Monitoring of earthquake precursors by multi-parameter stations in Eskisehir region (Turkey)

The objective of this study was to investigate the geochemical and hydrogeological effects of earthquakes on fluids in aquifers, particularly in a seismically active area such as Eskisehir (Turkey) where the Thrace–Eskisehir Fault Zone stretches over the region. The study area is also close to the North Anatolian Fault Zone generating devastating earthquakes such as the ones experienced in 1999, reactivating the Thrace–Eskisehir Fault. In the studied area, Rn and CO₂ gas concentrations, redox potential, electrical conductivity, pH, water level, water temperature, and the climatic parameters were continuously measured in five stations for about a year. Based on the gathered data from the stations, some ambiguous anomalies in geochemical parameters and Rn concentration of groundwater were observed as precursors several days prior to an earthquake. According to the mid-term observations of this study, well-water level changes were found to be a good indicator for seismic estimations in the area, as it comprises naturally filtered anomalies reflecting only the changes due to earthquakes. Also, the results obtained from this study suggest that both the changes in well-water level and gas–water chemistry need to be interpretated together for more accurate estimations. Valid for the studied area, it can be said that shallow earthquakes with epicentral distances of <30 km from the observation stations have more influence on hydrochemical parameters of groundwater and well-water level changes. Although some hydrochemical anomalies were observed in the area, it requires further observations in order to be able to identify them as precursors.     

Statistical approach for the geochemical signature of two active normal faults in the western Corinth Gulf Rift (Greece)

Soil–gas measurements of different gas species were performed in two distinct areas of the Corinth Gulf Rift (Greece): the Aigion-Neos Erineos-Lambiri (ANEL) fault zone and the Rion-Patras fault zone. Both zones lie in one of the most seismically active areas of the Euro-Mediterranean region, where a fast-opening continental rift is located. In particular, the geochemical investigations were focused on fault segments and fracture systems previously inferred by geomorphological, lithological and structural studies.In this work the applicability of soil–gas geochemistry surveys for the exploration of buried/hidden faults was tested by using various statistical methods. Moreover, a comprehensive geostatistical treatment of the collected data provided new insights into the control exerted by active structures on deep-seated gas migration towards the surface. In both investigated areas, the highest ²²²Rn and CO₂ concentration peaks correspond with zones where the interaction among fracture and fault segments was inferred by structural and morphological methods. This indicates a clear correlation between the shape and orientation of the anomalies and the different attitude and kinematic behavior of the faults recognized in the two areas. Furthermore, obtained results show that gases migrate preferentially through zones of brittle deformation by advective processes, as suggested by the relatively high rate of migration needed to obtain anomalies of short-lived ²²²Rn in the soil pores.     

A comparison of the sedimentary records of the 1960 and 2010 great Chilean earthquakes in 17 lakes: Implications for quantitative lacustrine palaeoseismology

