Previous Geophysical Studies Research Articles

Magnetotelluric image of the Chilean subduction zone in the Salar de Atacama region (23°-24°S): Insights into factors controlling the distribution of volcanic arc magmatism.

The Salar de Atacama region (23°-24°S) in the Central Andes of northern Chile represents an anomalous segment of the Andes, where a very large forearc basin is present (Salar de Atacama) and the axis of the active volcanic chain is deflected to the east from its regional trend. Here we used magnetotelluric (MT) data to infer the distribution of partial melts and hydrothermal fluids in order to better understand the mechanism controlling the magmatism in this subduction zone.Our 3-D inversion model shows a complex electrical resistivity structure, with several mid crustal conductors (<5 Ωm) imaged around the Salar de Atacama basin (SdAb). Although the forearc is dominated by high resistivities (>1000 Ωm), conductors were imaged below the Precordillera and in the northern SdAb, where major trench-parallel and transverse fault systems are present. We interpret these forearc conductors as evidence of active and fossil hydrothermal systems hosted by the Precordillera Fault System and the Calama-Olacapato-El Toro transverse fault. Conductors found along the arc are spatially coincident with active volcanoes likely representing partial melting in the continental crust. A large conductor was imaged in the backarc, indicating the presence of a significant volume of partial melts below the Altiplano-Puna plateau, as many previous geophysical studies have suggested. The model shows that the mid-lower crust and subcontinental mantle (i.e., >15 km depth) below the SdAb is dominated by resistivity values >100 Ωm., suggesting the presence of a relatively poorly hydrated continental crust and mantle wedge in this segment of the northern Chile subduction zone. These results, together with petrophysical interpretations derived from previous seismic and gravimetric studies, suggest that the lithosphere below the SdAb represents an impermeable block, which inhibits the upward flow of fluids and melts derived from deeper parts of the subduction system.

A seismicity revision and a probabilistic seismic hazard assessment using the Monte Carlo approach for the Canary Islands (Spain)

This study consists of a seismicity revision and the assessment of the probabilistic seismic hazard of the Canary Islands. In order to analyze its seismotectonics and associated seismic hazard, we performed a full revision of the seismicity of this region of oceanic intraplate seismicity, whose earthquake rate is low and its links to local tectonics are poorly understood. In this catalog, we designed a criterion for assigning the epicenter of historical events and homogenized the different magnitude values defined over time for earthquakes from the instrumental period. This analysis of the seismicity of the region and the structural lineaments obtained by previous geophysical studies enabled us to define two possible seismic zonings, which were then used in the probabilistic seismic hazard assessment. Given the special characteristics of the seismicity of the Canary Islands, we chose a Monte Carlo approach for calculating the hazard. We used the two alternatives provided by EqHaz open-source code (Assatourians and Atkinson, Seismol Res Lett 84:516–524, 2013; Seismol Res Lett 90:1407–1419, 2019), namely uniform and smoothed seismicity. Given that no ground motion prediction equation (GMPE) for the area is available, we selected the most robust and recent applicable relations from similar tectonic environment and checked the fit of the existing accelerometric data for the Canary Islands. We mixed the aleatory and epistemic uncertainties for each area of the two zonings in the hazard calculation. With this approach, the values obtained may be considered in the context of an extreme-value distribution of motions at a specific site after converting the simulated earthquakes into earth motion with the selected GMPEs. The final outcome is hazard maps for different periods in terms of horizontal acceleration at bedrock level, which we compared with the results deduced from recent hazard studies in the area.

Depth estimates of anomalous subsurface sources using 2D/3D modeling of potential field data: implications for groundwater dynamics in the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift

