Soil H<Subscript>2</Subscript> and CO<Subscript>2</Subscript> Surveys at Several Active Faults in Japan

Soil H₂ and CO₂ surveys were carried out along seven active faults and around the aftershock region of the 2000 Tottori-ken Seibu earthquake in Japan. Diffuse CO₂ effluxes were also measured along one fault and around the 2000 aftershock region. The results show highly variable H₂ concentration in space and time and it seems that the maximum H₂ concentration at each active fault correlates with fault activity as exemplified by the time of the latest big earthquakes. Even though observed H₂ concentrations in four faults were markedly lower than those collected previously in the latter half of the 1970s, it is evident that the higher H₂ concentrations in this study are due to the addition of the fault gases. Comparing the chemical composition of trapped gases (H₂: 5–20% and CO₂/H₂: 0.5–12) in fractured rocks of drill cores bored at the Nojima fault, a soil gas sample with the highest H₂ concentration showed large amounts of the trapped fault gas, diluted with atmospheric component. The profile experiment across a fracture zone at the Yamasaki fault showed higher H₂ concentrations and lower CO₂/H₂ ratios as was observed in soil gas from the fracture zone. A few days after the 2000 Tottori-kei Seibu earthquake, no CO₂ effluxes related to the occurrence of earthquakes were observed at the aftershock region. However, only above the epicenter zone, relatively high H₂ concentrations in soil gases were observed.     

松原5.7级地震震中区土壤氢气变化特征

2018年5月28日吉林松原发生5.7级地震,为了探究地震对断层气的影响以及震后断层气变化特征,震后在震中区进行了多期土壤氢气浓度观测工作,得到了震中区氢气浓度的最大值、背景值等多项指标。结合地震前后氢气浓度数据,分析震中区震前、震后氢气浓度趋势性变化情况,结果显示:在此次地震活动中,氢气浓度变化有"震前缓慢升高—临震下降—震后迅速升高—强余震前再次骤升"的现象;氢气浓度变化与地震活动间有很好的映震关系,主震对氢气浓度的上升起主要作用,而余震则起诱发作用,余震的活动会使赋存在地下岩石裂隙的氢气释放量增大,表明氢气对余震活动的响应较灵敏;另外,震后某一时刻空气中的氢气浓度也会升高,这可能与区域断层中的氢气逸散到大气中有一定关系。     

Measurement of radon gas on major faults in California,USA

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吉兰泰断陷盆地周缘断裂带气体释放及其对断层活动性的指示意义

为了研究吉兰泰断陷盆地周缘断裂带气体排放及其对断层活动性的指示意义,在盆地周缘4条活动断裂上布设了5条土壤气测量剖面和1条电磁测量剖面,观测了土壤气中Rn,Hg和CO₂的浓度、释放通量和地电阻率,对各测量剖面进行了土壤化学组分分析,计算得到了断层活动性相对指数K_Q。研究结果显示:土壤气体CO₂和Rn受渗透性较低的粉砂土阻挡,主要沿结构破碎的断层上盘逃逸,并形成浓度高峰;吉兰泰盆地南缘土壤气Rn,Hg和CO₂的浓度和释放通量最高,可能与盆地西南缘花岗岩中U和Ra的运移以及盆地南缘碳酸盐岩的分解有一定的关系。各测量剖面的断层活动性相对指数K_Q值的变化特征表明,正断层和逆断层的K_Q值大于走滑断层,且巴彦乌拉山山前断裂上的K_Q值最高,揭示其活动性最强,有可能是地震发生的潜在危险区。      

岷县漳县6.6级地震后申都乡地窖宏观异常现象及其机理探讨

2013年7月22日甘肃岷县漳县M_s6.6地震后岷县申都乡某地窖出现冒白色烟雾的宏观异常现象。本文通过土壤气Rn、Hg现场检测和采样后气相色谱检测、生物显微镜观察结果,探讨了气体来源、赋存和运移机理以及异常形成的物理化学机制。认为该点具有灵敏的断层活动响应,可作为今后开展断层活动及未来甘东南地震趋势预测判定的断层土壤气监测点进行长期监测。     