Seismically‐induced event deposits embedded in the sedimentary infill of lacustrine basins are highly useful for palaeoseismic reconstructions. Recent, well‐documented, great megathrust earthquakes provide an ideal opportunity to calibrate seismically‐induced event deposits for lakes with different characteristics and located in different settings. This study used 107 short sediment cores to investigate the sedimentary impact of the 1960 M_w 9·5 Valdivia and the 2010 M_w 8·8 Maule earthquakes in 17 lakes in South‐Central Chile (i.e. lakes Negra, Lo Encañado, Aculeo, Vichuquén, Laja, Villarrica, Calafquén, Pullinque, Pellaifa, Panguipulli, Neltume, Riñihue, Ranco, Maihue, Puyehue, Rupanco and Llanquihue). A combination of image analysis, magnetic susceptibility and grain‐size analysis allows identification of five types of seismically‐induced event deposits: (i) mass‐transport deposits; (ii) in situ deformations; (iii) lacustrine turbidites with a composition similar to the hemipelagic background sediments (lacustrine turbidites type 1); (iv) lacustrine turbidites with a composition different from the background sediments (lacustrine turbidites type 2) and (v) megaturbidites. These seismically‐induced event deposits were compared to local seismic intensities of the causative earthquakes, eyewitness reports, post‐earthquake observations, and vegetation and geomorphology of the catchment and the lake. Megaturbidites occur where lake seiches took place. Lacustrine turbidites type 2 can be the result of: (i) local near‐shore mass wasting; (ii) delta collapse; (iii) onshore landslides; (iv) debris flows or mudflows; or (v) fluvial reworking of landslide debris. On the contrary, lacustrine turbidites type 1 are the result of shallow mass wasting on sublacustrine slopes covered by hemipelagic sediments. Due to their more constrained origin, lacustrine turbidites type 1 are the most reliable type of seismically‐induced event deposits in quantitative palaeoseismology, because they are almost exclusively triggered by earthquake shaking. Moreover, they most sensitively record varying seismic shaking intensities. The number of lacustrine turbidites type 1 linearly increases with increasing seismic intensity, starting with no lacustrine turbidites type 1 at intensities between V½ and VI and reaching 100% when intensities are higher than VII½. Combining different types of seismically‐induced event deposits allows the reconstruction of the complete impact of an earthquake.     

Evidence of precursor phenomena in the Kobe earthquake obtained from atmospheric radon concentration

Atmospheric ²²²Rn concentrations were determined over a 10a period, which included the date of the Kobe, Japan earthquake, on January 17th 1995. It was found that the seismically related ²²²Rn anomaly was higher than the 99% confidence limits for the residual value of atmospheric ²²²Rn which had been observed 2 months before. The residual ²²²Rn concentration, in which residual values of the daily minimum are the difference between each normal ²²²Rn concentration (calculated from January 1984 to December 1993) and the daily minimum ²²²Rn concentration (January 1994 to January 1995), was calculated by applying the exponential smoothing method to the residual values for each day. It was found that the fluctuations of the residual values can be fitted very well to a log-periodic oscillation model. The real residual values stopped increasing at 1994.999 (December 31st 1994), which corresponds with the critical point (t_c) of best fit model. This anomalous ²²²Rn variation can be seen as the result of local stresses, not primary stresses which directly lead to the Kobe earthquake. On the other hand, when the critical exponent (z) and the radial frequency (ω) of the model were simultaneously fixed 0.2 ⩽ z ⩽ 0.6 and 6 ⩽ ω ⩽ 12, t_c (critical point) was between January 13th 1995 and January 27th 1995. The Kobe earthquake occurrence date (January 17th 1995) is within this range. Therefore this anomalous ²²²Rn variation can also be seen as the result of primary stresses which possibly led to the Kobe earthquake. There is a distinct possibility that similar statistical oscillations will be detected in other measurements such as microseismicity, tectonic strain, fluctuation in the ground level, or changes in groundwater elevations and composition.     

Plio-Quaternary stress states in NE Iran: Kopeh Dagh and Allah Dagh-Binalud mountain ranges

