带土壤气具有反映区域构造应力变化的潜力.同时断层土壤气定点观测台站的建设方案与技术指标为后期组建断层土壤气观测台网提供借鉴和参考,为进一步推广应用奠定良好基础.

The mid-eastern segment of the Qilianshan fault zone (QLF) on the northeastern margin of the Qinghai–Tibet Plateau is considered one of the key seismic hazard areas. The Zhangye Ms 5.0 earthquake and Menyuan Ms 6.9 earthquake are the two Ms ≥ 5.0 earthquakes in recent years. The spatio-temporal evolution of Rn across the fault before the two Ms ≥ 5.0 earthquakes were explored by combining a solid seismogenic model and numerical simulation results in this study. The results demonstrates the spatial distribution of Rn concentration intensity varies over time, indicating the evolving characteristics of fracture zone activity. The time-series variation characteristics are closely related the Zhangye Ms 5.0 earthquake and Menyuan Ms 6.9 earthquake. Overall, in the seismic source area and surrounding medium area of Zhangye Ms 5.0 earthquake, the soil gas Rn anomaly across faults characterized by a turning upward trend after continuous decline. The closer to the source area, the more obvious the upward trend. For Menyuan Ms 6.9 earthquake, the survey line (HT1) located in the main fracture zone of the earthquake and the survey line (HT7,30km from the epicenter) closer to the epicenter also showed a similar trend, while the other measurement lines in far-field exhibit declining trend before the Menyuan Ms 6.9 earthquake. Therefore, the continuous decline trend of soil gas may be crucial information for medium-term earthquake preparation in the seismogenic zone, and the trend of turning upward after continuous decline is a significant signal of short-term seismogenic event in far-field. This research could improve the understanding of the anomalous features of soil gas precursors and tracking the active sections of the fault. According to the model, the earthquake area canseismic source area, the surrounding medium area be divided into three sections: the seismic source area, the surrounding medium area, and the fracture fragmentation area.

The Xiahe Ms5.7 earthquake occurred in Xiahe county, Gannan prefecture, China (35.10°N, 102.69°E) on October 28, 2019, with a source depth of 10 km. This study investigates the spatial and temporal evolution characteristics of cross-fault soil gas concentrations prior to the Xiahe Ms5.7 earthquake by analyzing Rn, Hg, H2, and CO2 data collected from 11 profiles across the northern margin of the West Qinling fault zone from 2016 to 2019. The spatial distribution of these gases showed varying trends, with Rn concentration intensity decreasing from the Wushan segment to the east and west sections, while Hg, H2, and CO2 all broke the trend in the West Qinling fault zone's northern margin. The soil gas concentration intensity demonstrated a significant response to the Xiahe Ms5.7 earthquake, particularly in the west Ganjia sections. By integrating the seismogenic model and numerical simulation results, we explored the physical mechanism underlying these abnormal trends. Our findings suggest that the continuous decline characteristic of fault gas could be a valuable indicator of fracture tectonic activity, while an upward trend after continuous decline may signal a medium and short-term seismogenic event in the source area. These results provide a foundation for improved tracking of earthquake location and timing in a fault zone through cross-fault soil gas methods, highlighting the importance of enhancing deep fluid flow monitoring and seismogenic model research in fault zones.

基于祁连山断裂带中东段9个观测点2016-2021年的土壤气体氡浓度观测数据,深入分析其浓度强度时空分布特征.同时结合历史大震背景、现今地震活动与断裂带滑动速率的对比分析,从地球化学的角度对祁连山断裂带中东段进行地震危险区段判定.研究表明:祁连山断裂中东段土壤气体氡浓度强度的空间分布特征为东强西弱,其时间序列变化特征总体呈下降趋势;断层土壤气氡浓度强度的空间分布特征、历史及现今地震活动和断裂滑动速率具有较好的耦合性.研究结论可以提供研究区深部地下流体活动的证据,对进一步研究追踪未来可能发生地震的断层和活动段具有重要意义.