Natural sources of methane and carbon dioxide on Sakhalin Island and their role in the formation of ecological gas-geochemical zones

Two major groups of the lithosphere sources of methane are distinguished, i.e., the lithosphere sources proper (metamorphism, and serpentinization and spreading zones) and organic fuel deposits in the Earth’s crust. Hydrocarbon deposits, mud volcanoes, geothermal systems, and mineral sources are located on Sakhalin Island. All investigated objects are associated with large tectonic faults: the Yuzhno-Sakhalinsky and Main Pugachiovsky mud volcanoes are controlled by the Central Sakhalin fault; the Shakhterskoe coal deposit is confined to the West Sakhalin fault, while the Daginskaya geothermal system and the studied oil- and-gas fields are associated with the Hokkaido-Sakhalin fault. The authors’ studies of natural gases allow allocating two basic zones on the Sakhalin island with different types of hydrocarbon-carbon dioxide sources: Oil-and-gas (mostly methane; north-east) area and Coal-Gas area (carbon-dioxide-methane; west, south-west). Those areas differ in both chemical and isotope composition of natural gases. Some gashazardous areas have also been identified along tectonic faults, where they are due to gas release on the land surface. The populated localities on Sakhalin are shown to be situated near fault zones; therefore, the problem of gas hazard control is of critical importance.     

Abnormal Variation of H2, Hg and Other Fault Soil Gases Before Earthquake

By observing the variation of fault soil gases,we can understand the activity of faults and information on earthquake precursors.Two fault soil gas observation spots were set up at the Xiadian fault east of Beijing and at the Houhaoyao geothermal area in Huailai(Hebei Province)during November-December 1989.Concentrations of fault soil gases H2,Ar,CO2,CH4,Hg,etc.were observed.Before several moderate-strong earthquakes in 1990,the concentrations of fault soil gases such as H2 and Hg all showed abnormal variations quite significantly.In this paper,we mainly discuss the relation of these fault soil gases to earthquake activity.     

乌鲁木齐市城区隐伏断裂气体地球化学探测

利用气体地球化学方法对乌鲁木齐市城区７条隐伏断裂的重点地段进行了１１个剖面的定位探测。结果发现,断裂带中地球脱气现象十分强烈,而远离断裂脱气则迅速减弱。１１个剖面中９个剖面的土壤气氡在断裂上方异常显著,６个剖面的土壤气汞对断裂反映明显。经过数据数据处理和对比分析,有６条隐伏断裂在地面延伸的位置得到了较精确的判定。应当指出,地下水和人为开挖等因素对城市隐伏断裂的气体地球化学探测有较明显的干扰。     

Inducing action of hydrogen migrating along faults on earthquakes

The reduction of the microhardness and the crystal constants of some non-metallic materials, such as calcite, dolomite, antigorite, etc., are observed after a short time of hydrogen permeating treatment at low pressure. It means that hydrogen diffusion can cause their strength dropping or weakening. The hydrogen, which is produced under the earth by various chemical reactions or accumulated when the earth formed, is migrating up continuously along faults, causing weakening of rocks and faults at the same time. So it is possible that rocks and faults break under lower tectonic stress condition. Hydrogen anomalies are passive reflection, precursor and accompaniment of fault activities or earthquakes on the face of it, but hydrogen migrating has active influence on faults and its moving. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 229–235, 1992.     