Quantitative analysis of potential field data are made in the Ziway-Shala lakes basin over an area bounded by 38°00′ E - 39°30′ E and 7°00′ N - 8°30’ N. Most previous geophysical studies in the region under consideration focus on mapping the deep crustal structures and undulation of the Moho depth. Only few studies are targeted at mapping the shallow subsurface structures. The main focus of this paper is mapping geometries of the major lithological and structural units of the shallow subsurface using gravity and magnetic data. The ultimate objective of the research is to understand the hydrogeological dynamics of the region through mapping interfaces geometries. Automatic inversions, 2D joint forward modeling and 3D inversion are the major techniques employed. The 2D Werner de-convolution based on both gravity and magnetic data along the rift axis showed source depths tending to deepen northwards. Source depths estimates determined by Source Parameter Imaging also showed similar tendency. This is further strengthened by the joint 2D forward modeling of gravity and magnetic data which showed the top of the basement is sloping northwards. The result of the 3D gravity interface inversion agrees with results of the above mentioned depth estimation techniques. Finally, the gravity power spectral analysis resulted in two depth estimates, 1.53 km and 2.87 km which approximate the positions of two density interfaces. The shallow depth interface is thought to presumably delineate the low density Fluvio-lacustrine sediments including the rift floor volcanic units and crystalline basement. Our investigation results agree with the results of previous seismic studies which identified low velocity (“sediment-volcanic”) horizon in the rift floor with low resolution. The information obtained with regard to water balance of the basin, salinity level of the lakes and the conceptual hydrological flow model appears to reveal that the groundwater flow in the study region is controlled by subsurface structures, particularly, the mapped interface topographies.

Appraisal of Veldurti–Kalva–Gani (VKG) fault, Cuddapah Basin, India: Gravity and magnetic approach

A ~60 km long Veldurti–Kalva–Gani (VKG) fault is one of the identified strike-slip faults extending from Eastern Dharwar Craton (EDC) to Cuddapah basin in South Indian Shield. The recorded recent seismic activity during year 2012–2016 show occurrences of three microseismic events (< Mw 2.0) in the vicinity of this fault. Historically, no major seismic events are recorded near this fault except magnitude of 5.0–5.9 (1843) earthquake at about ~80 km west of this fault near Bellary. In the present study, analysis of available gravity, aeromagnetic and newly acquired ground gravity and magnetic data in the vicinity of the fault has been carried out to understand subsurface characteristics of this VKG fault and nearby structural features related to recent seismic activity. Analysis of aeromagnetic and gravity data shows shallow origin of the fault and earthquakes are associated with the zone of intersection like cross faults/lineaments which are parallel and perpendicular to the VKG fault. The calculated log normalized radially averaged power spectrum of the available gravity and aeromagnetic data shows four average depths h0 (12.7 km), h1 (6 km), h2 (2.0 km) and h3 (0.5 km). These estimated depths are possibly, bottom of the upper crust, thickness of the Cuddapah basin sediments, horizon of the basic sills, flows and the ferruginous quartzites and cumulative stratigraphic thickness of the Tadpatri shales and the Kurnools in the areas, respectively. The jointly inverted 2-D model from the ground gravity and aeromagnetic data along 2.7 km profile across VKG fault shows, faulting between Banganapalli Quartzite and Tadpatri Shales. The estimated average focal depth from the observed microseismic events is around 13 km. It is concluded from the present study that the observed microseismic events in the vicinity of VKG fault are associated with the intersection zones of cross faults/lineaments near the VKG fault and originated at an average depth of 13 km might be bottom of the upper crust. The estimated depths from the present analysis are well corroborated with previous geophysical studies.

Sedimentary thickness of the northern Indo-Gangetic plain inferred from magnetotelluric studies

Knowledge of the sedimentary thickness and configuration of the basement structure are important to estimate the seismic hazards in active foreland basin. We present sedimentary thickness of the northern Indo-Gangetic plain estimated from impedance tensors of 12 magnetotelluric sites. Occam’s one dimensional inversion scheme was applied to invariant of the magnetotelluric tensors for the period range of 0.001–1 sec at each site. Inverted one dimensional model corroborates north-easterly dipping Indian basement and accordingly increased thickness of the sedimentary column towards north and east direction. The top sedimentary layers of varying thickness and resistivity are correlated with the known borewell logs and previous geophysical studies around the study area. Significant difference is observed in the resistivity of the Indian basement and thickness of the sedimentary column across the Delhi Hardwar Ridge. The difference in resistivity may be an indicative of variation in compactness and degree of saturation of the sedimentary cover and the nature of the Indian basement rock across the Delhi Haridwar Ridge.

