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20 June 2023, Volume 45 Issue 3

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Review

RESEARCH PROGRESS AND PROSPECT OF SEISMIC FLUID GEOCHEMISTRY IN SHORT-IMMINENT EARTHQUAKE PREDICTION

LI Ying, FANG Zhen, ZHANG Chen-lei, LI Ji-ye, BAO Zhi-cheng, ZHANG Xiang, LIU Zhao-fei, ZHOU Xiao-cheng, CHEN Zhi, DU Jian-guo

2023, 45(3):  593-621.  DOI: 10.3969/j.issn.0253-4967.2023.03.001

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Establishing the method of short-imminent earthquake prediction is the most effective way to reduce losses caused by earthquakes and is also an important scientific issue. In the 1960s and 1970s, research on earthquake prediction was carried out successively in China and other countries in the world, and after over 50 years of development, abundant precursor observation data and earthquake cases have been accumulated, and significant progress has been made in the research of formation mechanisms of precursor anomalies and prediction methods.
Fluid is the most active component in the earth’s interior, and the fluids in various layers of the earth often carry characteristic geochemical information. The composition and variation of seismic fluid geochemistry are sensitive to changes of underground physical and chemical conditions, making them powerful indicators of seismic and tectonic activities. The formation mechanisms of fluid geochemical precursor anomalies mainly include liquid mixing, water-rock reaction, deep magma upwelling, seismic wave vibration, pore compression and pressure solubility mechanism. The fluid chemical anomalies associated with earthquakes can be attributed to the migration process of liquid mixing and the water-rock reaction mechanism caused by crustal stress changes.
This paper systematically summarizes the empirical formulas on the duration of anomaly, earthquake magnitude and epicentral distance, as well as the seismic fluid geochemical models and methods for short-imminent prediction established both domestically and internationally. In addition, four types of seismic fluid geochemical techniques and methods currently used in earthquake situation consultation in China are described. Nine of the most widely used prediction methods are selected to inspect the twenty-seven cases of earthquakes containing water radon or gas radon anomalies in the Earthquake Cases of China from 1997 to 2020. Generally, these methods all show strong applicability. However, empirical formulas based on different regions of the world selected to inspect the above cases generally show weak applicability. It indicates that current earthquake prediction models or methods are only representative to a certain extent, and there are still great difficulties in practical application, which also directly affects the prediction efficiency of the fluid geochemical models applied to the judgment of earthquake three elements.
Combined with our previous results, the paper puts forward the applicable theory for the precursor mechanism-based short-imminent prediction by seismic fluid geochemistry, that is, acquiring the dynamic change characteristics of the geochemical field based on the spatio-temporal dense and multi-item observation network, establishing a deep-shallow coupling anomaly genetic model based on the material cyclic reaction, and determining the temporal and spatial relationship between the evolution of regional fluid geochemical field and fluid geochemical changes at each measuring point in the fault zone. The construction of the geochemical subsystem of China Seismic Experimental Site provides a platform for capturing the short-imminent earthquake anomalies and constructing effective fluid geochemical anomaly mechanisms and models. The causes and abnormal mechanism of fluid geochemistry can be revealed and the seismic fluid geochemical short-imminent prediction method can be established in the light of the principle of seeking the source by field and combining the field and source.