嘉峪关气氡浓度在多年上升的背景下,于2017年出现了转折异常变化,但祁连山地震带内一直没有发生与异常幅度和持续时间相匹配的地震,直到2021年5月22日,在距离嘉峪关气氡测点570 km的玛多发生Ms7.4地震.为了判断嘉峪关气氡浓度异常与玛多地震的关系,结合震例,从异常信度、震前异常特征、震后异常变化及地质构造背景等方面对其进行深入分析.结果发现:嘉峪关气氡浓度的异常特征与地震所处的构造有关,发生在祁连山地震带内的地震,气氡浓度异常表现为1年或半年尺度的年畸变,发生在该地震带以外远距离的地震,气氡浓度异常表现为多年的趋势性变化;玛多地震发生后,嘉峪关气氡浓度下降速率减缓之后转折,呈恢复状态;嘉峪关气氡测点与玛多地震都位于青藏高原东北部,具有相同的动力背景且在构造上具有关联性.综合分析认为嘉峪关气氡多年趋势异常与玛多Ms7.4地震有关,研究对建立可靠的异常指标体系、提高地震预测水平具有重要意义.

在综合分析1991年以来祁连地震区及周边7次MS6.0以上地震前嘉峪关气氡异常特征的基础上,进一步分析嘉峪关气氡趋势异常与门源MS6.9地震的关系.分析结果显示:(1)在该区域,有些地震发生前气氡出现了明显的响应,表现为破年变和短期成组突跳异常变化;有些地震发生前气氡无响应,且地震发生前气氡是否有响应与震中距没有明显的关系.(2)地震发生前气氡是否出现异常,与地震发生的断裂紧密相关,同一断裂的地震,气氡异常具有相似性特征.地震前分布在大柴旦—宗务隆山断裂上的气氡都出现了破年变和成组突跳的配套性异常,而分布在托莱山—冷龙岭断裂上的气氡通常没有异常显示.(3)嘉峪关气氡的趋势异常可能与整个区域构造活动的增强有关,它可能是玛多MS7.4及其后发生在祁连山地震区及其周边包含门源MS6.9在内一系列MS 5.5以上地震的响应.该研究对地震前兆异常性质的判定、地震跟踪监测和预测效能的提高具有重要意义.

The Qilian–Haiyuan fault zone (QHF) is located in a highly deformed and seismically active area of the northeastern margin of the Tibetan Plateau. This study investigated the major elements, strontium, hydrogen, and oxygen isotopes of 22 sites in the thermal springs along the QHF from October to November 2020. The QHF hydrochemical system is recharged by meteoric water primarily infiltrating between 2.8 and 4.4 km a.s.l. Based on quartz geothermometers, the geothermal reservoir temperature variation ranged from 25.5 to 111.3°C, and the circulation depth ranged from 1.3 to 5.6 km. In the QHF zone, the highest spring water temperature values were correlated with deep groundwater circulation circuits in areas where earthquake foci are concentrated. A conceptual model of the hydrologic cycle of thermal springs explains the spatial distribution of earthquakes associated with tectonic movements. The fluid circulation of the QHF corresponds well with the seismicity, which indicates that the hydrological characteristics of the thermal spring in a fault zone are vital in receiving information on seismic activity to assess the seismic risk of the QHF in the future.

The Weixi–Qiaohou Fault (WQF) is considered an important zone of the western boundary of the Sichuan–Yunnan block, and its seismicity has attracted much attention after a series of moderate–strong earthquakes, especially the Yangbi Ms6.4 earthquake that occurred on 21 May 2021. In the present research, we investigate major and trace elements, as well as hydrogen and oxygen isotopes, of 10 hot springs sites located along the WQF, which are recharged by infiltrated precipitation from 1.9 to 3.1 km. The hydrochemical types of most analyzed geothermal waters are HCO3SO4-Na, SO4Cl-NaCa, and SO4-Ca, proving that they are composed of immature water and thus are characterized by weak water–rock reactions. The heat storage temperature range was from 44.1 °C to 101.1 °C; the circulation depth was estimated to range between 1.4 and 4.3 km. The results of annual data analysis showed that Na+, Cl−, and SO42− in hot springs decreased by 11.20% to 23.80% north of the Yangbi Ms5.1 earthquake, which occurred on 27 March 2017, but increased by 5.0% to 28.45% to the south; this might be correlated with the difference in seismicity within the fault zone. The results of continuous measurements of NJ (H1) and EYXX (H2) showed irregular variation anomalies 20 days before the Yangbi Ms6.4 earthquake. In addition, Cl− concentration is more sensitive to near-field seismicity with respect to Na+ and SO42−. We finally obtained a conceptual model on the origin of groundwater and the hydrogeochemical cycling process in the WQF. The results suggest that anomalies in the water chemistry of hot spring water can be used as a valid indicator of earthquake precursors.