Detection of Gas Hydrates in Faults Using Azimuthal Seismic Velocity Analysis, Vestnesa Ridge, W‐Svalbard Margin

AbstractJoint analysis of electrical resistivity and seismic velocity data is primarily used to detect the presence of gas hydrate‐filled faults and fractures. In this study, we present a novel approach to infer the occurrence of structurally controlled gas hydrate accumulations using azimuthal seismic velocity analysis. We perform this analysis using ocean‐bottom seismic data at two sites on Vestnesa Ridge, W‐Svalbard Margin. Previous geophysical studies inferred the presence of gas hydrates at shallow depths (up to ~190–195 m below the seafloor) in marine sediments of Vestnesa Ridge. We analyze azimuthal P‐wave seismic velocities in relation with steeply dipping near‐surface faults to study structural controls on gas hydrate distribution. This unique analysis documents directional changes in seismic velocities along and across faults. P‐wave velocities are elevated and reduced by ~0.06–0.08 km/s in azimuths where the raypath plane lies along the fault plane in the gas hydrate stability zone (GHSZ) and below the base of the GHSZ, respectively. The resulting velocities can be explained with the presence of gas hydrate‐ and free gas‐filled faults above and below the base of the GHSZ, respectively. Moreover, the occurrence of elevated and reduced (&gt;0.05 km/s) seismic velocities in groups of azimuths bounded by faults suggests compartmentalization of gas hydrates and free gas by fault planes. Results from gas hydrate saturation modeling suggest that these observed changes in seismic velocities with azimuth can be due to gas hydrate saturated faults of thickness greater than 20 cm and considerably smaller than 300 cm.

Eastward extrusion and northward expansion of the Tibetan plateau—discussions for the deep processes of the plateau uplift

The weak materials beneath the Tibetan Plateau have been escaping and extruding eastward from beneath the thickened and elevated central plateau driven by continuous convergence between India and Eurasia. This paper presents a detailed shear wave velocity cross-section image of crustal and uppermost mantle structure across the Tibet-Qinling transition zone that was generated from a dense linear seismic array. Based on our inverted seismic image together with previous geological and geophysical studies, we discussed about the material extrusion of the Tibetan Plateau, unveiling the depth extent in which the Qinling belt (which could serve as a flow channel) may accommodate the extrusion of the ductile material beneath the Tibetan Plateau, and suggested that how these dynamic processes might accommodate plateau uplift and expansion in the eastern margin of the Tibetan Plateau; furthermore, we compared the mechanisms of crustal deformation/thickening and the behavior patterns of lithospheric mantle between the eastern and northern margins of the Tibetan Plateau. Integrating our seismic image with previous observations, we can obtain the following findings: (1) A vigorous low velocity zone (LVZ) ( V s V s<4.2 km/s) resides beneath the Tibet-Qinling/Sichuan boundary area, contrasting to the high-velocity upper mantle to the east beneath the Qinling block and Sichuan basin. A synthetic analysis based on these findings and previous geological/geophysical observations leads to the corresponding conclusions: (1) Middle to lower crustal flow accompanied by regional fault-related strike-slip/thrusts may be occurring beneath the eastern margin of the Tibetan Plateau, and may be a significant deep seismogenic mechanism of the large earthquakes occurrence in the eastern margin of the plateau, but it has not vastly extruded farther beyond the Tibet-Qinling boundary into the lower crust of the Qinling belt. (2) The hot asthenospheric mantle material beneath the Tibetan Plateau may escape eastward driven by convergence/squeezing between India-Eurasia continents and flow to the Qinling belt, accelerating the delamination or thermal erosion of the base of the lithosphere beneath the Tibet-Qinling/Sichuan boundary area. Combination of the preceding two mechanisms, i.e., the extrusion of ductile middle to lower crustal materials accompanied by regional fault-related strike-slip/thrusts and isostatic buoyancy resulting from lithospheric detachment (triggered by asthenospheric flow), may have jointly engendered the plateau uplift and expansion in the Tibet-Qinling and Tibet-Sichuan transition zones. In contrast, uplift of the Qilian Shan in the northern margin of the Tibetan Plateau significantly depends upon upper crustal shortening processes above a weak midcrustal decollement, which is subject to the North China plate underthrusting beneath the plateau. Obviously, the deep process of material extrusion in the eastern margin of the Tibet Plateau is distinguishable from the deep mechanism responsible for the northward expansion within the Tibet-Alxa transition zone in the northern margin of the plateau. The fundamental geodynamic mechanism leading to the mode discrepancy of plateau uplift between the eastern and northern margins of the Tibetan Plateau may be attributed to the lateral variation of the nature of the lithospheres from central and eastern Tibet (interior part of the plateau) to the northern margin of the plateau. A relatively hot/weak Tibetan lithosphere underlies the Songpan-Ganzi and Qiangtang blocks in the central and eastern Tibet, while a relatively cold/rigid Asian lithosphere underlies the Qaidam and Qilian blocks (as well as the northern bordering Alxa block) in the northern margin of the Tibetan Plateau, with the boundary extending along the Kunlun-West Qinling range on a large-scale view.