福州市隐伏断层地球化学试验探测及研究

在城市环境中进行隐伏断层地球化学探测试验是一次新的尝试。试验探测内容包括 :不同地球化学测项 (土壤气汞和土汞、土壤气氡等 )、不同类型测氡仪器 (FD - 12 5 ,FD - 30 17RaA)、不同探测场地 (稻田、菜地、林区、道路、回填土和市区街道绿化带等 )。地球化学与浅层地震勘探结果的比较表明 ,2类异常点的对应率约为 70 % ,表明地球化学方法在福州市环境下的隐伏断层探测中是一种有效的方法 ;地球化学“伪异常”出现的可能部位是小桥边、废弃建筑物地基、路边垃圾堆放地等     

Fault Seal Analysis in an Onshore Unconventional Gas Target,North Perth Basin

During the 1980s, hydrocarbons were logged in aquifers during drilling of conventional gas wells in the Woodada gasfield. The gasfield is located in the North Perth Basin in Western Australia. Using Fault Seal Analysis Technology, our goal was to test the hypothesis that faults in the Kockatea Shale that are currently being reactivated may be leak prone. Wells proximal to faults with a fracture stability of greater than 5 MPa logged only methane. Wells proximal to faults with a fracture stability ≤5 MPa logged both methane and condensate in aquifers confirming that hydrocarbon leakage is correlated with critically stressed faults. This assessment assumes that fault rocks in the Kockatea Shale, which is a regional source rock and seal, comprise uncemented phyllosilicate rock. For the normal stress case, faults oriented west-north-west with moderate dip have the lowest integrity. For the strike slip stress case, faults oriented north-west and west-south-west, with moderate to steep dip have the lowest integrity. If the Kockatea Shale fault rock is assumed to be a cemented phyllosilicate, then the fracture stability increases to 14 MPa for both the normal and strike slip case. In this case, Jurassic-Permian fault intersections may be contributing to hydrocarbon leakage, however, this would require numerical modeling for confirmation. Based on leak off tests, the increase in pressure required to hydraulically fracture the formation varies between 10.7 and 13.8 MPa. The treatment pressures used during hydraulic fracturing may potentially exacerbate leakage in areas such as the Woodada gasfield.     

依兰—伊通断裂北段断层氢气的地球化学特征分析

2017—2018年在依兰—伊通断裂黑龙江段开展断层氢气调查工作,沿断裂选取8个剖面进行跨目标断层观测及土壤氢气采样。结果表明：①断层氢气浓度常存在异常变化;②不同类型的断层,氢气浓度异常变化幅度没有明显差异;③走滑断层异常特征一般为断层两侧低、中间高,呈现“低—高—低”的特点;倾滑断层异常特征一般为断层的上盘略高、下盘略低;④断层氢气排放不仅受微观局部断层的影响,更受宏观的地震活动性大环境的影响。     

Geological Observations of Damage Asymmetry in the Structure of the San Jacinto, San Andreas and Punchbowl Faults in Southern California: A Possible Indicator for Preferred Rupture Propagation Direction

We present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the San Andreas, San Jacinto, and Punchbowl faults in southern California. The observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters, a fault zone scale of meters to tens of meters and related geomorphologic features. The observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters. In all of those scales, more damage is found on the side of the fault with faster seismic velocities at seismogenic depths. The observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks. The data are consistent with a preferred northwestward propagation direction for ruptures on all three faults. If our results are supported by additional observations, asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section. The property of a preferred rupture direction can explain anomalous behavior of historic rupture events, and may have profound implications for many aspects of earthquake physics on large faults.     

合作市隐伏断层控制性地球化学探测场地试验

近年来在城市活断层探测中广泛开展了土壤气探测的工作.本文在合作市周边地区选择2个位于西秦岭北缘断裂西段具有典型断层露头的甘加、黄香沟场地开展了现场土壤气汞、氡浓度测试.主要结论有：（1）断层带存在明显汞、氡浓度异常,曲线的峰值能够很好地对应断层位置,而且其曲线形态能够较好的反映出断层性质与几何形态;（2）异常段上方氡、汞的峰值异常一般比背景值高出1.5倍以上,且具有成组特征（即在断层带上方超过背景值1.5倍以上的点2个以上）,并具有典型正弓形曲线形态;（3）合作市地处草原,地表覆盖层匀一、稳定,外界干扰较小,植被覆盖好,不仅有利于土壤气汞、氡浓度地球化学方法进行隐伏断裂的控制性探测,同时也有利于对已知断裂活动性时空变化特征进行跟踪探测研究.     