NE Iran, including the Kopeh Dagh and Allah Dagh-Binalud deformation domains, comprises the northeastern boundary of the Arabia–Eurasia collision zone. This study focuses on the evolution of the Plio-Quaternary tectonic regimes of northeast Iran. We present evidence for drastic temporal changes in the stress state by inversion of both geologically and seismically determined fault slip vectors. The inversions of fault kinematics data reveal distinct temporal changes in states of stress during the Plio-Quaternary (since ～ 5 Ma). The paleostress state is characterized by a regional transpressional tectonic regime with a mean N140 ± 10°E trending horizontal maximum stress axis (σ₁). The youngest (modern) state of stress shows two distinct strike-slip and compressional tectonic regimes with a regional mean of N030 ± 15°E trending horizontal σ₁. The change from the paleostress to modern stress states has occurred through an intermediate stress field characterized by a mean regional N trending σ₁. The inversion analysis of earthquake focal mechanisms reveals a homogeneous, transpressional tectonic regime with a regional N023 ± 5°E trending σ₁. The modern stress state, deduced from the youngest fault kinematics data, is in close agreement with the present-day stress state given by the inversions of earthquake focal mechanisms. According to our data and the deduced results, in northeast Iran, the Arabia–Eurasia convergence is taken up by strike-slip faulting along NE trending left-lateral and NNW trending right-lateral faults, as well as reverse to oblique-slip reverse faulting along NW trending faults. Such a structural assemblage is involved in a mechanically compatible and homogeneous modern stress field. This implies that no strain and/or stress partitioning or systematic block rotations have occurred in the Kopeh Dagh and Allah Dagh-Binalud deformation domains. The Plio-Quaternary stress changes documented in this paper call into question the extrapolation of the present-day seismic and GPS-derived deformation rates over geological time intervals encompassing tens of millions of years.     

Geochemistry of soil gas in the seismic fault zone produced by the Wenchuan Ms 8.0 earthquake, southwestern China

The spatio-temporal variations of soil gas in the seismic fault zone produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake were investigated based on the field measurements of soil gas concentrations after the main shock. Concentrations of He, H₂, CO₂, CH₄, O₂, N₂, Rn, and Hg in soil gas were measured in the field at eight short profiles across the seismic rupture zone in June and December 2008 and July 2009. Soil-gas concentrations of more than 800 sampling sites were obtained. The data showed that the magnitudes of the He and H₂ anomalies of three surveys declined significantly with decreasing strength of the aftershocks with time. The maximum concentrations of He and H₂ (40 and 279.4 ppm, respectively) were found in three replicates at the south part of the rupture zone close to the epicenter. The spatio-temporal variations of CO₂, Rn, and Hg concentrations differed obviously between the north and south parts of the fault zone. The maximum He and H₂ concentrations in Jun 2008 occurred near the parts of the rupture zone where vertical displacements were larger. The anomalies of He, H₂, CO₂, Rn, and Hg concentrations could be related to the variation in the regional stress field and the aftershock activity.     

Seismicity of Gujarat

Paper describes tectonics, earthquake monitoring, past and present seismicity, catalogue of earthquakes and estimated return periods of large earthquakes in Gujarat state, western India. The Gujarat region has three failed Mesozoic rifts of Kachchh, Cambay, and Narmada, with several active faults. Kachchh district of Gujarat is the only region outside Himalaya-Andaman belt that has high seismic hazard of magnitude 8 corresponding to zone V in the seismic zoning map of India. The other parts of Gujarat have seismic hazard of magnitude 6 or less. Kachchh region is considered seismically one of the most active intraplate regions of the World. It is known to have low seismicity but high hazard in view of occurrence of fewer smaller earthquakes of M????6 in a region having three devastating earthquakes that occurred during 1819 (M _w7.8), 1956 (M _w6.0) and 2001 (M _w7.7). The second in order of seismic status is Narmada rift zone that experienced a severely damaging 1970 Bharuch earthquake of M5.4 at its western end and M????6 earthquakes further east in 1927 (Son earthquake), 1938 (Satpura earthquake) and 1997 (Jabalpur earthquake). The Saurashtra Peninsula south of Kachchh has experienced seismicity of magnitude less than 6.     

Activité du gaz radon dans l'air du sol et sismicité locale : exemple du bassin de l'Arax (Arménie)Radon soil-gas concentrations and local seismicity: case of the Arax basin (Armenia)

A survey of soil gas radon concentrations has been carried out at three sites, in the seismic area of Armenia, from 1996 to 1999. The seismicity generates opposed behaviours at the different sites. This heterogeneity is related to the sites locations in the tectonic frame. An increase of radon concentration occurs inside the tectonic micro-blocks during local seismic activity. On the contrary, an abrupt co-seismic decrease is recorded close to the fault area. These variations can be due to ‘pore-collapse’ phenomena which expulse the pore-gas and increases Rn concentration in soil at intra-blocs sites, but also to pore-elastic deformations and microfracturing modifications in the fault area. To cite this article: K. Kharatian et al., C. R. Geoscience 334 (2002) 179–185.     