2D and 3D MT in the central Skellefte Ore District, northern Sweden

New broadband magnetotelluric (MT) data have been acquired along two parallel profiles in the central part of the metallogenic Skellefte district in northern Sweden. The data were recorded as part of the Swedish 4D modelling of mineral belts project and cover an area with several economical and sub-economical deposits. The dimensionality and quality of the data were carefully analyzed and new error floors were systematically determined prior to inverse modelling in 2D and 3D. The algorithms used were EMILIA and WSINV3DMT. For the 2D inversion, only the determinant of the impedance tensor was used, while for the 3D inversion all elements were considered. The obtained models fit the inverted data, and image the main regional features. A detailed comparison reveals the superiority of the 3D model, both in model structures and data fit. After assessing the main features in the model, an interpretation is proposed and refined with the support of previous geophysical studies. The most interesting features are large and medium-sized conductors associated with crustal-scale shear zones and faults within the Skellefte Group rocks. These may be depicting a network of fossil pathways for hydrothermal fluid transport and as such, provide new insight into past processes in the area.

Three-dimensional forward modelling of gravity data using meshfree methods with radial basis functions and unstructured nodes

A study is presented using a mesh-free approach and a radial basis function generated finite difference (RBF-FD) method for numerically modelling 3-D gravity data. The gravity responses, that is, vertical gravity and gravity gradients, are obtained by solving the partial differential equation (PDE), that is, the Poisson’s equation for gravitational potential. The mesh-free approach discretizes PDEs using exclusively a cloud of unconnected nodes, instead of traditional tessellated meshes as used by mesh-based numerical methods such as finite difference, finite element and finite volume. Thus, the potentially computationally expensive and unstable creation and manipulation of 3-D meshes can be entirely avoided. A new type of finite-smoothness radial basis functions (RBFs), namely, the quintic-order polyharmonic spline (PHS) RBF, is proposed here in the RBF-FD frame for solving the gravity problem. Previous geophysical data modelling studies using RBF-FD have employed the infinitely smooth RBFs, such as the popular Gaussian (GA) RBFs. Here, both GA and PHS RBFs were tested with different numbers of nodes per mesh-free subdomain and with various shape parameter values (only GA RBFs have a shape parameter). The test results show that the PHS RBF-FD method is more computationally efficient than the GA RBF-FD counterpart. To achieve more efficiency, unstructured node distributions are proposed in discretizing the density models. For both quasi-uniform and unstructured node distributions, numerical results from the proposed PHS RBF-FD demonstrate that the computed vertical gravity and gravitational potential values agree well with analytical solutions with a reasonable number of degrees of freedom. A comparison study of modelling a complex density model with the PHS RBF-FD scheme and nodal finite-element method shows that the RBF-FD scheme generates sparse, asymmetric, linear systems of equations, supports unstructured nodal discretization and local refinement, and can have nonlinear h-convergence under refinement. Finally, the proposed RBF-FD method was applied to obtain the vertical gravity and gravity gradients over a real-world density model, where the benefits of mesh-free discretization are clearly illustrated.