STATUS OF RESEARCH AND OBSERVATION ON UNDERGROUND FLUID HYDROGEN IN SEISMIC FAULT ZONES IN CHINA

JIANG Yu-han, WANG Zi-si, LIU Jia-qi, LIANG Hui, ZHOU Qi-chao, GAO Xiao-qi

2023, 45(3):  622-637.  DOI: 10.3969/j.issn.0253-4967.2023.03.002

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Large-scale observation network has been set up in China, including the observations of groundwater dynamics, geothermal water, and geochemical parameters, and long-term observation data has been obtained for underground fluids. Hydrogen observation is considered to be one of the methods that are most likely to make a breakthrough in the aspect of earthquake precursor monitoring and prediction, thus, plays an important role in earthquake monitoring and forecast in China. Many scholars have carried out research on the relationship about hydrogen and earthquake precursors, and proved that abnormal hydrogen concentrations are related to and have certain correlations earthquake activities. The main objects of hydrogen observation in China include the escaping gas from fault soil and the escaping gas from deep wells and hot springs near the fault. Different analytical methods are used for different types of hydrogen, and the main methods include gas chromatograph analysis and digital high-precision hydrogen analyzer analysis. Through years of observation practice, a large number of typical examples have been obtained in China. The relationship between the abnormal hydrogen concentration and the earthquake has a correspondence. The main manifestation is that the hydrogen concentration increases several times or even tens or hundreds of times in a few months or a few days before the earthquake. It is mainly divided into two cases: First, it rises rapidly to several times in a short time before the earthquake. The concentration reaches about hundreds of times the background value in more than ten to a few days immediately before the earthquake, and then the earthquake occurs. The concentration quickly declines and restores the background value after the earthquake. Second, the hydrogen concentration continues to increase in fluctuation, and decreases after reaching the maximum value, then, the earthquake occurs after recovery. This kind of anomaly is short in time, mostly, they are imminent or medium and short-term abnormalities. Therefore, the hydrogen response to the earthquake precursor is an important short-imminent earthquake prediction indicator, and can be used as an important approach to explore the short-impending earthquake prediction.
The hydrogen in the crust mainly comes from biochemical and chemical actions. The hydrogen on the surface layer of the crust is mainly produced by microbial decomposition of organic matter and mineral salts. It is regularly symbiotic with gases such as methane and carbon dioxide. The hydrogen in the crustal fault belts, especially in the active fault zones, also comes from the failure and deformation of rock. The formation mechanism of hydrogen in the crust can be summarized into 3 categories: 1)Under normal circumstances, the hydrogen content is very low, and most of them exist in the pores of the rock and soil layer in a free state, or are adsorbed on the surface of the rock. When the external conditions remain unchanged, the gas is in a balanced state; when the environment changes, especially the underground stress changes, the cracks develop continuously under the action of tectonic stress, resulting in interconnecting each other, and subsequently, the deep hydrogen also changes and emits to the ground surface, including the imminent rupture stage in the earthquake preparation and rock oscillation; 2)The chemical reactions occur between the crushed rock's fine particles and water, generating hydrogen; 3)The temperature gradient causes the hydrogen attached in the crack to escape.
In short, hydrogen is a better method for studying earthquake reflecting ability among the underground fluid observation methods. Representative earthquake cases are obtained from observations of both dissolved hydrogen in the water or soil hydrogen. This observation item plays an important role and has practical significance in the geochemical observation means. In the observation of earthquake underground fluids, hydrogen observations can provide data support for future earthquake risk zoning and earthquake tendency tracking and analysis.

Research paper

USING SLUG TEST METHOD TO STUDY HYDROGEOLOGICAL PARAMETERS OF WELL-AQUIFER AND THE CORRESPONDING RELATIONSHIP WITH EARTHQUAKE

LÜ Fang, MU Hui-min, LI Yan, GUO Wen-feng, YAO Lin-peng, GONG Jing-zhi

2023, 45(3):  638-651.  DOI: 10.3969/j.issn.0253-4967.2023.03.003

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Hydraulic parameters of aquifers are important parameters for studying the rule of groundwater movement, and also key parameters for regional groundwater resource evaluation, groundwater flow numerical simulation and groundwater quantitative calculation. Aquifer hydraulic parameters also play an important role in the study of groundwater dynamic characteristics and their relationship with earthquakes. Among all kinds of hydrological responses caused by earthquakes, the change in well water level is the most common. Stress accumulation in the process of earthquake preparation, static stress caused by fault dislocation after the earthquake, and dynamic stress caused by seismic wave propagation will lead to the change of crust-media structure at local or regional scales, which will inevitably lead to the change of media characteristics(such as permeability)of the aquifer in the rock mass. As a result, the well water level will change. Therefore, the study of aquifer hydraulic parameters is of great significance to understand the role of groundwater in the process of earthquake preparation and occurrence to understand the hydrological response mechanism related to earthquakes.
China has the largest seismic underground fluid observation network in the world, but most of the observation wells are transformed from geological and petroleum wells, lack complete basic data, and most of the hydraulic parameters of the aquifer are unknown. For underground water level real-time monitoring wells or seismic precursor information observation wells, hydraulic parameters can be obtained by traditional pumping test methods when the well is completed, but it is difficult to implement in order to ensure the continuity of observation data after the well is put into use. The hydraulic parameters of the-well-aquifer system often change with the change in the regional groundwater environment. In order to more accurately interpret the dynamic changes of the observed well water level, we need to obtain the hydraulic parameters of the-well-aquifer system under the current state. As a single well hydraulic test, the slug test has the characteristics of convenient operation, short test time, and low disturbance to aquifer, which can easily and relatively quickly obtain the hydraulic parameters of well-aquifer. With the advent of the digital water level measuring instrument of second sampling rate and its widespread use in the observation of underground fluid, the slug test method is more widely used in the determination of hydraulic parameters of the underground fluid aquifer.
In this paper, the water-level time response data of 8 wells in Shanxi area were obtained by using the Slug test method, and the water conductance coefficient of the observed aquifer was estimated by selecting the corresponding data analysis model according to the attenuation type of water level. The coefficient of transmissivity of each well-aquifer is compared and analyzed by statistics of the co-seismic response of each well and discusses the reliability of the Slug test estimation results and the applicability of the method. The following conclusions are obtained: 1)The co-seismic response of wells with large transmissivity is usually of vibration type. 2)The hydrogeological parameters of the well-aquifer can be obtained dynamically by the Slug test, and the subtle changes of aquifer medium state can be captured to interpret the dynamic changes of well water level more accurately. 3)With the popularization of water level high-frequency sampling observation instruments, the Slug test has been well used in the parameter determination of high permeability medium in underground fluids.
The results show that the slug test method can quickly and accurately obtain the hydraulic parameters of the well-aquifer, capture the change of aquifer medium state in time, interpret the dynamic change of well water level more accurately, and conduct quantitative analysis of the abnormal change of water level.

