Groundwater Anomalies Research Articles

Expulsion of geopressured hydrothermal system along active faults and its relation to the occurrence of earthquakes in the Shinanogawa seismic belt, Japan

The 1995 Northern Niigata Earthquake (M 6.0) occurred at a shallow depth in the Niigata seismic gap. The anomaly areas in temperature, electrical conductivity and Cl- concentration of groundwater trend northeast as linear distribution in the epicentral area and are approximately coincident with the area of the seismic intensity 6 (JMA scale). The distributions of seismic intensity 6 and groundwater anomalies convincingly imaged the presence of a buried active fault beneath the epicentral area. The occurrence of this earthquake and the anomalies of groundwater were related to the expulsion of geopressured hydrothermal system (GHS). All epicenters of the destructive earthquakes along the Shinanogawa seismic belt are actually located in the buried active fault zones characterized by the areas of temperature and geochemical anomalies of groundwater. These earthquakes might have been triggered by the activity of GHS. The expulsion of GHS along an active fault in combination with the thermal softening of fault zone attributed to high rock temperature may reduce fracture strength of the rock, and trigger earthquake occurrence. The dimension of the anomaly area of groundwater temperature at the epicentral area reflected the scale of the earthquake fault. The linear anomaly areas of groundwater resulting from the expulsion of GHS and having no historical destructive earthquake are of the predicated areas of future destructive earthquakes. Monitoring of groundwater conditions in these areas may provide useful information regarding the future occurrence of earthquakes.

Recurrence anomaly of ground water behavior before strong earthquakes in North China

By analyzing the relationship between ground water behavior and strong seismic activity during the past more than 20 years in North China, we have found similar water level descending variation of a part of wells in the short-term stage before several strong earthquakes. The characteristics of anomaly are: at the beginning, water level dropped abruptly or accelerated to drop; then it turned to slow rising with a smaller amplitude than that of descending; earthquakes occurred during the slow-rising process of water level, and at that time or before earthquake occurrence, water level rose with a large amplitude. Among more than 100 wells in North China, the descending anomalies were not recorded for many times, but similar variation processes of water level were noted at different wells before several strong earthquakes, which proves that seismic precursory anomalies of ground water are of certain recurrence features, occurring repeatedly before different strong earthquakes. Therefore, it is necessary to study the genesis of this type of anomaly and its relationship with strong seismic activity.

A clear case of radon anomaly associated with a micro-earthquake event in a Stable Continental Region

Though many worldwide attempts to establish radon anomalies in soil and groundwater as precursors for major earthquakes are made, yet practically no such work has been reported for micro‐earthquakes in a Stable Continental Region (SCR). From the continuous radon measurements for five months at a depth of 1 m in fractured granitic area, we report for the first time that enhanced radon concentration preceded seventeen hours the occurrence of micro earthquake. The peak radon level (twice the normal concentration) was reached thirteen hours before the occurrence of the event and decayed exponentially during the next 5 days to come to normal level. This radon precursor pertains to a dry period, unaffected by meteorological factors, thus exemplifies clearly its association with micro earthquake.

Groundwater flow in playa lake environments: impact on gold and pathfinder element distributions in groundwaters surrounding mesothermal gold deposits, St. Ives area, Eastern Goldfields, Western Australia