Lithospheric Signature of Late Cenozoic Extension in Electrical Resistivity Structure of the Rio Grande Rift, New Mexico, USA

AbstractWe present electrical resistivity models of the crust and upper mantle from two‐dimensional (2‐D) inversion of magnetotelluric (MT) data collected in the Rio Grande rift, New Mexico, USA. Previous geophysical studies of the lithosphere beneath the rift identified a low‐velocity zone several hundred kilometers wide, suggesting that the upper mantle is characterized by a very broad zone of modified lithosphere. In contrast, the surface expression of the rift (e.g., high‐angle normal faults and synrift sedimentary units) is confined to a narrow region a few tens of kilometers wide about the rift axis. MT data are uniquely suited to probing the depths of the lithosphere that fill the gap between surface geology and body wave seismic tomography, namely the middle to lower crust and uppermost mantle. We model the electrical resistivity structure of the lithosphere along two east‐west trending profiles straddling the rift axis at the latitudes of 36.2 and 32.0°N. We present results from both isotropic and anisotropic 2‐D inversions of MT data along these profiles, with a strong preference for the latter in our interpretation. A key feature of the anisotropic resistivity modeling is a broad (~200‐km wide) zone of enhanced conductivity (&lt;20 Ωm) in the middle to lower crust imaged beneath both profiles. We attribute this lower crustal conductor to the accumulation of free saline fluids and partial melt, a direct result of magmatic activity along the rift. High‐conductivity anomalies in the midcrust and upper mantle are interpreted as fault zone alteration and partial melt, respectively.

3D electrical imaging of the inner structure of a complex lava dome, Puy de Dôme volcano (French Massif Central, France)

Lava domes result from extrusion of massive lava, frequent explosions and collapses. This contribution focuses on a complex trachytic lava dome, the Puy de Dôme volcano, located in the Chaîne des Puys volcanic field (French Massif Central, France). We performed Electrical Resistivity Tomography (ERT) acquisitions on the entire edifice in order to investigate its overall inner structure as well as to detail its summit area. The resulting large ERT dataset integrated a recently developed 3D inversion code based on an unstructured discretization of the geometrical model. The 3D inversion models obtained refine the existing geological model of the Puy de Dôme's inner structure obtained by previous geophysical studies. These results also highlight the strong fracturing and fumarolic alteration that affect the summit part of the volcano.

Geophysical Contribution to the Determination of the Limit between Laayoune and Foum El Oued Aquifers, Origin of Supply Sources of Oued Sakia El Hamra, Laayoune Province, Morocco

Laayoune and the Foum El Oued aquifers are in hydraulic communication only at the level of Oued Saguia El Hamra. The present study has accordingly made use of all the hydrogeological, hydrological, geological and geophysical data that preceded the watershed of Oued Saguia El Hamra in its downstream part. These data are by no means omplementary with the objective of having a better understanding of the boundary line between Laayoune and Foum El Oued aquifers and the origin of feeding the sources of Oued Saguia El Hamra. This study will focus only on the previous geophysical studies where a reinterpretation of electrical soundings has proved useful as a result of the recent well-logging results. It makes it possible to highlight the presence of a significant rise in the truncated marly substratum of Oued Saguia El Hamra and depressions (left and right banks) which could correspond to stream channels or depressed areas. At the level of the Wadi bed, there has been a regular immersion of the conductive level roofs from east to west towards Foum El Oued favoring the flow of wastewater from the zone and spraying the brackish water sources towards the groundwater of Foum El Oued. In the light of the reinterpretation of electric polls, plus as well as the geophysical surveys by electrical tomography and high definition made at the right and left banks of the Oueed Saguia El-Hamra, it was possible to verify the existence of dry ridge separating the two webs of Laayoune and Foum El Oued and stream channels or depressed areas of the left bank for drainage of brackish water to sources located along Oued Saguia El Hamra. The true resistivity models tomography profiles confirm the presence of the backbone at the left and right banks and the graben of the left bank for underground water drainage of the web Laayoune to sources welling the river Saguia El Hamra. They reveal the presence of a quaternary plio-cover (coastal platform Moghrebian) as being heterogeneous and affected by many electrical discontinuities, particularly in the level resistant R2 (coquina sandstone). These discontinuities could correspond to lateral changes in facies and/or synsedimental faults compartmentalizing the plio-quaternary formations into a system of Horsts and Grabens that relies on the whole (D1, Cs) attributed to formations from the Miocene to the Upper Cretaceous. The contours of the map show the distribution of the three families of electrical soundings A, B and C limited by two main electrical discontinuities D and M. East of discontinuity million, this map reflects the look of the wall of the Pliocene-Quaternary resting on the impermeable upper Cretaceous floor. To the west of this electrical discontinuity M, the map reflects the behavior of the roof of marl deposits of Miocene age.