Seismicity and source parameters of moderate earthquakes in Sikkim Himalaya

In this study, we accurately relocate 360 earthquakes in the Sikkim Himalaya through the application of the double-difference algorithm to 4?years of data accrued from a eleven-station broadband seismic network. The analysis brings out two major clusters of seismicity??one located in between the main central thrust (MCT) and the main boundary thrust (MBT) and the other in the northwest region of Sikkim that is site to the devastating Mw6.9 earthquake of September 18, 2011. Keeping in view the limitations imposed by the Nyquist frequency of our data (10?Hz), we select 9 moderate size earthquakes (5.3????Ml????4) for the estimation of source parameters. Analysis of shear wave spectra of these earthquakes yields seismic moments in the range of 7.95?×?10²¹ dyne-cm to 6.31?×?10²³ dyne-cm and corner frequencies in the range of 1.8?C6.25?Hz. Smaller seismic moments obtained in Sikkim when compared with the rest of the Himalaya vindicates the lower seismicity levels in the region. Interestingly, it is observed that most of the events having larger seismic moment occur between MBT and MCT lending credence to our observation that this is the most active portion of Sikkim Himalaya. The estimates of stress drop and source radius range from 48 to 389?bar and 0.225 to 0.781?km, respectively. Stress drops do not seem to correlate with the scalar seismic moments affirming the view that stress drop is independent over a wide moment range. While the continental collision scenario can be invoked as a reason to explain a predominance of low stress drops in the Himalayan region, those with relatively higher stress drops in Sikkim Himalaya could be attributed to their affinity with strike-slip source mechanisms. Least square regression of the scalar seismic moment (M ₀) and local magnitude (Ml) results in a relation LogM ₀?=?(1.56?±?0.05)Ml?+?(8.55?±?0.12) while that between moment magnitude (M _w ) and local magnitude as M _w ?=?(0.92?±?0.04)Ml?+?(0.14?±?0.06). These relations could serve as useful inputs for the assessment of earthquake hazard in this seismically active region of Himalaya.     

Identifying space–time clustering properties of the 1983–1997 Irpinia–Basilicata (Southern Italy) seismicity

The fractality of the earthquake sequence (1983–1997) of Irpinia–Basilicata (Southern Italy), one of the most seismically active regions of the Mediterranean area, has been analysed by temporal and spatial fractal tools. The fractal exponent α, estimated by the Allan Factor method, characterises the time-clustering behaviour of the set of earthquakes, while the correlation dimension D_C, calculated by means of the correlation integral method, gives information on the space-clustering behaviour of the sequence of seismic events. Analysing the variations of both the parameters, we recognised the presence of a strong space–time clusterisation associated with the major events that occurred in the investigated area.     

A Study on Crack Damage Stress Thresholds of Different Rock Types Based on Uniaxial Compression Tests

