ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉In Northern Apennines, upper Lutetian-Bartonian sedimentary mélanges, corresponding to ancient mud-rich submarine mass transport deposits, are widely distributed along the exhumed outer part of the External Ligurian accretionary wedge (ELAW), over an area of ∼300 km long and tens of kilometres wide. The occurrence of CH〈sub〉4〈/sub〉-derived carbonate concretions (septarians) in a specific tectono-stratigraphic position below sedimentary mélanges, allows us to document the relationships between a significant period of regional-scale slope failure, climate change (Early and Middle Eocene Optimum stages; EECO and MECO), gas hydrates dissociation, and accretionary tectonics during the Ligurian Tectonic Phase (early-middle Lutetian). The distribution of septarians at the core of thrust-related anticlines documents that gas hydrates dissociation was triggered by accretionary tectonics rather than climate changes. The different ages of slope failure emplacement and septarians formation support that gas hydrates dissociation was not the most important triggering for slope failure. The latter occurred during a tectonic quiescence stage associated with a depositional regressive trend, and probably minor residual tectonic pulses, which followed the Ligurian Tectonic Phase, favoring the dynamic re-equilibrium of the ELAW. Our findings provide useful information for better understanding factors controlling giant slope failure events in modern accretionary settings, where they may cause tsunamis.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The investigation of using a novel radial basis function-based meshfree method for forward modelling magnetotelluric data is presented. The meshfree method, which can be termed radial basis function-based finite difference (RBF-FD), uses only a cloud of unconnected points to obtain the numerical solution throughout the computational domain. Unlike mesh-based numerical methods (for example, grid-based finite difference, finite volume and finite element), the meshfree method has the unique feature that the discretization of the conductivity model can be decoupled from the discretization used for numerical computation, thus avoiding traditional expensive mesh generation and allowing complicated geometries of the model be easily represented. To accelerate the computation, unstructured point discretization with local refinements are employed. Maxwell’s equations in the frequency domain are re-formulated using $\mathbf {A}$-ψ potentials in conjuction with the Coulomb gauge condition, and are solved numerically with a direct solver to obtain magnetotelluric responses. A major obstacle in applying common meshfree methods in modelling geophysical electromagnetic data is that they are incapable of reproducing discontinuous fields such as the discontinuous electric field over conductivity jumps, causing spurious solutions. The occurrence of spurious, or non-physical, solutions when applying standard meshfree methods is removed here by proposing a novel mixed scheme of the RBF-FD and a Galerkin-type weak-form treatment in discretizing the equations. The RBF-FD is applied to the points in uniform conductivity regions, whereas the weak-form treatment is introduced to points located on the interfaces separating different homogeneous conductivity regions. The effectiveness of the proposed meshfree method is validated with two numerical examples of modelling the magnetotelluric responses over three-dimensional conductivity models.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Recently, the effectiveness of the use of traditional methods of searching for kimberlite pipes in the Arkhangelsk diamondiferous province (subarctic zone of Russia) has decreased greatly. Thus, new methods of kimberlite exploration need to be developed. This study discusses new patterns of the distribution of natural radioactive isotopes within the Arkhangelsk diamondiferous province (Zolotitskoe kimberlite field), in particular the isotopes of uranium 〈sup〉234〈/sup〉U and 〈sup〉238〈/sup〉U. A variety of isotopic radiogeochemical studies has shown that the kimberlite pipes are characterized by local radioisotope anomalies, on the surface and in exploration drill core. These irregularities are clearly manifested in the formation of non-equilibrium anomalous isotopic composition of uranium in the surrounding rocks and groundwater of the near-contact zone of the pipes. These uranium isotopes can be used to explore for kimberlites in other areas with similar complex landscape-geological and climate conditions of the subarctic zone.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉This thematic set of papers is associated with a two-day conference on Ground-related Risk to Transportation Infrastructure held on October 26 and 27, 2017 at the Geological Society of London, an event that attracted nearly 200 attendees from around the world. This excellent level of interest, coupled with the extremely high standard of presentation and discussion at the event, gives a good indication of the level of interest in the subject area.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The ‘Karst of Ireland: Landscape Hydrogeology Methods’ contains everything you could wish to know about karstic processes and the resulting landforms found in Ireland. Published by Geological Survey Ireland, it has been produced to a very high standard as a 318-page A4 hardbacked book, using high quality glossy paper, and is richly illustrated with full colour figures, maps and photographs. Initiated as a legacy project at the end of the author’s long and successful professional career working on karst, the book works well both as a reference text on (Irish) karst, and as a beautifully illustrated coffee table book for those with a more general interest in geology.