HYDROLOGICAL PROPERTIES ESTIMATION BY WATER LEVEL IN RESPONSE TO BAROMETRIC PRESSURE

LIU Wei, BAI Xi-min, LÜ Shao-jie, SHI Zhe-ming, QI Zhi-yu, HE Guan-ru

2023, 45(3):  652-667.  DOI: 10.3969/j.issn.0253-4967.2023.03.004

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Groundwater, as one of the most active components of the earth's crust, has a sensitive reflection to crustal stress as well as solid deformation. Previous studies have shown that fluctuations in barometric pressure cause corresponding dynamic changes in well water level, and the response of well water level to barometric pressure signal can reveal a lot of hydrogeological information such as groundwater movement law and aquifer water storage mechanism, and can also provide a new way to estimate the hydraulic parameters of the aquifer. The method of using well water level in response to barometric pressure to estimate the hydraulic parameters is in-situ, low cost, and low disturbance, which has certain advantages compared with traditional field hydrogeological tests.
We analyze the water level and barometric pressure data of the monitoring wells in the fault zone, and the change characteristics of the permeability of the fault zone can be obtained. The seismicity of the Qujiang fault is strong, and studying its permeability and evolution characteristics plays an important role in understanding the seismicity in this area. Therefore, in this study, we took the Gaoda well in the Qujiang fault zone in Yunnan Province as the object of study, and collected minute level data of well water level and barometric pressure from February 2019 to July 2020, based on the response of the well water level to the barometric pressure signal in different periods to calculate the hydraulic parameters of the aquifer using barometric response function and analyzed the change characteristics of the permeability of the fault zone after the earthquake. At the same time, compared and analyzed the parameter estimation results with the results calculated by previous methods using well water level in response to earth tide and slug test. The results show that:
(1)The aquifer permeability of the Gaoda well ranges from 8.89×10^-15 to 11.10×10^-15m² and the transmissivity ranges from 2.44×10^-6 to 3.05×10^-6m²/s during different observation periods, and the overall variation is not significant and fluctuates within a certain range, indicating the aquifer permeability of the Gaoda well did not change significantly after the earthquake, and the permeability of the Qujiang fault zone was relatively stable. Meanwhile, previous studies on the tidal analysis of the Jiangchuan well near the southern section of the Xiaojiang fault zone, which is 16.6km away from Tonghai, showed that the permeability of its aquifer did not change significantly after the Tonghai 5.0 earthquake in 2018, indicating that the permeability of the southern section of the Xiaojiang fault zone and the Qujiang fault zone are relatively stable, and the hydraulic characteristics of the two have a certain similarity, and the comparison result between the two wells is referential to some extent.
(2)The hydraulic parameters of the aquifer calculated based on the response of well water level to the barometric pressure are somewhat different from those calculated by previous authors using earth tide responses and slug tests, and the obtained parameters are slightly lower than those obtained by earth tide responses and slug tests, this may be due to the different factors considered by different methods and the different degrees of fracture development in the part of the fault zone where the well is located, resulting in a certain degree of heterogeneity in the aquifer, causing differences in the results obtained by different methods, which reflect the differences in the spatial scales, the applicability of the models, and the parameter range represented by the aquifer hydraulic parameters inferred by the response models of different well-aquifer systems. In addition, the barometric pressure signal acts in a wider frequency band, more parameters are inferred by the model, and its response can be recorded under different hydrogeological conditions, making the response model to barometric pressure more widely applicable.