Radon anomalies on three kinds of faults in California

Radon emanation is known to be anomalously high along active faults in many parts of the world. We tested this relationship in California during July and early August 1992, using a portable radonmeter to conduct soil-air radon surveys at 5 sites across three kinds of faults: Creeping, locked, and freshly broken.Along a 350-m long survey line across a creeping segment of the San Andreas fault at Nyland Ranch in San Juan Bautista, we found anomalous radon concentrations not in the creep zone itself as determined by a creepmeter, but on the adjacent sides, 10 and 30 meters from the center line of the fault. The anomalous values were 5 times higher than the background values measured farther away from the fault. A similar radon anomaly was observed along a 420-m long survey line across a creeping segment of the Calaveras fault near 7th Street in Hollister. There, the anomalous values were about 6 to 11 times the background values and about 40 and 50 m from the center line of the fault. The double-peaked featire of the anomalies may be indicative of a relatively low gas permeability of the fault-gouge materials in the creeping zones and high permeability of fractured rocks in the adjacent shear zones.Along a 144-m survey line across the currently locked segment of the San Andreas fault at the Earthquake Trail near Olema, the radon concentration was indeed anomalously high in the fault zone, by a factor of two above background values. However, the maximum values (3 to 6 times background) again were recorded about 10 meters from the center line.Three weeks after the magnitude 7.5 Landers earthquake of 28 June 1992, we conducted a survey along a 300-m line across the earthquake fault alongside Encantado Road in the epicenter area. The radon values measured at the two main fault breaks were an order of magnitude higher than the background values. A similar result was found along a 420-m line alongside Reche Road about 1.7 km south of Encantado Road.     

CORRELATION BETWEEN GAS GEOCHEMICAL EMISSION AND FAULT ACTIVITY OF THE YUXIAN-GUANGLING AND KOUQUAN FAULTS

Soil gas emission is closely related to tectonic and seismic activity and has been widely used to track active faults and monitor seismicity in the upper crust. Because active fault plays an important role as the channel of the earth's deep gas upward migration due to its high permeability and porosity, the geochemical characteristics of soil gas in fault zone is a good indicator of tectonic fracture and activity. In order to study the soil gas geochemical emission intensities and its correlation to fault activity, fluxes of Rn, Hg and CO₂ in soil gas and the ground resistivity were surveyed across the Yuxian-Guangling Fault and Kouquan Fault which are both Quaternary active faults in the border area of Shanxi Province, Hebei Province and Inner Mongolia Autonomous Region. In 2017, soil gas fluxes were measured in 2 profiles consisting of 10 and 9 wells of depth of 3.0m across the fault scarps in Yuxian-Guangling Fault and Kouquan Fault, respectively. Resistivity tomography sections were attained by ground resistivity survey with electrode spacing of 5.0m along the profiles of soil gas measurement. The gas geochemical data show that there exist two abnormal flux peaks across the Yuxian-Guangling Fault and one in the Kouquan Fault. The high density resistivity measurement shows that fault breccia and fractured rocks zones are developed under the measured faults, where higher values of soil gas flux are also observed. Fractures with high gas permeability in the strata favor the transfer and migration upward of soil gases, which results in the anomalies of gas flux value. In addition, the anomalies of gas flux values are spatially identical with the occurrence of the fault scarps. The soil gas degassing rate of Yuxian-Guangling Fault is higher than that of Kouquan Fault. The research results of high density electrical prospecting and previous tectonic activity show that low-resistance bodies are more developed and the fault activity is stronger with higher slip rate, which leads to the more intense emission of soil gas in Yuxian-Guangling Fault. The conclusions can be made that soil gas geochemical characteristics and degassing rate in fault zone is closely correlated to the tectonic activity and fracture degree. Combination of geochemical and geophysical methods is an efficient way for the monitoring and study of fault activity to estimate the possible earthquake hazards.     