Magma Chamber and Crustal Channel Flow Structures in the Tengchong Volcano Area From 3‐D MT Inversion at the Intracontinental Block Boundary Southeast of the Tibetan Plateau

AbstractBased on magnetotelluric (MT) array data, we have obtained the first three‐dimensional (3‐D) electrical resistivity model at the Gaoligong intracontinental block boundary in southeastern Tibetan Plateau where the Quaternary intraplate Tengchong volcanism and seismic activities occur. Comparing with results of previous geophysical studies in the area, our MT model clearly reveals three conductive bodies in the depth ranges of 10–30 km in the Tengchong volcano area, which we interpret as three middle‐lower crustal magma chambers associated with the Tengchong volcanism. Seismogenic faults in the Gaoligong Shear Zone (GLGSZ) are characterized by subvertical conductive zones bounded by resistive upper crustal layer on both sides. Earthquakes of moderate magnitudes near the GLGSZ have all occurred within the conductive fault zones at the bottom of the upper resistive crust. More importantly, one large resistive body was imaged at middle‐lower crustal depth beneath the GLGSZ, which seems to block the previously proposed horizontal crustal channel flow along this intracontinental block boundary. Our 3‐D model indicates that crustal channel flow could take place east of the GLGSZ. The current study provides evidence from electrical resistivity structure for the middle‐lower crustal magma chambers in the Tengchong volcano area and detailed 3‐D electrical structure of crustal channel flow in this active tectonic region.

Magma Plumbing System at Izu-Oshima Volcano, Japan: Constraints From Petrological and Geochemical Analyses

The Izu-Oshima volcano is one of the most active volcanoes in Japan, and has generated relatively large-scale eruptions every 30–40 years for the past 200 years. As more than 30 years have passed since the last eruptions in 1986–87, volcanic activity is expected to resume in the near future. To help elucidate the current and future state of the volcano’s magma system, the temporal evolution of the recent magma plumbing system was investigated through a petrological and geochemical analysis of its basaltic lavas and pyroclastics (<~53 wt.% of SiO2) that were erupted during the last ~1.5 kyrs. The basaltic products have variable phenocryst contents, ranging from ~0 to ~20 vol.%, and phenocryst-bearing samples commonly contain plagioclase, orthopyroxene, and clinopyroxene phenocrysts. The whole-rock compositions are significantly scattered in the Harker variation diagrams, suggesting that the compositional diversity was established by at least two independent magmatic processes. The application of principal component analysis on the whole-rock major element data suggests that one magmatic process was crystal fractionation of crystal-poor magmas, and the other process was either plagioclase accumulation or mixing of plagioclase-rich magmas. Based on this observation, and combined with the petrological analysis and previous geophysical studies, we propose that aphyric magmas, stored in an 8–10 km-deep magma chamber, progressively differentiated over time from the 7th to 20th century. Furthermore, the compositional variations in basalts resulted from the mixing of the differentiating aphyric magmas with variable proportions of porphyritic magmas derived from a 13–18 km-deep magma chamber. Because recent eruptions have been triggered by the ascent of porphyritic magma from the 13–18 km-deep magma chamber, and its injection into the 8–10 km-deep magma chamber, it is important to monitor the deeper magma chamber to predict future volcanic activity.

Karst Detection Beneath the Pyramid of El Castillo, Chichen Itza, Mexico, by Non-Invasive ERT-3D Methods

Currently, archaeologists perform excavations determined by previous geophysical studies to accurately establish the prospective targets and minimize site disturbance. Among others, one of the methods most widely employed is the Electrical Resistivity Tomography (ERT-2D, -3D). However, investigation of the subsoil of archaeological buildings is not possible to carry out with traditional geophysical methods, because the structure itself prevents it. Therefore, it is necessary to design non-invasive special arrays capable of characterizing the subsoil of such buildings, while preserving their historical context. Here we show how this procedure combined with sequences of resistivity observations at depth allowed us to detect a low resistivity body beneath the pyramid of El Castillo in Chichen Itza (Mexico). This feature may be associated with a cavity (karst) partially filled with sweet water. On the other hand, a natural cavity was discovered under El Osario pyramid (south of El Castillo), at the end of the 19th century. Therefore, this pyramid was also studied to validate the effectiveness of this methodology, obtaining outstanding results. This method provides an interesting procedure to investigate the subsoil of archaeological structures for unveiling evidences that allow specialists to understand the religious meaning of these temples.