Geochemical sampling of groundwater may be an effective tool in exploring for Au deposits in areas of transported overburden. However, to use hydrogeochemistry effectively, we need to understand which elements are useful pathfinders and how their distribution patterns are affected by groundwater flow and geochemistry, especially in areas that have hypersaline groundwater. A hydrogeochemical survey was completed over the strongly Au-endowed St. Ives area of the Eastern Goldfields of Western Australia. Extensive geochemical and hydrogeological data were measured from 80 holes drilled on a kilometre-spaced grid. A numerical model was developed to quantify the groundwater flow regime in the study area. Also, hydrogeological data in combination with the geochemistry of the waters were used to identify a complex flow regime adjacent to Lake Lefroy, a playa lake in the area. Regional brine flows towards the salt lake, evaporation occurs and hypersaline (playa) brines are formed that sink and flow away from Lake Lefroy and beneath the regional brines. Density-driven, convective mixing of these two brines is responsible for forming a zone of brines with transitional compositions located near the edge of Lake Lefroy. Knowledge of the groundwater flow regime was used to interpret the distribution of elements around bedrock Au mineralization. An Au-only groundwater anomaly (>10 ppt, maximum 52 ppt) is located downstream of the Junction Au ore body. No anomaly was identified in deep brines associated with the nearby Argo/Apollo Au ore body; however, shallow regional brines sampled in a previous survey showed elevated Au values overlying and downstream of the mineralization. The lack of elevated Au concentrations in deeper groundwater associated with Argo/Apollo is because the samples collected were hypersaline playa brines sourced from Lake Lefroy that had not flowed over the mineralization. Possible pathfinder elements, such as Sb, Bi and Te, were either present in concentrations too low to detect or showed no spatial pattern related to known mineralization; however, a strong correlation was noted between some of their concentrations and salinity. Evaluation of the groundwater anomalies observed in this study suggests that hydrogeochemical exploration is most effective at a resolution of 0.5–4 km spacing, and does not require fully cased and packed wells, at least in cases where no confined aquifers are intersected in drilling. The results of this study illustrate the importance of understanding groundwater flow regimes in planning and interpreting hydrogeochemical surveys.

The 1995 Northern Niigata Earthquake with special attention to a geopressured hydrothermal system

The 1995 Northern Niigata Earthquake of magnitude (M) 5.5 occurred at the eastern margin of the Niigata seismic gap and might have been a precursor of a large destructive earthquake. The anomaly areas in temperature, electrical conductivity and Cl‐ concentration of groundwater were approximately coincident with the area of the seismic intensity 6 on the Japan Meteorological Agency scale, and convincingly demonstrated the presence of a buried active fault beneath the epicentral area, as was suggested by a linear distribution of seismic intensity 6. These anomalies of groundwater were created by the expulsion of geopressured hydrothermal water along an active fault. Anomalies in local groundwater and hotspring systems associated with the earthquake and the proximity of the earthquake to the Niitsu oil field led to an interpretation that the earthquake might have been triggered by activity within the geopressured hydrothermal system. The accumulation of geopressured hydrothermal water in combination with high rock temperature might reduce fracture strength of the rock, and trigger earthquake occurrence.

Test-sites for earthquake prediction experiments within the Colli Albani region

In this paper we discuss some geochemical data gathered by discrete and continuous monitoring during the 1995–1996 period, carried out for earthquake prediction test-experiments throughout the Colli Albani quiescent volcano, seat of seismicity, selecting some gas discharge sites with peri-volcanic composition. In particular we stressed the results obtained at the continuous geochemical monitoring station (GMS I, BAR site), designed by ING for geochemical surveillance of seismic events. The 12/6/1995 (M=3.6–3.8) Roma earthquake together with the 3/11/1995 (M=3.1) Tivoli earthquake was the most energetic events within the Colli Albani - Roma area, after the beginning of the continuous monitoring (1991) up today: strict correlation between these seismic events and fluid geochemical anomalies in groundwater has been discovered (temperature, Eh, 222Rn, CO 2, NH 3). Separation at depth of a vapour phase, rich in reducing-acidic gases (CO 2, H 2S, etc...), from a hyper-saline brine, within the deep geothermal reservoir is hypothesised to explain the geochemical anomalies: probably the transtensional episodes accompanying the seismic sequences caused an increasing and/or triggering of the phase-separation process and fluid migration, on the regional scale of the Western sector of the Colli Albani, beyond the seismogenic depth (2–4 Km) up to surface. We draw the state of art of GMS II monitoring prototype and the selection criteria of test-sites for earthquake prediction experiments in the Colli Albani region.

Groundwater Anomalies Associated with the 1995 Hyogo-ken Nanbu Earthquake.