THE RELATIONSHIP BETWEEN TIDAL RESPONSE OF WELL WATER LEVEL AND MODULATION OF SMALL EARTHQUAKES

LI Ji-ye, YAN Rui, ZHANG Si-meng, HU Lan-bin, MENG Ling-lei, ZHOU Chen

2023, 45(3):  668-688.  DOI: 10.3969/j.issn.0253-4967.2023.03.005

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Underground fluid plays a vital role in the process of earthquake preparation and occurrence. Water level observation wells with good pressure and sealing are usually called crustal strain-sensitive indicators. In groundwater micro-dynamic observation, information such as earthquake precursor anomalies can be identified by using tidal response characteristics of well water level. In this paper, the water level data of Yanshou Station, Tonghe Station, Zhaodong Station, Gannan Station and Suihua Beilin Station, which can be used for tidal analysis in 18 water level observation wells in the Heilongjiang area since 2016, are selected, and the tidal factors of diurnal waves in the diurnal wave group of water level tidal response of each well are obtained by using Vinidkov harmonic analysis method. Based on the tidal analysis of the whole-point observation data in Heilongjiang and its surrounding areas without earthquakes above M_S4.0, with no obvious interference of water level and high accuracy and continuity of water level, the maximum tidal factor of the diurnal wave is extracted as the background value of diurnal wave of well water level tidal response at each station. Combined with the significant earthquakes around each station, the abnormal variation characteristics of diurnal wave height before and after Ningjiang M_S5.0, M_S5.7 and M_S5.1 earthquakes in Songyuan, Jilin Province are extracted.
When the earthquake preparation reaches the final stage, the triggering effect of the external environment will become the key factor. This research shows that the horizontal tidal force of the day and month is closely related to the occurrence of large earthquakes, and the tidal force of the earth's tide before moderate and strong earthquakes has obvious modulation and triggering effect on seismicity. In the study of the relationship between earthquakes and tidal triggering, it is the most intuitive and effective method to analyze the degree of modulation triggering of earthquakes in a certain region and the modulation anomaly characteristics of small earthquakes near the epicenter before earthquakes. Using M_L3.0 earthquakes in Heilongjiang Province and its adjacent areas, this paper selects a time window length: 1 year, time step length: 3 months, space window length: 150km, space step length: 0.5°×0.5°, and a lower limit of the number of earthquakes as 5, calculates the spatial anomaly area of modulation ratio one year before the earthquake, extracts the stress modulation anomaly near the focal area before the moderate and strong earthquake in Songyuan, Jilin Province, and further discusses the relationship between diurnal wave anomaly and small earthquake modulation in the process of earthquake preparation and occurrence.

The results show that: 1)The background change of diurnal wave tide factor of well water level tide response is relatively stable, the anomaly is easier to identify and has a high signal-to-noise ratio. 2)Before the Ningjiang earthquake in Songyuan, Jilin Province, the diurnal wave anomaly of the tidal response of the well water level was synchronous and morphologically consistent, mainly represented by the matching anomaly of three or more stations. 3)The Ningjiang earthquake in Songyuan, Jilin, occurred within 2.6 months after the end of the matching anomaly of the Sunday wave height, with the shortest of only 7 days, and has obvious short-term and imminent characteristics. The duration and amplitude of the anomaly are related to the magnitude of the earthquake. 4)Before the Ningjiang earthquake in Songyuan, Jilin Province, there was a low-value anomaly in the modulation ratio of small earthquakes with M_L≥3.0 in the focus area, which was mainly characterized by short-impending features. It may have the same physical meaning as the lower or lower b-value before the earthquake.
The diurnal wave anomalies in the tidal response of the well water level reflects the change in the stress state within the structure. The modulation ratio of small earthquakes can better reveal that the tectonic stress in the focal area reaches or approaches a critical state. The combined analysis of the two helps identify and capture short-term and imminent anomalies of earthquake precursors. Studying the tidal response of well water level and the modulation of small earthquakes may be an effective way to explore earthquake precursor information related to tidal force triggering during earthquake preparation and occurrence.

GENESIS AND DEEP GEOTHERMAL PROCESS OF MAOYA HOT SPRINGS IN LITANG, WESTERN SICHUAN

SHEN Hua-liang, YANG Yao, ZHOU Zhi-hua, RUI Xue-lian, LIAO Xiao-feng, ZHAO De-yang, LIANG Ming-jian, CHEN Meng-die, GUAN Zhi-jun, REN Hong-wei