Characterization and assessment of groundwater resources using hydrogeochemical analysis, GIS, and field data in southern Wadi Qena, Egypt

An integrated approach using hydrogeochemical analysis, remote sensing, GIS, and field data was employed to characterize the groundwater resources in southern Wadi Qena, Egypt. Various thematic maps showing topography, lineaments, wadi deposits, slope, and stream networks were combined through GIS analysis to discriminate groundwater potential zones on the valley floor. The resulting map classifies the area into five groups of groundwater potentiality from very high to very low zones, supported by the groundwater level, well locations, and by the results of previous geophysical studies. Thirty-seven groundwater well data were tested from the Quaternary and Nubian Sandstone aquifers and analyzed for physio-chemical parameters. Results of hydrochemical analysis show that water quality varies widely through the aquifers, and groundwater in the Quaternary aquifer shows the highest salinity values and a predominance of Na and Cl in water chemical facies. Overlay GIS maps of alkalinity (SAR and RSC) and salinity hazards (EC and Cl) of the Quaternary aquifer were prepared. The resulting maps show that samples do not present an alkalinity hazard in most areas but are potentially salinity hazard. Therefore, the water is fit for agricultural use with certain restrictions, but is not suitable for direct human consumption because it is either very hard or too saline.

Qualitative appraisal of high resolution satellite derived free air gravity anomalies over the Maldive Ridge and adjoining ocean basins, western Indian Ocean

High resolution satellite derived Free Air Gravity (FAG) anomalies over the Maldive Ridge segment of the Chagos-Laccadive Ridge and adjoining areas of the Arabian Basin and Central Indian Basin was analyzed to understand their structure. Indian Ocean Geoidal Low corrected FAG depict the N-S trending relative highs associated with the Maldive Ridge, the lows associated with the Vishnu Fracture Zone, and the seafloor fabrics associated with spreading Central Indian Ridge in the Arabian Basin. Transformation operations for enhancing the short, intermediate and long wavelength anomalies were carried out. The northward continuation of the Vishnu Fracture Zone, the eastward extension of the fracture zones of the Arabian Basin into the Deep Sea Channel (DSC) region, between the Maldive Ridge and the Chagos Bank, and the signatures related to north-south trending coral reefs have been deciphered from the enhancement of short wavelength anomalies. The long wavelength anomalies show N-S highs along the axis of the Maldive Ridge and the DSC, which on comparison with previous studies, is inferred to be the signature related with the underplated material from the passage of the Indian plate over the Reunion Plume. Depth to top of two interfaces over the Maldive Ridge and the DSC were computed using the power spectral method by assuming a window of 110 km dimension and 50 km overlap. Depths were thus calculated for 44 blocks along the ridge axis. The average depth to the top of the shallow interface was found to be 5.5 km while that of the deeper was 11.0 km. These depths and interfaces were confirmed by undertaking 2D forward modelling along a 300 km long N-S profile along the western segment of the Maldive Ridge. Integrating with the available previous geophysical studies, it is inferred that the shallow interface represents the depth to the top of the acoustic basement underlying 1–1.5 km thick lava flow unit while the deeper is associated with the interface between the crust and initial Moho before the underplating took place (presently top of the underplated material). From the present study, it has been inferred that the Deep Sea Channel between the Maldive Ridge and Chagos Bank can be purely oceanic in nature.

Post-Deccan Trap stress reorientation under transpression: Evidence from fault slip analyses from SW Saurashtra, Western India