There have been many reports on earthquake-induced groundwater anomalies, but the mechanism of these anomalies is not very clear. The 1995 Hyogo-ken Nanbu earthquake, which occurred on January 17, 1995, was one of the greatest shallow inland earthquakes in Japan since the modern Japanese observation system for monitoring seismicity was established. Many groundwater anomalies have been observed following this earthquake. Our purpose was to study these anomalies systematically and to gain some knowledge of their mechanisms. We initiated a questionnaire survey in and around the source region on March 8, 1995. We also investigated the discharging waters which appeared on Awaji Island after the Hyogo-ken Nanbu earthquake. These results were compared with the volumetric strain changes estimated from fault models for the earthquake which stretch in a SW-NE direction. Many post-seismic rises in the level or discharge of groundwater as well as a few post-seismic drops were detected by the questionnaire and the investigation. However, their distribution did not coincide with the distribution of the volumetric strain changes, which are of a quadrantial type because the fault models are of a right-lateral strike-slip type. Several precursory groundwater anomalies were also reported, although few pre-seismic crustal strain changes were observed. Therefore, the volumetric strain changes do not always explain the groundwater anomalies that accompanied the 1955 Hyogo-ken Nanbu earthquake.

Development of subcracks betweenen echelon fractures in rock plates

Uniaxial compression tests on marble plates containing two prefabricated en echelon fractures were performed in this study. Photographs showing the typical characteristics of subcrack development were taken under direct scanning electron microscope (SEM) observation during the test. From these photographs, the effects of the length L of a single fracture, the separation distance d and overlapping proportion a/L between two prefabricated en echelon fractures on the development of subcracks were analyzed. The results show that the interaction between en echelon fractures strengthened with decreasing d. For d ≥ L, there was little interaction and the development of subcracks near one of the en echelon fractures was unaffected by the existence of the other. Whereas for d < L, the number of subcracks in the area intermediate between en echelon fractures tended to increase with increasing a and decreasing d. In order to ascertain whether the experiment can furnish some implications for research on earthquake prediction and neotectonic activity, the test results were compared with the spatial-temporal development of foreshocks and ground-water anomalies before the 1975 Haicheng earthquake and the structural framework of the Ganzi pull-apart basin. The results of the comparison are encouraging. According to the similarity between the test results in the laboratory and the natural phenomena in the field, the fault on which the Haicheng earthquake would occur, could be inferred immediately from the relative geometry of the two sub-parallel active faults in the area. Thus, it is considered that the test results would advance our understanding of the process of neotectonic activities and give us inspiration for earthquake prediction.

SEM study of fracture development in a compressed marble specimen and implications for earthquake precursors : Zhao Yong-Hong, Huang Jie-Fan & Wang Ren, Acta Geophysica Sinica, 36(4), 1993, pp 453–462

Uniaxial compression tests of marble plates each containing a slot were conducted under direct SEM observation. The specimen was 20× 10×2.5 mm3 in size. The slat was 7 mm long and 0.44 mm wide and cut through the specimen normally at an inclination of 45° to the loading axis. The initiation, propagation, plan distribution and coalescence of cracks on the specimen surface were monitored throughout the whole test process. From the microphotog-raghs taken at different stages of loading, the opening and closure of microcracks in the frontier area of a propagating fracture were studied, and the variations of fracture width with the loading level and with the position of measuring point were analysed. Based upon the test results, the significant deviation of the fracture energy needed to propagate a macrocrack in brittle rock materials from the value estimated by the theory of linear elastic fracture mechanics was explained, the concept of using a scalar as the damage parameter in rock damage mechanics was put forward and the possible significance of the test results for understanding various earthquake precursors, such as crustal deformation and groundwater anomalies, etc., was investigated. By comparing the test results with the precursory anomalies of groundwater level prior to the 1975 Haicheng earthquake (Ms = 7.3), it was found that the temporal-spatial distribution of groundwater level anomalies had showed many features closely resembling the sequence of crack activities observed in our test.