2023, 45(3):  689-709.  DOI: 10.3969/j.issn.0253-4967.2023.03.006

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Maoya hot spring, as a famous earthquake monitoring site, is seldomly studied in terms of its genesis and deep geothermal process. In this paper, we investigated the chemical and isotopic composition of thermal water in Maoya and Maohuo in Litang to elucidate the hydrochemical characteristics and genesis of the geothermal waters.
The study results show that Maoya hot springs and Maohuo hot spring are of the Na-HCO₃ type as a result of dissolution processes involving feldspars from the reservoir rocks due to the water-CO₂-rock interaction during the deep circulation of the geothermal waters. According to the diagram of Cl^- and Na⁺ concentrations of the geothermal water samples, Cl^- in Maoya hot spring originates from the mixing of granodiorite and basalt aqueous solutions in the process of water rock interaction, while Cl^- in Maohuo hot spring mainly originates from granodiorite aqueous solutions. The stable isotope δD and δ¹⁸O composition of geothermal waters indicates that they are recharged by meteoric precipitation. The Maoya hot springs have the characteristics of higher concentration of ion components and slightly oxygen drifting compared with the Maohuo hot spring, indicating that they have a deeper circulation depth and experience a stronger water-rock interaction. In addition, the ratio of Cl^-that comes from deep source in Maoya hot springs is higher than that in Maohuo hot spring.
The high temperature geothermal water formed by deep circulation of meteoric water is mixed by the shallow cold water during the ascending process. We employed SiO₂ geothermometer and Si-enthalpy model to estimate the temperature of shallow reservoir after mixing with cold water and the temperature of deep reservoir and the mixing ratio of cold water, respectively. The results suggest that the temperature of shallow reservoir in Maoya thermal field is in the range of 75~103℃ and the temperature of deep reservoir in Maoya thermal field is about 235℃ and the mixing ratio of cold water ranges from 87% to 94%. Based on the temperature of deep reservoir, we calculated the depth of the geothermal cycle in Maoya area, which is close to 5km.
The heat source triggering the formation of this geothermal system mainly originates from mantle and partial melting body of the crust. In addition, Cenozoic granitoid magmatic residual heat and upper crust radioactive heat can also provide additional heat sources. During the process of surface cold water circulation from shallow to deep, on the one hand, it forms deep geothermal water through normal geothermal gradients, and on the other hand, the mantle fluid upwelling below the Litang Basin and partial melting in the middle crust further heat the groundwater to form a high-temperature deep reservoir. The deep geothermal water is transported to the surface along the Litang Fault under the effect of hydrostatic pressure and hydrothermal convection. During ascending process, the first mixing of groundwater with superficial cold water occurred due to the presence of structural cracks in the crust, and the temperature of the mixing water is about 100℃. When the geothermal water migrates to the near surface, it mixes with the pore water and bedrock fissure water in the basin for the second time, and the mixing proportion of cold water increases(about 90%). Finally, it emerges to the surface, forming a group of medium-low temperature hot springs.

GEOCHEMICAL CHARACTERISTICS OF SOIL GAS IN ACTIVE FAULT ZONE AND ITS IMPLICATION TO THE ADJUSTMENT OF REGIONAL STRESS FIELD IN THE SOUTHERN AREA OF LIAONING PROVINCE

WANG Xi-long, LUO Yin-hua, JIN Xiu-ying, YANG Meng-yao, KONG Xiang-rui

2023, 45(3):  710-734.  DOI: 10.3969/j.issn.0253-4967.2023.03.007

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As an important geochemical observation method, the measurement of gas concentration in fault soil is an important geochemical method to reveal fault properties, fault activities and the adjustment of regional stress field.

Historical moderate-strong earthquake activity is frequent in the southern Liaoning Province, especially in Jinzhou Fault and Haicheng Fault, where many mid-strong earthquakes occurred. So, carrying out continuous observation of soil gas concentration in different sections of Jinzhou Fault and Haicheng Fault in the southern Liaoning Province can provide an important means for analyzing fault activity status and regional stress field adjustment and the characteristics of seismicity.
In this study, we laid out 4 cross-fault measurement lines of soil gas in Jiuzhai-Gaizhoubei section and Gaizhoubei-Anshannan section of Jinzhou Fault and 2 measurement lines on the Haicheng Fault. Based on the results of cross-fault soil gas Rn, CO₂ and H₂ concentrations measured 7 times from 2018 to 2022 at totally 6 profiles across the Jinzhou and Haicheng faults, and combined with the research results of geological structure, seismic activity, abnormal development of hydrogen in Panjin NO.1 well, GPS observation, b-value variation in the study area and cross-fault leveling in Jinzhou Fault, the paper discusses the concentration intensity and distribution characteristics of fault soil gases of the study area, and the results are as follows:
(1)The concentrations and distribution patterns of soil gas Rn, CO₂ and H₂ vary greatly in different faults. The concentration of Rn varies from 2.45kBq·m^-3 to 140.73kBq·m^-3, while that of CO₂ is between 0.19%-9.55%, and H₂ are from 0.48ppm to 1 527ppm.
In most cases, the distribution of soil gas concentrations on each profile is of single-peak shape and the soil gas concentrations are higher at the outcrops or hanging wall of the fault. The main reason causing this phenomenon is the difference of the fault properties in different sections and the geological features.
(2)The soil gas Rn, CO₂ and H₂ for the same profile with different periods has a similar consistent variation. The soil gas Rn has a high correlation with CO₂, which means that the soil gas Rn is affected by carrier gas of CO₂ when migrating and redistributing to the ground along the structural crack of the fault.
(3)The spatial patterns for the mean concentrations of soil gas Rn, CO₂ and H₂ in the northern area are higher than those in the southern region within the study area, which could be attributed to the characteristics of geologic structure, underground medium structure and vertical crustal deformation, as well as the geological condition of each site.
(4)The temporal variation for the mean concentration values of soil gas Rn, CO₂ and H₂ of each site has a similar consistent trend, and its dynamic changes show two stages: The mean concentrations of soil gas Rn, CO₂ and H₂ have a gradual increase trend from May, 2018 to May, 2020, but with a downward recovery trend from October, 2020 to 2022. Combined with seismic activity, abnormal development of hydrogen in Panjin NO.1 well, GPS observation, b-value variation in the study area, cross-fault leveling in Jinzhou Fault and other analysis results, we found that this temporal variation of geochemical characteristics in the study area might be mainly controlled by the properties of fault activity, seismic activity and adjustment of regional stress field. When the fault is under the action of continuous tensile stress, the increase of seismic activity and regional stress field could promote the release of fault soil gas, which might be the reason causing the concentration abnormalities of soil gas.
In a word, it is of great scientific significance to carry out the continuous observation of soil gas on the main active faults in the southern area of Liaoning Province. This research proved that the tectonic geochemical observation with frequent multiple phases not only can provide geochemical data for analyzing the characteristics of gas released on the fault zone in the southern area of Liaoning Province, but also can reflect tectonic activity status and the changes of the regional stress field effectively, which means it might be a good tool for earthquake prediction and monitoring of fault movement.