玛曲断裂带土壤气汞、氡地球化学特征

根据汞、氡土壤气在断层上的形成机制,分析了玛曲断裂带土壤气中汞、氡地球化学异常特征,并依据异常判断了断层的位置、产状、性质.与断层的实际剖面所反映的断层规模与性质基本耦合.为评价活断层提供了有益的参考.     

Discharge of H2 from the Atotsugawa and Ushikubi Faults,Japan, and its relation to earthquakes

The concentration of H₂ in soil gases has been measured weekly at five stations on the Atotsugawa and Ushikubi faults in northern central Main Island, Japan, since 1981 in search of possible relationship with earthquakes. The observed H₂ concentration varies from lower than 1 ppm to 7.8% in time and place. When a large earthquake (M: 7.7, epicenter distance: 486 km) occurred on 26 May 1983, an outstanding discharge of H₂ was observed at all five stations, preseismically at three of them, and coseismically at the other two. Simultaneous H₂ emission was also observed at some stations in seven other occasions. These periods of unusual H₂ discharge nearly coincided with occurrences of major earthquakes in Japan, but not of local minor earthquakes along the Atotsugawa fault. This fault, being a deep fracture zone, may be sensitive to large-scale crustal stress changes which incidentally cause the major earthquakes. Increased H₂ may be produced by rock fracture caused by the increased stresses on the fault and by the earthquakes themselves. Local minor earthquakes along Atotsugawa fault with magnitude lower than 3 may be unable to cause sufficient rock fracture to produce significant H₂.     

A study of the depth of weathering and its relationship to the mechanical properties of near-surface rocks in the Mojave Desert

Weathered granite extends 70 m deep at Hi Vista in the arid central Mojave Desert of southern California. The low strength of this granite is due to the alteration of biotite and chlorite montmorillonite. Deep weathering probably occurs in most granites, although we cannot rule out some anomalous mechanisms at Hi Vista. Geophysical instruments set in these slightly altered rocks are limited by the unstable behavior of the rocks. Thus, tectonic signals from instruments placed in shallow boreholes give vague results. Geophysical measurements of these weathered rocks resemble measurements of granitic rocks near major faults. The rheology of the rocks in which instruments are placed limits the useful sensitivity of the instruments.     

HIGH-ACCURACY ANALYSIS OF SOIL HYDROGEN ANOMALY IN FAULT ZONE

Hydrogen is recognized as one of the most useful gases to detect fault activities. Based on long-term high-accuracy soil hydrogen observation data in fault zones, the paper evaluates the reliability of data according to the distribution of measurements. Through the evaluation of earthquake-reflecting ability of hydrogen concentration, we consider that there is a certain corresponding relationship between hydrogen concentration and seismic activity and we present the judging index for this anomaly. Hydrogen concentration characteristics with the earthquakes within the range of 350km around the station were analyzed, especially the two earthquakes, which occurred on October 24, 2010 and March 8, 2011 in Taikang, Henan Province, with magnitude M_S4.6 and M_S4.1 respectively. The observation station is located at Xiaxian in Shanxi Province, 300km away from the epicenter. In a week before the two earthquakes, high-accuracy soil hydrogen concentration measurements showed similar anomaly variation, which was increasing abruptly, then decreasing, and after the earthquakes it returned to background level. Overall, the changing scope was more than 20 times of the background value. We concluded that the anomaly was affected by tectonic setting of the earthquakes. The similar hydrogen distribution pattern recorded at the same station is attributed to the same tectonic position and focal mechanism solution. The hydrogen could be an effective tool for short-term and imminent earthquake prediction, which provides reference for short-term and imminent earthquake prediction in areas with high earthquake risk.