Radon anomalies in soil-gas and groundwater as earthquake precursor phenomena : H. S. Virk & B. Singh, Tectonophysics, 227(1–4), 1993, pp 215–224

Radon anomalies in soil-gas and groundwater as earthquake precursor phenomena : H. S. Virk & B. Singh, Tectonophysics, 227(1–4), 1993, pp 215–224

Numerical Relationship between Surface Deformation and a Change of Groundwater Table Before and after an Earthquake

AbstractThe purpose of this study is to estimate the effect of earthquakes upon a groundwater flow around a repositories for high-level radioactive wastes. Estimation of a groundwater flow change before and after an earthquake or a volcanic eruption is one of the issues for a long-term safety assessment of the repositories. However, almost any systematic investigation about the causality between a groundwater flow change and an earthquake or an eruption was not found, and as well no estimation formula has been published. We succeeded to obtain a primitive relationship between a groundwater change and an earthquake in this study.Our study consists of three stages. First, several survey reports which describe field observation results of groundwater anomalies caused by earthquakes or eruptions have been collected. The necessary data have been read from the literature and systematically arranged. Second, source mechanisms of the corresponding earthquakes were inspected and static displacements at the well positions were calculated by the dislocation theory in the seismology. Third, parametric studies among the parameters of groundwater anomalies and earthquakes were carried out to find a numerical relationship between a couple of them. Then, a preliminary relationship between water table change in a well and static displacement at the well position was found.We can conclude that temporary change of water table seems to depend on the norm of displacement vector. In this relationship, the maximum value of water table change would be approximately one hundred times of the displacement.

Radon anomalies in soil-gas and groundwater as earthquake precursor phenomena

Earthquake-related changes in soil-gas and groundwater radon have been detected at a sensitive observation site in the Kangra valley, Himachal Pradesh, India, using both instantaneous and time-integrated techniques. The recording station at Palampur in the Kangra valley is located near the main boundary fault of the Himalayas. Eleven earthquake-related anomalies have been recorded since August, 1989 in both the soil-gas and in the groundwater simultaneously. In order to differentiate the real signal from noise, the effect of meteorological variables (soil and air temperature, rainfall, pressure, humidity and wind velocity) on radon emanation was also studied. Most of the radon anomalies are found to be correlated with earthquakes rather than to changes in meteorological conditions.

地震予知のための地下水テレメータ観測システム

Since the Tokai district was designated as an“ntensified Observation Area” in 1974, many kinds of observation for the purpose of earthquake prediction have been done by various institutes. Among them, automatic data collection and real time monitoring of the changes in groundwater level and radon concentration were started in 1976 by Geological Survey of Japan (GSJ). These data are automatically transmitted through the telephone line, and received by an engineering work station at GSJ, in Tsukuba. During these observation period, we have detected some ground water anomalies caused by earthquakes.This system is constructed with taking the following points into account.1) Water level is observed in pressured ground water. The ground water, that is sensitive to barometric pressure and earth tide, is expected fairly sensitive to an earthquake.2) Radon is observed in self-flowing ground water, because it is difficult to control pumping power for non-self-flowing ground water and distinguish radon anomalies from pumping rate fluctuations.3) Aiming low operation cost and flexibility of the system, data transmitting path has been changed from simple conventional telemetering equipment to computer communication. These computers on the observation sites are sampling data every 2 minutes, and transmitting data every hour.4) The computer at data collecting center has been substituted from mini-computer system to engineering work station with LAN system. So, high quality data processing can be performed on specified independent computer system.

地震に伴う地下水・地球化学現象

Hydro-geochemical anomalies associated with large earthquakes are reviewed and discussed from the viewpoints of (1) the response of confined groundwater to minute deformations of crustal rocks, (2) ground-water anomalies related to earthquake source mechanisms, (3) origins of groundwaters and dissolved gases in them, and (4) factors to cause groundwater gas anomalies.Theoretical models for deformations of water-saturated porous or cracked rocks under the undrained condition indicate that compressive and extensive stress changes induce increasing and decreasing changes in the pore water pressure, respectively. The pore pressure change is to be the same order of the external stress change, but the maximum change of pore pressure cannot exceed the stress change. These are consistent with observed responses of confined groundwaters to the earth tides.Coseismic groundwater anomalies accompanied by the 1946 Nankaido earthquake and the 1923 Great Kwanto earthquake are directly related to their residual fields of megathrusting faults. Preseismic groundwater anomalies of these great earthquakes commonly suggest that megathrusting fault motions began to start preceding main seismic events.Chemical compositions of groundwater gases are controlled by admixing of the dissolved atmospheric air with the subsurface gases enriched in CH4, He and others. Groundwater gas anomalies can be described by the change in mixing ratio of the two different gases and/or by the compositional change of the admixing subsurface gas itself. Such an admixing process could be affected by anomalous pore pressure distributions induced by the deformation of crustal rocks related to earthquakes. H2 gas anomalies are distinctive among subsurface gas anomalies reported so far, but the origins are not fully understood. The characteristics of H2 anomalies are discussed.