1 研究背景 地球内部发生的各种物理、化学场变化以及构造体之间的相互作用,都会使地下深部气体沿着活动断裂带、板内块体边界、火山等地壳薄弱带处向上逸散.断层土壤气指沿着断裂破碎带逸出地表的地下气体,其浓度以及逸出速率值大小能灵敏、客观地反映地下应力、应变状态以及构造活动状态,因此常被用来鉴识断裂带活动状态、探寻隐伏断层以及地震危险性研究等.

针对现阶段地震人员死亡评估模型在评估特定区域地震时评估结果误差较大的问题,结合中国地震灾害特点、人口分布情况等,将中国大陆划分为西北、西南和大陆东部三个地区,并对各区域按照人口密度进行分级.采用多元非线性回归方法,选取震中烈度、震区面积、抗震设防烈度参数建立分区域的地震灾害人员死亡评估模型.研究结果表明,该模型在西北、西南地区的验证结果较好,适用于中国地震灾害情况,可以用于震后快速盲评估,在一定程度上能够避免评估不同震级、相同震中烈度和人口密度的地震时,出现相同结果的情况;虽然该模型在震例较少的大陆东部地区以及城市直下型地震中评估结果需要进一步改进,但为解决特定区域地震死亡评估中误差较大问题提供了思路,能够为震后应急指挥和救援提供数据参考.

基于甘东南地区2019年地球物理场年度异常,对该区域的地球物理定点观测资料进行全时空扫描,9个台站18个台项在夏河Ms5.7地震前出现异常变化.从异常的重复性、多学科前兆的协调性、异常的时空演化及震后异常变化4个方面对各个异常进行信度划分,并根据划分结果定量计算了各个异常与夏河地震关系的信度值.结果显示:18项异常的信度都在50％以上,表明作为夏河地震的前兆异常基本可信,但异常信度存在差异,其中临夏水位、临夏钻孔应变NS向和武都两水水位的信度最高,达到80％,临夏水温和天水钻孔应变NS、NW向的信度最低,低于60％.空间分布上,分布在光盖山—迭山断裂的异常信度较高,而位于西秦岭北缘断裂北侧的异常信度较低,这与该地区构造应力的集中和孕震机制有关.地球物理异常信度分析对建立有效的地震预报指标体系具有很好的促进意义.

基于中国1993—2017年的历史地震资料,通过对四个现行地震灾害人员死亡评估模型进行对比,深入分析不同模型在不同震级(M)及不同地区的适用性和精度.研究结果表明:(1)当M<6.0时,四个模型评估结果与实际死亡人数数量级基本一致,均适用于震后人员伤亡快速评估;当6.0≤M<7.0时,四个模型评估结果与实际死亡人数差距较大,经过参数修正后的刘金龙模型评估结果相对较好;当M≥7.0时,四个模型的评价结果都不理想,需要结合震区地理环境、经济条件等进行人工修正才能使用;(2)在地震较多的云南、四川、甘肃、新疆、西藏五个省份中,GB/T 30352-2013评估结果优于其他三个模型;在四川、云南、甘肃地区评估时需考虑次生地质灾害因素的影响,不然可能会导致评估误差较大.研究结果可为各级政府和应急管理部门地震灾害应急业务能力的提升提供技术支持.

梳理了现有3种地震灾害人员死亡评估模型,并基于甘肃地区历史中强地震(MS4.7～6.0),对人员死亡评估模型进行震例计算.结果表明:①针对甘肃地区中强地震,3种模型的评估结果与实际结果基本处于同一数量级,可用于地震后快速盲评估计算,为地震后政府和应急管理部门应急指挥决策提供科学依据;②当中强地震震中烈度达到Ⅷ度时,这3种评估模型的计算结果与实际人员死亡人数存在较大误差,需要根据专家经验进行人机交互修正,依据地震灾区地理环境、社会经济、人口空间分布等资料进行综合分析支撑应急决策;③在甘肃地区中强地震的人员死亡评估中,李雯模型因考虑了地震灾区面积、区域适用性要优于其他两类模型.