When rock samples are loaded until macroscopic fractures develop, the failure process can be divided into several stages based on axial and lateral strain responses or the acoustic emission sequence during uniaxial compression tests. Several stress thresholds may be identified: the crack closure stress σ _cc, crack initiation stress σ _ci, crack damage stress σ _cd, and uniaxial compressive strength σ _ucs; these may be used as a warning indicator for rock rupture. We investigated the crack damage stress σ _cd, its threshold, and a possible relationship between σ _cd and the uniaxial compressive strength. The σ _cd of different rock types were compiled from previous studies based on uniaxial compression tests. The results showed that the overall averages and standard deviations of σ _cd /σ _ucs for igneous, metamorphic, and sedimentary rocks were ~0.78 (±0.11), ~0.85 (±0.11), and ~0.73 (±0.18), respectively. There were no significant differences in σ _cd /σ _ucs between the different rock types, except that the sedimentary rock had a slightly larger standard deviation attributed to the variation of porosity in the samples, while the metamorphic rock had higher average σ _cd /σ _ucs resulting from the small statistical sample size. By excluding the higher-porosity (>10 %) rock samples, the averages and standard deviations of σ _cd /σ _ucs for igneous, metamorphic, and sedimentary rocks were ~0.78 (±0.09), ~0.85 (±0.09), and ~0.78 (±0.11), respectively. The results imply that the rock origin process (i.e., igneous, metamorphic, and sedimentary) has a minimal effect on σ _cd /σ _ucs. The ratio σ _cd/σ _ucs could be an essential intrinsic property for low-porosity rocks, which could be used in rock engineering for predicting the failure process.     

Seismic interferometry and ambient noise tomography in the British Isles

Traditional methods of imaging the Earth's subsurface using seismic waves require an identifiable, impulsive source of seismic energy, for example an earthquake or explosive source. Naturally occurring, ambient seismic waves form an ever-present source of energy that is conventionally regarded as unusable since it is not impulsive. As such it is generally removed from seismic data and subsequent analysis. A new method known as seismic interferometry can be used to extract useful information about the Earth's subsurface from the ambient noise wavefield. Consequently, seismic interferometry is an important new tool for exploring areas which are otherwise seismically quiescent, such as the British Isles in which there are relatively few strong earthquakes. One of the possible applications of seismic interferometry is ambient noise tomography (ANT). ANT is a way of using interferometry to image subsurface seismic velocity variations using seismic (surface) waves extracted from the background ambient vibrations of the Earth. To date, ANT has been used successfully to image the Earth's crust and upper-mantle on regional and continental scales in many locations and has the power to resolve major geological features such as sedimentary basins and igneous and metamorphic cores. Here we provide a review of seismic interferometry and ANT, and show that the seismic interferometry method works well within the British Isles. We illustrate the usefulness of the method in seismically quiescent areas by presenting the first surface wave group velocity maps of the Scottish Highlands using only ambient seismic noise. These maps show low velocity anomalies in sedimentary basins such as the Moray Firth, and high velocity anomalies in igneous and metamorphic centres such as the Lewisian complex. They also suggest that the Moho shallows from south to north across Scotland which agrees with previous geophysical studies in the region.     

In-situ Rock Spalling Strength near Excavation Boundaries

It is widely accepted that the in-situ strength of massive rocks is approximately 0.4 ± 0.1 UCS, where UCS is the uniaxial compressive strength obtained from unconfined tests using diamond drilling core samples with a diameter around 50 mm. In addition, it has been suggested that the in-situ rock spalling strength, i.e., the strength of the wall of an excavation when spalling initiates, can be set to the crack initiation stress determined from laboratory tests or field microseismic monitoring. These findings were supported by back-analysis of case histories where failure had been carefully documented, using either Kirsch’s solution (with approximated circular tunnel geometry and hence σ _max = 3σ ₁ ?σ ₃) or simplified numerical stress modeling (with a smooth tunnel wall boundary) to approximate the maximum tangential stress σ _max at the excavation boundary. The ratio of σ _max /UCS is related to the observed depth of failure and failure initiation occurs when σ _max is roughly equal to 0.4 ± 0.1 UCS. In this article, it is suggested that these approaches ignore one of the most important factors, the irregularity of the excavation boundary, when interpreting the in-situ rock strength. It is demonstrated that the “actual” in-situ spalling strength of massive rocks is not equal to 0.4 ± 0.1 UCS, but can be as high as 0.8 ± 0.05 UCS when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic “as-built” excavation boundary condition is honored, the “actual” in-situ rock strength, given by 0.8 UCS, can be applied to simulate progressive brittle rock failure process satisfactorily. The interpreted, reduced in-situ rock strength of 0.4 ± 0.1 UCS without considering geometry irregularity is therefore only an “apparent” rock strength.     

