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.

The soil gas concentration and escape rate value can sensitively and objectively reflect the underground state of stress, strain and tectonic activity. In addition, abnormal phenomena of fault soil gas often occur before and after seismic activity. It is often used to identify the active state of fault zones, explore hidden faults and assess earthquake risk.
As an important geochemical method, the soil gas measurement is an important geochemical method to reveal fault properties and fault activities and other tectonic activities. In this study, we laid out 8 measurement lines of soil gas along the Borokonu-Aqikkuduk Fault, the Kusongmuchike piedomont fault, the Dushanzi-Anjihai Fault, the Horgos-Tugulu Fault, the Kashi River Fault and the Nalati Fault in two earthquake risk areas of the north Tianshan Mountains in Xinjiang, namely, the “North Tianshan Wenquan-Jinghe M7 earthquake risk area” and the “Wusu-Hejing M6 earthquake risk area”. From 2017 to 2020, a total of 6~7 phases of measurements were carried out to make clear the distribution characteristics of Rn, CO₂ and Hg concentrations along these faults. There have been many moderate/strong earthquakes near the above-mentioned faults, and it is of great significance of soil gas measurement on these faults for us to gain a deep understanding of the fault activity characteristics and earthquake risk.
In this paper, the spatial distribution characteristics of fault soil gas are analyzed based on the multi-period measurement results, and the activity of the fault zone and the regional earthquake risk are discussed respectively. The results show that: 1)The Rn concentrations are more stable than that of CO₂ and Hg along each measurement line, which can be used as an effective indicator gas for analyzing the distribution of fault zones, indicating the location of fault fractures and judging the activity of faults. Since there are many interference factors of CO₂ and Hg concentrations, they can be used as an auxiliary means. In most cases, the distribution of Rn concentrations on other measuring lines is of single-peak shape, indicating that the soil gas concentration is higher at the outcrops of the fault. However, the concentrations of Rn in the Kusongmuchike piedmont fault and the Horgos-Tugulu Fault are higher, and the distribution curve of Rn concentrations shows multiple high-value forms, indicating that there are other fractures and broken positions on the fault zone besides the fault exposure position. 2)The highest Rn concentrations on the measuring lines of the Kusongmuchike piedmont fault, the Nalati Fault and the Horgos-Tugulu Fault are 99 802Bq/m³, 80 549Bq/m³, 78 834Bq/m³, which are not only higher than the Rn concentration of other measurement lines in the same period, but also higher than the highest Rn concentration of 58 205Bq/m³ in the Hutubi North Fault. The fault activity is relatively stronger. 3)The earthquake risk of Wenquan-Jinghe area is relatively low, with relatively high regional stress accumulation. In addition, the fault activity in this area is intensive, and moderate to strong earthquakes are more likely to occur, so there is a certain earthquake risk.
In a word, it is of great scientific significance to carry out the activity detection and seismic risk assessment of the main active faults in the northern Tianshan area of Xinjiang. In the future, monitoring and in-depth research on the geochemistry of fault soil gas in the “North Tianshan Wenquan-Jinghe earthquake risk zone” is of great significance for judging the earthquake risk in the north Tianshan area. The results of this paper provide geochemical data for analyzing the characteristics of gas released by the fault zone in the northern Tianshan area of Xinjiang, and for guiding the selection and layout of seismic stations, as well as for seismic situation tracking and anomaly ascertainment.

Mud volcano is a conical sedimentary body formed by high-pressure mud and gas-dominated fluid migrated to the surface through faults and other channels deep underground, which looks like a volcanic cone formed by magma-volcanism. As a product of crustal movement, mud volcano can bring a large amount of valuable information from deep to the surface when erupting. Therefore, mud volcano is called "god-given borehole" with a depth of 7~12km. Mud volcanoes are the result of upthrust of trapped gases released by the pressure in the stratum and also the channel for the upward migration of gases in the earth. The submarine mud volcano is one of the signs of hydrate and the living evidence of hydrate. The Wusu mud volcanoes are located in the northern Tianshan tectonic belt. Since the mud volcamoes locate in the active part of the tectonic belt and are well connected to the underground, their active degree has a good correlation with the seismicity. The earthquake cases studies based on the 7a long real-time macroscopic monitoring data and the more than 3a long geochemical monitoring data of the Wusu mud volcanoes show that in the earthquake cases of M_S ≥ 5.0 within the range of 300km around the Wusu mud volcanoes, the abnormal mud gushing quantity obviously increased by macroscopic monitoring before 9 out of 13 earthquakes. The geochemical microcosmic monitoring data showed obvious abnormal changes before 3 out of 6 earthquakes. The anomalous duration from the emergence of the anomaly to the occurrence of the earthquake is mainly of the mid-term(6~12 months). Before the Jinghe M_S6.6 earthquake on August 9, 2017, the Wusu mud volcanoes spewed violently and the chemical components showed an obvious high value anomaly. In January 2017, there was a significant increase in the amount of mud spewing in Aiqigou No.1mud volcano and Baiyanggou No.1mud volcano, and one month before the earthquake, there was the phenomenon that mud gushing amount of Aiqigou No.2 mud volcano gradually increased and the volcano was from dormant to active. There were obvious high values appearing before the earthquake in F^-and SO₄^2- in the Aiqigou No.1mud volcano and in F^-, CO₃^2-, SO₄^2-, Rn(gas), CH₄, Ar and N₂ in Baiyanggou(No.1 and 2)mud volcanoes. The values of F^-, CO₃^2-, SO₄^2-, Ar and N₂ showed short-term anomalies, while CH₄ and Rn(gas)showed medium term anomalies. Giggenbach triangular diagram (Na-K-Mg) indicates that the water-rock reaction of Baiyanggou mud volcanoes is complete and little disturbed by the outside. The water-rock reaction of the Aiqigou mud volcanoes is still ongoing, which can explain why the precursor anomaly of the chemical components of the Baiyanggou mud volcanoes is more obvious than that of the Aiqigou mud volcanoes. The geothermal reservoir temperature of the study area is estimated by using a cationic (Na-K, K-Mg, Na-K-Ca) geothermometer. The geothermal reservoir temperature of the Wusu mud volcanoes is about 70℃, and the circulation depth is about 3km. In the process of earthquake preparation, the mud carries deep chemical components to the ground surface due to the effect of compression stress(the result of focal mechanism)or the concentration of regional tectonic stress with earthquake preparation; Or the rock strata in or near the seismogenic area are deformed, the depth of liquid circulation will increase, and the water-rock reaction will be accelerated, which will increase the concentration of some ionic components, and the squeezing process will cause a large number of mud to gush out of the ground, carrying geochemical components. Therefore, the gushing quantity and some chemical components of the mud volcanoes were obviously abnormal before the earthquake.