地震前兆异常性质的准确判断是提高地震预测水平的前提.据国内外相关统计,真正具有地震预测意义的地震前兆异常数量十分有限.因此,厘清震前异常与地震的关系对今后异常识别意义重大.2017年8月8日九寨沟Ms7.0地震前4个月,甘肃清水温泉水氡出现大幅波动异常变化.为了判断这次异常与九寨沟地震的关系,在异常持续期间,通过抽水实验、水化学组分以及氢氧同位素组成分析,并结合九寨沟地震发震构造以及其他前兆异常空间分布特征,对水氡异常成因机理进行了深入分析.结果 表明:对于水氡与水位的变化关系,实验与实际观测结果一致,都呈正相关变化;实验中水氡变化幅度与实际观测的相当;水化学组分以及氢氧同位素测试结果显示清水温泉水氡异常期间,来自地壳深部的流体并未增加;清水温泉区其他前兆没有出现明显的短期异常变化;在空间上清水温泉与九寨沟地震震源区被两条深大断裂隔开.因此,清水温泉水氡大幅异常变化并非九寨沟地震的短期前兆异常,而是企业抽水开始时间、持续时间以及用水量不固定引起的.该实例为地震前流体前兆异常性质的准确判定提供了更为可靠的方法和思路.

为了掌握MS 7以下强震近场流体的同震变化特征,以岷县漳县MS 6.6地震为例,分析震中300 km范围内数字化水位和水温的同震变化特征及与未来4级以上地震发震区域的关系,结果显示:MS 7以下强震引起的水位、水温同震相对变化幅度不大,且持续时间短,形态以突跳型变化为主;同震变化的相对幅度在空间上没有显著的差异;同震变化的初始方向在空间上具有四象限分布特征,且与震源机制解的四个区域配套;同震变化初始方向向上台站集中的区域与未来4级以上地震的发生区域有关.由于震级偏低的地震相对于MS 7以上地震其发生频率高,且近场研究范围较小,因此其在未来地震的预测上具有更加重要的意义.

Eight soil gas measurements were performed in the Liupanshan fault zone (LPSFZ) to observe the concentration and flux of soil gas radon (Rn) and CO2 in October 2017 and October 2018. By combining the historical strong earthquake background and modern seismic activity of the fault zone, the relation between the geochemical distribution characteristics of soil gas and the seismicity of the fault zone was studied herein. Furthermore, the strong seismic hazard potential of the fault zone was discussed. Results show that the concentration of soil gas Rn and CO2 considerably varies in the northern segment of the LPSFZ and is relatively stable in the southern segment. The spatial distribution of the concentration intensity and flux is strong in the north and weak in the south. However, the southern segment of the LPSFZ has a seismic gap that has not been ruptured by strong earthquakes with Ms≥6.5 for the last 1400 years, whereas the seismic activity in the northern segment is relatively frequent, indicating that the fault zone locking degree of the southern segment is higher than that of the northern segment. This observation is completely consistent with the geochemical characteristic distribution of soil gas. Therefore, the southern segment of the LPSFZ should be considered a hazardous segment, where major or strong earthquakes can occur in the future.

白杨河断裂是酒西盆地内部一条重要的活动断裂,断裂长约25 km,整体走向近EW,倾向N,倾角约25°.以往的研究认为白杨河断裂为一条全新世活动的隐伏断裂,其持续的活动造成了上覆阶地变形,形成白杨河背斜.通过卫星影像解译和野外实地考察,在断裂西段和中段发现连续发育的低断层陡坎,表明断裂活动已至地表.古地震探槽揭露白杨河断裂全新世以来至少发生过2次地震事件,年代分别为距今(8.7±0.6)ka和(3.9±0.5)ka,每次地震事件的垂直断距都在约0.6 m,利用经验公式,估算震级约为6.8级.