PRELIMINARY STUDY ON CHARACTERISTICS OF SOIL GAS Rn AND CO₂ DEGASSING IN THE MAIN FAULT ZONES OF XIONG'AN NEW AREA

WANG Jiang, CHEN Zhi, ZHANG Fan, ZHANG Zhi-xiang, ZHANG Su-xin

2023, 45(3):  735-752.  DOI: 10.3969/j.issn.0253-4967.2023.03.008

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Based on the analysis and processing of field mobile observation data, soil gas Rn and CO₂ degassing characteristics in the main fault zones of Xiong'an New Area(XNA)and its effects on the regional environment was preliminarily studied. Repeated observations of soil gas concentrations and fluxes in 2020 and 2021 for the seven measurement lines on the buried faults show that strong degassing characteristics exist in the main fault zones of XNA.
The range of variation of the mean soil gas Rn fluxes for each profile is from 71.44 to 335.35mBq/m²·s, and the range of variation of the mean CO₂ fluxes is from 25.96 to 78.23g/m²·d; the range of variation of the mean Rn concentration intensity is from 0.91 to 2.30, and the range of variation of the mean CO₂ concentration intensity is from 1.13 to 2.61, which are comparable to the degassing intensity of soil gas in other typical fault and earthquake zones in the world. Except for the flux of Rn in the Niudong branch fault 1 and CO₂ in the Niudong branch fault 2, the average values of Rn and CO₂ fluxes in 2021 are higher than those in 2020.The maximum variation of soil gas Rn flux is 116%in the Niudong Fault, and the maximum variation of CO₂ flux is 370%in the Niudong Fault; the variation of soil gas Rn concentration intensity is 119%in the Niudong Fault, but there is no significant variation of concentration intensity in other faults in both periods.
The observation and analysis found that the areas of high soil gas concentration anomalies on the three seismic profiles in XNA are highly coincident with the distribution of deep faults, showing a concentrated degassing phenomenon along the fault zones. The AA' seismic profile on the west side exposes three hidden fractures, which are the Taihangshan Fault, Rongcheng Fault, and one unnamed fault on the west side. The AA' soil gas Rn and CO₂ concentration profiles, corresponding to the location of the upward trend line of the Rongcheng Fault and the unnamed fault on the west side, show the characteristics of simultaneous single-peak type high-value anomalies of Rn and CO₂ concentrations. This phenomenon may indicate that the Rongcheng Fault and the unnamed fault on the west side are still highly active.
The BB' soil gas profile shows two areas of high soil gas concentrations respectively in the east and west. The western high value area shows synchronous peak anomalies of Rn and CO₂ concentrations, and the location of the anomalies is basically consistent with the upward extension direction of the Xushui-Dacheng Fault. In the east, only single-peak anomalies in CO₂ concentration are observed, and the magnitude of the anomalies is more significant than that in the western section, but there is no corresponding fault. Therefore, the synchronous peak anomalies of Rn and CO₂ concentrations in the western section should be related to the fault activity of Xushui-Dacheng Fault, while the single-peak anomalies of CO₂ concentrations in the eastern section may be non-tectonic.
The CC' seismic profile contains the Taihangshan Fault, Niudong Fault, Niudong branch fault 1, Niudong branch fault 2, and several secondary faults. The peak anomalies of soil gas Rn concentration in the soil gas profile are detected at different degrees and near the locations corresponding to the upward extension trend lines of Niudong Fault, Niudong branch fault 1, and Niudong branch fault 2. All three peak anomalies of soil gas Rn concentration may be related to the activities of Niudong Fault, Niudong branch fault 1, and Niudong branch fault 2. However, the single-peak CO₂ concentration anomaly is detected only above the Bazhou Depression, and the anomalous area basically overlaps with the area where the Bazhou Depression is located. The Bazhou Depression is one of the main areas of concentrated population in XNA, and the biological activity is relatively strong. According to the results of the existing study that soil gas CO₂ generated by biological activities may also be the main recharge source of CO₂ gas released from fault zones in the basin, the large-scale CO₂ concentration anomalies detected above the Bazhou Depression may also be generated by biological activities in the basin.
The results show that strong degassing characteristics exist in the main fault zones of XNA. The environmental effects of gas release from the main fault zones in XNA are evaluated by combining with the comprehensive prevention and control standards for indoor gas environmental pollution. The highest value of radon gas release in the main fault zone of XNA reaches 675mBq/m²·s in the Rongcheng Fault, and 395.70mBq/m²·s and 334.84mBq/m²·s in the Nudong branch faults respectively, the results indicate that it is necessary to carry out comprehensive radon prevention treatment for the buildings above the Rongcheng fault and Nudong fault zone. The preliminary estimation results of CO₂ release show that the daily contribution of CO₂ degassing from the main fault zones of XNA to the atmosphere is about 1 622.56t, and the annual contribution is as high as 0.59×10⁶t. Attention should be given to its impact on the regional environment.
The research results in this paper are significant for urban planning, environmental management and comprehensive assessment of the environmental impact of gas release from the fault zone in XNA.