The migrating and enriching of fault gas during dynamic load-unload process are important indexes to evaluate the stress state and tectonic activity of underground medium. The Hutubi underground gas storage provides a natural experiment site for the analysis of the relationship between the gas geochemistry and the stress-strain status. In this paper, the soil gas concentrations of Rn, CO₂, Hg and H₂ during the gas injection in the Hutubi underground gas storage were analyzed. The results show that the soil gas contents and changing trend are close to the background value in the non-reservoir area and fault zone, which may reveal the weak activity of the fault. Significantly higher concentrations of soil gas H₂ and Hg are observed in the gas storage area, where H₂ maximum reaches 5.551×10^-4 and Hg maximum reaches 53ng/m³. Moreover, the abnormal soil gas H₂ and Hg measurement locations are more consistent. The variation trends of soil gas Hg, H₂, Rn, and CO₂may be related to the different gas generation and response mechanisms. The concentrations of soil gas H₂ and Hg are sensitive to the variation of pressure and the development of cracks in the underground gas storage, and they can reveal gas injection's effect on fault activity. This study provides a new basis for analyzing the influence of gas injection and withdrawal in Hutubi underground gas storage on fault activity.

Mud volcano is a kind of structural geological phenomena under certain hydrogeological environment and can bring plenty of valuable information to the ground when it erupts, therefore, many researchers call it as "Heaven granted well" whose depth can be up to 12km. Mud volcanoes in Xinjiang are distributed in the central-west region of North Tianshan, and five of them are representative, namely, Horgus, Dushanzi, Wenquan, Poplar valley, and Sailetike. We tested the gas, fluid and solid components of these mud volcanoes through investigations and studies of topography and geomorphology, geological and hydrogeological conditions, and mud debris characteristics, and preliminary obtained the origin of these mud volcanoes based on geochemical features. Finally, the paper describes briefly that the continuous enchancement of regional crustal tectonic stress can not only give rise to the seismogenesis and earthquake occurrence, but also break the original cycle of mud volcano to bring about significant activity, therefore, the two have a certain homology relationship.

The mud volcanoes are located in the piedmont downwarp belt of the North Tianshan Mountains in Xinjiang, and all developed at the axis of the anticlinal fold. In this study, based on the geological background and activity of mud volcanoes, we carried out a detailed field investigation on these mud volcanoes at four sites, including Baiyanggou, Aiqigou, Dushanzi and Huoerguosi, and analyzed the particle size, microscopic morphology and composition of the collected samples. The results show that the eruptive types of these mud volcanoes in this area include mud cone, mud shield, mud basin, mud pool and mud hole. The mud liquid surfaces are calm, and bubbling frequency is low. The largest diameter of the nozzle is 2.5m, and the farthest distance of discharged mud flow is about 6.5 meters. The particle sizes of the samples with the similar average sizes are mostly in the range of 0.3～100μm, the medium sizes focus on 5～10μm, and all of the samples are characterized by poor sorting. The column diagrams are generally unimodal distribution and the samples show quite positive skewness in particle size distribution symmetry. The matrix is composed mainly of clay minerals, containing <15% mineral grains mainly of quartz and feldspar, and there are some iron particles in Baiyanggou and Aiqigou samples. According to the analyses, it is proposed that the mud volcanoes in North Tianshan Mountains have the same material source, which is from the middle-lower Jurassic strata, and the high pressure in the strata can be attributed mainly to tectonic activity in the region. At present, the activity of mud volcanoes is relatively weak, and it is unlikely to cause a serious disaster in the near future.

The new No.5 well is a petroleum well with 2260m depth, and it spewed nine times from 1978 to 2004. During a blowout, a lot of natural gas with a little of oil and oil wax were spewed together with underground water; the tiptop height of water column was more than 10m. Based on the statistics, the blowout phenomenon of well did not have obvious cycle characteristics.Based on the analysis of earthquake cases, the authors selected the earthquakes that occurred in the half year after the No.5 well had spewed to study the relation between earthquake and blowout of well. The result shows that the corresponding rate for M5 earthquake is 33.3% in the range of 100km, 0 for M5 earthquake in a range of 100km to 300km, 0 for M6 earthquake in the range of 500km, and 60% for large earthquake or strong earthquake swarm in the range of 1000km.The probable reason was explained. Finally, the authors raised their standpoint about earthquake tendency concerning the blowout of No.33 well in Wusu and the new No.5 well again in 2005.