Interpretation of radon anomalies in seismotectonic and tectonic-gravitational settings: the south-eastern Crati graben (Northern Calabria, Italy)

The study area is located in the south-eastern part of the Crati valley (Northern Calabria, Italy), which is a graben bordered by N–S trending normal faults and crossed by NW–SE normal left-lateral faults. Numerous severe crustal earthquakes have affected the area in historical time. Present-day seismic activity is mainly related to the N–S faults located along the eastern border of the graben. In this area, much seismically induced deep-seated deformation has also been recognised.In the present paper, radon concentrations in soil gas have been measured and compared with (a) lithology, (b) Quaternary faults, (c) historical and instrumental seismicity, and (d) deep-seated deformation.The results highlight the following:

(a) There is no evidence of a strong correlation between lithology and the radon anomalies.

(b) A clear correlation between the N–S geometry of radon anomalies and the orientation of main fault systems has been recognised, except in the southernmost part of the area, where the radon concentrations are strongly affected by the superposition of the N–S and the NW–SE fault systems.

(c) Epicentral zones of instrumental and historical earthquakes correspond to the highest values of radon concentrations, probably indicating recent activated fault segments. In particular, high radon values occur in the zones struck by earthquakes in 1835, 1854, and 1870.

(d) Deep-seated gravitational deformation generally coincides with zones characterised by low radon concentrations.

In the studied area, the anisotropic distribution of radon concentrations is congruent with the presence of neotectonic features and deep-seated gravitational phenomena. The method used in this study could profitably contribute towards either seismic risk or deep-seated gravitational deformation analyses.     

Seismic velocity and Poisson’s ratio tomography of the crust beneath East Anatolia

Eastern Anatolia is a region in the early stages of continent–continent collision and so provides a unique opportunity to study the early development of continental plateau. Located within the Alpine–Himalayan fold-thrust fault belt, the Anatolian plateau is geologically very complex, with over half of the surface area covered with late Cenozoic volcanics of diverse composition. The plateau is also seismically active and is dissected by numerous seismogenic faults predominantly of strike-slip motion. In this study, we determine 3-D tomographic images of the crust beneath eastern Anatolia by inverting a large number of arrival time data of P- and S-waves. From the obtained P- and S-wave velocity models, we estimated the Poisson’s ratio structures for a more reliable interpretation of the obtained velocity anomalies. Our tomographic results are generally consistent with the major tectonic features of the region. High P- and S-wave velocity anomalies are recognized near the surface, while at deeper crustal layers, low seismic wave velocities are widely distributed. Poisson’s ratio exhibits significant structural heterogeneities compared to the imaged velocity structure. The seismic activity is intense along highly heterogeneous zones and is closely associated with pre-existing faults in the central and western parts of the study area. Results of the checkerboard resolution test indicate that the imaged anomalies are reliable features down to a depth of about 40 km. The low-velocity/high Poisson’s ratio zones in the middle to lower crust are consistent with many geophysical observations such as strong Sn attenuation, low Pn and Sn velocity, and the absence of mantle lid, implying the presence of partial melt in the uppermost mantle.     

An archaeo-seismological study of the Nîmes Roman aqueduct,France: indirect evidence for an <Emphasis Type="Italic">M</Emphasis> > 6 seismic event?