SPATIAL DISTRIBUTION CHARACTERISTICS OF SOIL GAS HE CONCENTRATION IN THE EASTERN LIUPANSHAN FAULT ZONE AND ITS RELATIONSHIP WITH TECTONIC ACTIVITY

ZHANG Wen-liang, LI Ying, LIU Zhao-fei, HU Le, LU Chang, CHEN Zhi, HAN Xiao-kun

2023, 45(3):  753-771.  DOI: 10.3969/j.issn.0253-4967.2023.03.009

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The geochemical characteristics of soil gas in fault zone are closely related to regional tectonic evolution and seismic activity. He is a noble gas element with high chemical inertness, diffusivity and mobility, which can migrate from the deep underground to the surface along fractures. Its escape level in the crust is generally controlled by crustal stress. Therefore, soil gas He is an effective tracer, which can be used to study the process before the occurrence of dangerous earthquakes. Monitoring the He in a seismically active area may provide evidence of stress field changes caused by tectonic activity, and better understand the seismic genesis process within the region. The eastern Liupanshan fault zone is located between the Ordos block and the northeastern margin of the Qinghai-Tibet Plateau and is an important part of the northern section of the North-South Seismic Belt. The medium-strong earthquakes in this zone are frequent, and there have been many strong earthquakes above 7 in history.
In order to explore the spatial distribution characteristics of soil gas He concentrations in the eastern Liupanshan fault zone and its relationship with tectonic activities, eight cross-fault soil gas measurement profiles were deployed in the eastern Liupanshan Fault, along which soil gas He concentrations were measured. In the meantime, in order to carry out comparative studies, the Xiaoguanshan Fault located 14km away from the eastern Liupanshan Fault was selected, and three cross-fault soil gas measurement profiles were also set to measure the soil gas He concentration along the survey line. The measurement results showed that the concentration of soil gas He in the eastern Liupanshan Fault is 3.786~7.472ppm, and the average He concentration of eight measurement profiles is between 4.983~6.335ppm. The He concentration range in Xiaoguanshan Fault is 2.168~7.043ppm, and the average He concentration of the three measurement profiles is between 4.784~5.235ppm. The soil gas He concentration in the eastern Liupanshan Fault has a decreasing trend from north to south. The soil gas He concentration in the Xiaoguanshan Fault is the highest in the north of the fault and smaller in the south-central part of the fault. The spatial distribution of He concentration in the north parts of the two faults is higher than that in the middle and south parts of the fault, which is closely related to the difference of regional activity of the faults. In combination with previous research results on the age of fault activity, locking degree, slip rate and tectonic stress in the eastern Liupanshan Fault, it is considered that the activity of the northern segment of the fault is more intense than that of the middle and southern segments of the fault. The comparative analysis shows that the tectonic evolution process of the eastern Liupanshan Fault is similar to that of the Xiaoguanshan Fault, and both faults are thrust type. Therefore, the spatial distribution characteristics of soil gas He concentration in the two faults are consistent. Combined with previous studies, it was found that the spatial variation of soil gas He concentration measured in the eastern Liupanshan Fault was consistent with the spatial variation of soil gas Rn and CO₂ flux. The He concentration of soil gas in the eastern Liupanshan fault zone is close to that in Tangshan area, lower than that in Yanhuai Basin, the middle and southern section of Xiadian Fault and the rupture zone of Wenchuan earthquake, and higher than that in Xinbaoan-Shacheng Fault.