Uranium anomaly evaluation in groundwaters: a hydrogeochemical study in the Nisa region, Portugal

The hydrogeochemistry of the Nisa region, central Portugal, an area of known U mineralization, was studied. The geology of the region comprises an S-type high-level granite intrusion with vein-type U-mineralization,its contact-metamorphic aureole developed in a schist host rock containing disseminated U-mineralization. An integrated multi-variate statistical, hydrogeochemical and thermodynamic approach was adopted for the interpretation of groundwater data from the granite and its contact aureole. The U groundwater anomalies are evaluated within the local hydrogeochemical context. One-hundred-and-fifteen water samples were collected and analyzed for pH, Ec, Eh, Na, K, Ca, Mg, Fe, Mn, Al, U, Li, Ba, Sr, Cl, HCO 3, SO 4, NO 3, PO 4, F, DOC and H 4SiO 4. Uni- and bi-variate statistical studies of these dilute waters indicate that mainly surface and land-use characteristics control the dominant chemical processes, with water-rock interaction playing a lesser role. Granite/schist compositional differences show up especially in the Mg and HCO 3 content of the waters. Multi-variate statistical analyses (Principal Component Analysis and Q-mode Cluster Analysis) were carried out separately on the granite and schist water groups. Evapotranspiration is an important process both in the schist and granite environment. The impact of fault/fracture zones, land-use, oxidation/reduction reactions and water-rock interaction is different for the waters from each bedrock type. For both rock types, the waters can be classified into genetic groups or clusters ranging from surface runoff/shallow interflow, through surface, agriculture, and infiltrated, to village-pollution type water. The distribution pattern of these groundwater types is not disrupted by the granite-schist contact, allowing for an interpretation of the U distribution in the entire study area. Thermodynamic calculations indicate that the predominant U species in solution is UO 2(HPO 4) 2 2−. Virtually all waters are undersaturated with respect to U minerals. The distribution of U in the waters is thus not influenced by precipitation of U-bearing minerals. The distribution of U in the water samples as well as that for U/Ec (in an attempt to correct for evapotranspiration) are evaluated. The U- and U/Ec-anomaly patterns are considerably biased by bedrock differences and varying genetic history. Interpretability is greatly improved if these differences are taken into account by defining U-threshold values for each genetic water cluster. All known radiometric anomalies are reflected in the hydrogeochemical U-anomaly pattern. The general hydrogeochemical features allow for a refined evaluation of the known radiometric anomalies and of the additional anomalies found in the present study. Additional hydrogeochemical anomalies from waters reflecting depth characteristics are attractive exploration targets.

Geochemical precursors to seismic activity

Studies of earthquake precursory phenomena during the last several decades have found that significant geophysical and geochemical changes can occur prior to intermediate and large earthquakes. Among the more intensely investigated geochemical phenomena have been: (1) changes in the concentrations of dissolved ions and gases in groundwaters and (2) variations in the concentrations of crustal and mantle volatiles in ground gases. The concentration changes have typically showed no conanomalies trend (either increasing or decreasing), and the spatial and temporal distribution of the observed anomalies have been highly variable. As a result, there is little agreement on the physical or chemical processes responsible for the observed anomalies. Mechanisms proposed to account for precursory groundwater anomalies include ultrasonic vibration, pressure sensitive solubility, pore volume collapse, fracture induced increases in reactive surfaces, and aquifer breaching/fluid mixing. Precursory changes in soil gas composition have been suggested to result from pore volume collapse, micro-fracture induced exposure of fresh reactive silicate surfaces, and breaching of buried gas-rich horizons. An analysis of the available field and laboratory data suggests that the aquifer breaching/fluid mixing (AB/FM) model can best account for many of the reported changes in temperature, dissolved ion and dissolved gas concentrations in groundwater. Ultrasonic vibration and pressure sensitive solubility models cannot reasonably account for the geochemical variations observed and, although the pore collapse model could explain some of the observed chemical changes in groundwater and ground gas, uncertainties remain regarding its ability to generate anomalies of the magnitude observed. Other geochemical anomalies, in particular those associated with hydrogen and radon, seem best accounted for by increases in reactive surface areas (IRSA model) that may accompany precursory deformation around the epicenter of an impending earthquake. Analysis of the probable response of these models to the earthquake preparation process, as well as to other environmental factors, suggests that geochemical monitoring programs can provide information that may be valuable in forecasting the probability of an earthquake; however, because of the complexity of the earthquake preparation process, the absolute prediction of seismic events using geochemical methods alone, does not presently appear to be feasible.