This paper presents a synthesis of a multidisciplinary study carried out along the Nîmes Roman aqueduct, located in the southeast part of France. The study was motivated by archaeologists attempting to explain the partial destructions of only one aerial bridge of the aqueduct (Pont de la Lône). Given its close proximity to the Nîmes seismically active fault, a possible seismic origin for the destructions was invoked. Seismologists and structural engineers thus carried out a variety of field and numerical investigations to test the seismic hypothesis. Supporting field evidence was found first along the aerial bridge section of the aqueduct: broken stalactites, arch warping, cracks and destruction of the bridge just above a breast wall shortly after its construction. Secondly, the underground part of the canal was analysed: irregularity in the thickness of calcite deposits of the canal walls, presence of numerous cracks, horizontal shift of the otherwise linear structure of the canal and presence of calcite twins in the deposits, found only where the canal crosses the Nîmes fault system. Numerical modelling and experimental results show that (1) only the Pont de la Lône would have suffered serious damage under seismic solicitation (assuming an M6 earthquake at 10 km distance). The second aerial bridge, the three levels arches Pont du Gard, an historical monument still standing today, would have suffered less damage, due to its very different fundamental frequency of around 0.4 Hz, far from the amplified seismic near-field spectrum; (2) the numerical models also show that a fall of the canal wall would require a higher magnitude event (M > 6); (3) the presence of calcite twinning requires a differential static stress of 4 MPa or greater, which can only be achieved very close to the fault rupture of a M > 6 earthquake; (4) finally, local surface rupturing of such a fault would also corroborate the hypothesis that the observed offset of the canal may be partly seismically induced. Although other possible origins for each individual evidence may not be excluded, the observed spatiotemporal concentration of architectural/geological anomalies together with the numerical results allow us to support a possible co-seismic origin for these disorders, indirectly attesting to the potential seismic activity (M > 6) of the nearby Nîmes fault. Furthermore, following the conclusions of archaeological studies, the disorders occurred between 250 and 350 year AD, thus providing timing for this possible seismic event, an essential parameter in seismic hazard analysis. These results validate the archaeo-seismological approach as a tool that may help improve the knowledge of major infrequent earthquakes in areas of moderate seismic activity.     

Seasonal variations in soil radon emanation: long-term continuous monitoring in light of seismicity

Soil gas radon release patterns have been monitored continuously for more than 3 years in the Eastern Mediterranean Province (EMP) (Southern Turkey), alongside regional seismic events, providing a multidisciplinary approach. In the period from January 2008 to January 2011, 14 earthquakes M _L ≥4 occurred in the study area. By monitoring the sites for more than 3 years, the site-characteristic patterns of soil radon emanation of each site have become evident. Radon emanation data show seasonal (semi-annual) variation characteristics; high soil radon values are between May and October and low soil radon values are between November and April. With available rainfall data, the soil gas radon data can be more reliably evaluated. It is shown in this paper that if radon emanation data are available over sufficiently long periods of time and baseline data (and their seasonal variations) are known with certainty for each monitoring site, then the observation of positive anomalies might provide a correlation or connection to seismic activity.     

Environmental influences upon mercury, radon and helium concentrations in soil gases at a site near Denver, Colorado

Variations in soil gas Hg, Rn and He concentrations and meteorological variables were monitored daily at one site over a period of 22 months. Air and soil temperature, humidity, barometric pressure, soil moisture, wind direction and velocity, soil freeze-thaw, water table elevation, crystal strain and gas emissions were determined simultaneously in order to assess the influence of the environmental variables on gas emission.Mercury concentrations were found to be higher in the summer while Rn and He concentrations were higher in the winter. It is hypothesized that adsorption-desorption controls the migration of Hg whereas Rn and He concentrations are controlled by diffusion and mass transport. Gas emissions respond to seasonal and shorter-term changes in environmental conditions. Stepwise multiple regression using gas emissions as the dependent variables suggests that environmental parameters account for 62% of the total Hg variance, 83% of the total Rn variance, and 33% of the total He variance. Temperature, barometric pressure and soil moisture exert the most influence on gas emissions with temperature effects dominating gas emissions throughout the year. Soil gas emissions display a predictable behavior during winter and summer when more stable meteorological conditions exist. During the transitional seasons of spring and fall, soil gas emissions become erratic and exhibit increased variability. Environmental variables are interrelated and appear to control the manner of gas migration.Increased adsorption by solids during the season of falling temperature decreases soil gas Hg<0.0001 ng L⁻¹, which is desorbed as the soil warms in the spring. Transport of Rn and He is primarily by convection. During the winter months, air temperatures are less than soil temperatures promoting upward movement of Rn and He by convection. During the summer, soil temperatures are less than air temperatures and an inversion layer below the level of sampling reduces upward flux and observed concentration.     