We analyze paleostress from a region in SW Saurashtra (India) using three free software, T-Tecto, WinTensor and FaultKin based on attitudes of ∼ E-W trending, sub-vertical, brittle fault planes and associated horizontal, inclined and subvertical slickensides, developed over them. We present a detailed fault slip analysis of a previously unmapped E trending fault (herein referred as the Katar Fault) that cuts across and offset the southeastward dipping Deccan lava flows with a dense NE-SW trending dyke swarm belonging to Deccan Trap near Rajula (Gujarat, India). Measured fault strikes do not match exactly with ∼ NW/NNW trending basement structures, deduced by previous geophysical studies, nor with faults reported from the offshore Saurashtra region. However, a few geomorphic features around Katar indicate a similar strike slip offset. A stress ratio of up to 0.2, and a SE maximum principal stress axis were deduced using T-Tecto, while WinTensor and FaultKin gave plunging stress axes. Therefore, stress analyses by T-Tecto seem more plausible. The SHmax direction usually remains close to one of the stress axes, and shows up to 36° rotation when compared with the offshore region. We attribute the formation of Katar Fault to post-Deccan Trap oblique slip reverse faulting with sinistral slip due to intraplate transpressional stresses induced by the anticlockwise rotation and collision of the Indian plate with the Eurasian plate at ∼ 55 Ma. This transpressional faulting post-dates the eastward tilting of lava flows and anticlockwise rotation in the dyke swarm patterns caused by the emplacement of large Girnar intrusive body. Detailed structural geological field-work in Saurashtra would be much required to understand its onshore tectonics.

High-resolution surface wave tomography of the European crust and uppermost mantle from ambient seismic noise

Taking advantage of the large number of seismic stations installed in Europe, in particular in the greater Alpine region with the AlpArray experiment, we derive a new high-resolution 3-D shear wave velocity model of the European crust and uppermost mantle from ambient-noise tomography. The correlation of up to 4 yr of continuous vertical-component seismic recordings from 1293 broad-band stations (10 • W-35 • E, 30 • N-75 • N) provides Rayleigh wave group velocity dispersion data in the period band 5-150 s at more than 0.8 million virtual source-receiver pairs. 2-D Rayleigh wave group velocity maps are estimated using adaptive parametrization to accommodate the strong heterogeneity of path coverage. A probabilistic 3-D shear wave velocity model, including probability densities for the depth of layer boundaries and S-wave velocity values, is obtained by nonlinear Bayesian inversion. A weighted average of the probabilistic model is then used as starting model for the linear inversion step, providing the final V s model. The resulting S-wave velocity model and Moho depth are validated by comparison with previous geophysical studies. Although surface wave tomography is weakly sensitive to layer boundaries, vertical cross-sections through our V s model and the associated probability of the presence of interfaces display striking similarities with reference controlled-source seismology (CSS) and receiver function sections across the Alpine belt. Our model even provides new structural information such as an ∼8 km Moho jump along the CSS ECORS-CROP profile that was not imaged by the reflection data due to poor penetration across a heterogeneous upper crust. Our probabilistic and final shear wave velocity models have the potential to become new reference models of the European crust, both for crustal structure probing and geophysical studies including waveform modelling or full-waveform inversion.

Cenozoic pulsed compression of Da'an-Dedu Fault Zone in Songliao Basin (NE China) and its implications for earthquake potential: Evidence from seismic data

The Da'an-Dedu Fault Zone (DDFZ) is a major tectonic feature cutting through the Songliao Basin from south to north in NE China. Pulsed compression deformation of DDFZ during the Cenozoic implies a complex geodynamic process, and the latest stage of which occurred in the Quaternary directly influences the present seismicity of the interior basin. Although most of the evidence for Quaternary deformation about the Songliao Basin in the past decades was concentrated in marginal faults, all five earthquake swarms with magnitudes over 5.0 along the buried DDFZ with no surface expression during the past 30years suggest it is a main seismogenic structure with seismic potential, which should deserve more attention of geologists. However, limited by the coverage of the Quaternary sedimentary and absence of strong historic and instrumental earthquakes records (M>7), the geometric pattern, Quaternary activity and seismic potential of the DDFZ remain poorly understood. Thus, unlike previous geophysical studies focused on crust/mantle velocity structure across the fault and the aim of exploring possible mineral resources in the basin, in this study we have integrated a variety of the latest seismic data and drilling holes from petroleum explorations and shallow-depth seismic reflection profiles, to recognize the Cenozoic pulsed compression deformation of the DDFZ, and to discuss its implication for earthquake potential. The results show that at least four stages of compression deformation have occurred along the DDFZ in the Cenozoic: ~65Ma, ~23Ma, ~5.3Ma, and ~1.8Ma, respectively, although the geodynamic process behind which still in dispute. The results also imply that the tectonic style of the DDFZ fits well with the occurrence of modern seismic swarms. Moderate earthquake potential (M≤7.0) is suggested along the DDFZ.