Salinity and Temperature Anomalies over Structural Oil Fields, Carter County, Oklahoma

The relationship between high ground-water salinity and high bottom-hole temperature anomalies and deep-seated structures was investigated in Carter County, Oklahoma. A modified spontaneous potential method was used to compute salinity from 700 well logs at a depth of 300 ft (90 m). This depth was chosen because it was within reach of truck-mounted rigs, yet below brine-polluted ground water. The hydraulics of the shallow aquifers were also investigated to determine the origin of the anomalies. Detailed mapping of the high-salinity anomalies gave good definition of the major structures at 300 ft (90 m) as did detailed mapping of geothermal gradients at 1,200 ft (370 m) over the Healdton oil field (T4S, R3W). Other structures had poorly defined temperature anomalies. Spontaneous potential data indicate that, in general, upward cross-formational flow is not occurring; hence, the high-salinity anomalies are related to partially flushed, tectonically raised structures. In the absence of upward flow, the temperature anomalies are caused by proximity to radioactive basement rocks.

Probabilistic approach to short- and intermediate-term earthquake forecasting: II test application to southern California

A method is proposed for assessing the modifying effects of precursory observations on long-term probabilities of strong earthquakes (M≥6). Estimated short-and intermediate-term probabilities so estimated rely on the mean precursory time and its uncertainty as a function of the mainshock magnitude and epicentral position. Short-and intermediate-term modification of long-term probabilities within a 120,000 km2 circular area covering most of Southern California are estimated for one or more observations of strain, resistivity, and ground water anomalies centered near Palmdale in the central Transverse Ranges. The principal assumptions employed are log normal distribution of globally reported waiting periods and assumed predictive reliability of precursory phenomena. As expected, the significance of short-and intermediate-term modifications scale with the reliability of precursory observations, and are particularly enhanced within window lengths of a precursory characteristic time.

Geochemical and isotopic characteristics of spring and groundwater in the State of São Paulo, Brazil

A study of spring water shows that a correlation exists between the physical and chemical characteristics of the water and the lithology from where it issues. Water from crystalline rock can be classified as Ca/1bMg-bicarbonate, with low conductivity and temperature; water from sediments and/or weathered crystalline rock as Ca/1bMg-chloride—sulfate; and from volcanic rock, diabase and basalt as Na-bicarbonate water. Monthly samples of eight springs and of rain water in the region of Águas da Prata were analyzed for the deuterium and 18O isotopic contents expressed as δD and δ 18O in order to determine the origin of these waters. The coincidence of the isotopic values of spring water with the regional meteoric line indicates a local source of recharge. Chemical anomalies of groundwater in the shallow Bauru and Basalto aquifers in the Paraná Basin are probably caused by ascending water from the confined deep Botucatu—Piramboia aquifer through fracture and fault zones.

Fluorite and fluorine in water

Abstract The content of fluorine in water in the neighbourhood of fluorite-containing mineralizations has been measured. The mineralizations contain skarn silicates, calcite, fluorite, scheelite and molybdenite. Increased contents were found both in streams and in ground water. The content in streams in the immediate neighbourhood of the mineralizations was found to be 0.3–0.5 ppm. The background value should be of the order of 0.15 ppm, provided that the fluorine is of atmospheric origin. In ground water, several ppm of fluorine have been measured. The anomalies in ground water seem to be restricted to an area close to the outcrops of the ore bodies. This is due to the reactivity of the fluoride ion, which makes it apt to be removed from solution.