Two-dimensional spatial analysis of the seismic b-value and the Bouguer gravity anomaly in the southeastern part of the Zagros Fold-and-Thrust Belt,Iran: Tectonic implications

Environmental managers and protection agencies try to assess the magnitudes of earthquakes in regions of seismic activity. For several decades they have used the seismic b-values and Bouguer anomalies for evaluating the crustal character and stress regimes. We have analyzed geostatistically data on both variables to map their spatial distributions in the southeast of the Zagros of Iran. We found a strong correlation between the distribution of the b-value and the Bouguer gravity anomaly in the region. The large Bouguer gravity anomaly values and small b-values all accord with there being a thinner crustal root and a larger concentration of stress in the center. The small to moderate Bouguer gravity anomaly values and intermediate to large b-values accord with the thicker crustal root and the smaller concentration of stress in the northeast. We conclude the southeast of the Zagros, consists of heterogeneous crust, such that accounts for its varied tectonics.     

Fluid geochemistry of the Sardinian Rift-Campidano Graben (Sardinia,Italy): fault segmentation,seismic quiescence of geochemically “active” faults,and new constraints for selection of CO2 storage sites

Sardinia is typically seismically quiescent, displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity. This evidence may be in agreement with the presence of a ductile layer in the northern sector of the island, as suggested by the He isotopic signature in fluids rising to the surface through quiescent fault systems. The fault systems have been found to be “segmented” and therefore isolated in fluid circulation. The study of fluid behaviour along fault systems becomes strategically important when applied to solve some geological risk assessments such as Rn-indoor, or to define geological structures like potential CO₂ storage sites. Both of these have been recently requested by the exploitation in Italy of the Euratom Directive and the evolution of the KyotoProtocol policy.Four water-dominated hydrothermal areas of Sardinia, located along regional fault systems, were considered: Campidano Graben, Tirso Valley, Logudoro and Casteldoria. A fluid geochemical survey was carried out taking into account physical–chemical and environmental parameters, major elements within gaseous and liquid phases, a few minor and trace elements, selected isotope ratios (²H, ¹⁸O, ¹³C, ³He/⁴He), ²²²Rn concentration, and some dissolved gases.Two different fluids have been recognised as regards both water chemistry and dissolved gases: (i) CO₂-rich gases, poor in He and Rn, with a relatively high ³He/⁴He ratio (up to R/R_a = 2.32), associated with Na–HCO₃–(Cl) thermal and cold groundwater; (ii) gases rich in He and N₂, poor in CO₂ and Rn, with a low ³He/⁴He ratio, associated with alkaline thermal and cold waters. The distribution of these two groups of fluids characterises the Sardinian tectonic systems. In fact, gas fluxes are not homogeneous, being mainly related to the different fault segments and to the areas where Quaternary basalts crop out. The underground geochemical evolution of the Sardinian fluids, as a function of the geological and tectonic systems, provides some suggestions for solving one of the most important problems: CO₂ geological sequestration. In order to reduce the CO₂ excess produced by human activity, the best geological disposal sites are reservoirs with low hydraulic conductivity, sealed to fluid movement, or aquifers characterised by maximum pH buffering capacity of their mineralogical matrix. The knowledge of the role of faults, as permeability barriers or as deep fluid uprising pathways, is prerequisite.