FAULT GAS OBSERVATION AND SURFACE RUPTURE FEATURE INTERPRETATION OF THE M_S7.4 MADOI EARTHQUAKE

WANG Bo, CUI Feng-zhen, LIU-ZENG Jing, ZHOU Yong-sheng, XU Sheng, SHAO Yan-xiu

2023, 45(3):  772-794.  DOI: 10.3969/j.issn.0253-4967.2023.03.010

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An M_S7.4 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 M_S7.4 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 M_S7.4 earthquake may be confined to the shallow crust.

Research paper

ANALYSIS ON EVOLUTION MECHANISM OF TIANZHUANG FAULT DEFORMATION ZONE

HAN Xiao-fei, SHI Shuang-shuang, DONG Bin, XUE Xiao-dong, FAN Xue-fang

2023, 45(3):  795-810.  DOI: 10.3969/j.issn.0253-4967.2023.03.011

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Active fault deformation zones are commonly referred to as fault failure zones. The width of the deformation zone is generally several meters to tens of meters, representing the strongest range of fault activity deformation and the degree of exposure of future surface fracture zones, that is, the severely damaged strip area, which is a key avoidance object for construction projects.
Modern ground buildings(structures)generally have underground engineering that requires excavation of foundations ranging from a few meters to several tens of meters. Hidden faults that cannot be exposed by foundation excavation and whose buried depth is less than 60m may also form fractures on the surface, but the location of the fractures is difficult to determine. At the same time, the long-term creep of the hidden faults and the historical multiple periods of seismicity have formed significant plastic deformation or weak displacement areas near the surface. Therefore, studying the range of hidden fault deformation zones of Quaternary sedimentary layers can provide scientific basis for scientific avoidance of active faults.
The local changes of the Fenhe River channel(surface deformation survey)reflect the stages and stages of tectonic activity in the Taiyuan Basin, mainly including the early stage of the third episode of the Xishan Mountains, the Huangkun movement, and the Gonghe movement, and there are adjustments in two modes of movement: strike slip and tension; Through the precise processing and interpretation of 15 shallow seismic survey lines, and combined with some geological deep hole profiles, the stratigraphic age of the seismic stratigraphic profile was marked, revealing the multi-stage expansion activity of the Tianzhuang fault inverted terrace, and further demonstrating the correctness of the basin's third-phase expansion activity revealed by the changes in the Fenhe River channel.
The Qinghai-Tibetan movement(3.4~1.66Ma BP)in the third episode of Xishan formed the front edge of Tianzhuang fault in Pliocene and the strike slip compression tectonic activity before the Qinghai-Tibetan movement formed the rear edge section. The Huangkun movement(1.2~0.7Ma BP)formed the front edge section of Tianzhuang fault in Middle Early Pleistocene Late Early Pleistocene and the tectonic activity between Qinghai-Tibetan and Huangkun movement formed the rear edge section of Middle Early Pleistocene, The Gonghe movement(after 0.15Ma)formed the front section of the Tianzhuang fault F_2-Qh(creep slip)and the rear section of the Tianzhuang Fault $\mathbb{F}_{1-\mathbb{Q}_{\mathbb{P}}^3} $ in the middle Pleistocene uplift tectonic activity. The section formed by the Qinghai-Tibetan movement was the oldest, the section formed by the Huangkun movement was the second, and the section formed by the Gonghe movement was the latest. The dislocation activity of the section formed by the Gonghe movement occurred in the middle Pleistocene and Late Pleistocene, and the Holocene was dominated by slow seismicity, Shown as weak creep movement, the front edge section of the Tianzhuang Fault F_2-Qh(creep)and the rear edge section of $\mathbb{F}_{1-\mathbb{Q}_{\mathbb{P}}^3} $ are active sections that require attention in urban planning.
In the structural history of the Tianzhuang fault, as a branch of Jiaocheng fault in the NEE direction, together with Jiaocheng fault, controlled the sedimentary process of the basin before the middle Pleistocene in the Qingxu sag. Through this work, the response of the Tianzhuang fault to the Qinghai-Tibetan movement, the Huangkun movement, and the Communist movement has been systematically revealed. Finally, it is considered that the evaluation of the deformation zone formed by the Tianzhuang fault since the latest Gonghe movement(Late Pleistocene)is of practical significance for engineering earthquake resistance and avoiding faults. Through the measurement of fault gas profiles and evaluation of fault gas anomaly zones of the Cross Tianzhuang Fault in Xizancun and Malianying Road, combined with the detailed stratigraphic faulting revealed by the joint drilling profiles of the Cross Tianzhuang Fault in Xizancun and Malianying Road, the extension of the main active section F_2-Qh on both sides of the front edge of the Tianzhuang Fault since the Republican Movement has been determined to be 30m, which is the deformation zone range. The extension of the main active section on both sides of the rear edge of the Tianzhuang Fault is 55m, which is the deformation zone range, Considering the specific needs of engineering seismic fortification, it is advisable to conduct seismic fortification and engineering design based on the deformation zone range on both sides of these two main sections.