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Receiver functions are sensitive to sharp seismic velocity variations with depth and are commonly used to constrain crustal thickness. The H-κ stacking method of Zhu and Kanamori (〈span〉2000〈/span〉) is often employed to constrain both the crustal thickness (H) and ${V_P}$/${V_S}$ ratio ($\kappa $) beneath a seismic station using P-to-s converted waves (Ps). However, traditional H-κ stacks require an assumption of average crustal velocity (usually ${V_P}$). Additionally, large amplitude reverberations from low velocity shallow layers, such as sedimentary basins, can overprint sought-after crustal signals, rendering traditional H-$\ \kappa $ stacking uninterpretable. We overcome these difficulties in two ways. When S-wave reverberations from sediment are present, they are removed by applying a resonance removal filter allowing crustal signals to be clarified and interpreted. We also combine complementary Ps receiver functions, Sp receiver functions, and the post-critical P wave reflection from the Moho (SP〈sub〉m〈/sub〉p) to remove the dependence on an assumed average crustal ${V_P}$. By correcting for sediment and combining multiple data sets, the crustal thickness, average crustal P-wave velocity, and crustal ${V_P}$/${V_S}$ ratio is constrained in geologic regions where traditional H-$\ \kappa $ stacking fails, without making an initial P-wave velocity assumption or suffering from contamination by sedimentary reverberations.〈/span〉

Description: 〈span〉In the original version of this article the author, Adrian Flores Orozco, was incorrectly listed. This has now been corrected and the publisher apologises for the error.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉At the 2015 United Nations International Climate Change Conference in Paris (COP21), 197 national parties committed to limit global warming to well below 2°C. But current plans and pace of progress are still far from sufficient to achieve this objective. Here we review the role that geoscience and the subsurface could play in decarbonizing electricity production, industry, transport and heating to meet UK and international climate change targets, based on contributions to the 2019 Bryan Lovell meeting held at the Geological Society of London. Technologies discussed at the meeting involved decarbonization of electricity production via renewable sources of power generation, substitution of domestic heating using geothermal energy, use of carbon capture and storage (CCS), and more ambitious technologies such as bioenergy and carbon capture and storage (BECCS) that target negative emissions. It was noted also that growth in renewable energy supply will lead to increased demand for geological materials to sustain the electrification of the vehicle fleet and other low-carbon technologies. The overall conclusion reached at the 2019 Bryan Lovell meeting was that geoscience is critical to decarbonization, but that the geoscience community must influence decision-makers so that the value of the subsurface to decarbonization is understood.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉The evolution of the Philippine Sea Plate (PSP) since Jurassic is one of the key issues in the dynamics of lithosphere and mantle. The related studies benefited mostly from seismic tomography which provides velocity structures in the upper mantle. However, the upper-mantle structure is not well resolved compared to the continental areas due to the lack of seismic data in the Philippine Sea. We employ a 3-D gravity inversion constrained by an initial model based on the 〈span〉S〈/span〉-wave tomography (SL2013sv; Schaeffer & Lebedev 2013) to image the density structure of the upper mantle of the PSP and adjacent region. The resulting model shows a three-layer pattern of vertical high-low-high density variation in the upper mantle under the PSP. The thin high-density layer evidences for strong oceanic lithosphere in the West Philippine Sea. The relatively low dense mantle located below the PSP possibly originates from the asthenosphere. The PSP differs from the Pacific and the Indian-Australian plates in the whole depth range, while its structure is similar to the eastern Eurasian and Sunda plates. In the depth range, 200–300 km, the relative high-density zone beneath PSP extends to the Sunda Plate and to the eastern Eurasian Plate. We further estimated the conversion factor of our density model and the velocity model (SL2013sv; Schaeffer & Lebedev 2013) in order to locate the changes of compositional effects in the upper mantle. The negative conversion factor indicates that the compositional changes primarily affect the density anomalies beneath the PSP. We, therefore, describe the layered density structures as ‘sandwich’ pattern, which is unique and different from adjacent regions.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Low-velocity layers within the crust can indicate the presence of melt and lithologic differences with implications for crustal composition and formation. Seismic wave conversions and reverberations across the base of the crust or intracrustal discontinuities, analysed using the receiver function method, can be used to constrain crustal layering. This is commonly accomplished by inverting receiver functions jointly with surface wave dispersion. Recently, the proliferation of model-space search approaches has made this technique a workhorse of crustal seismology. We show that reverberations from shallow layers such as sedimentary basins produce spurious low-velocity zones when inverted for crustal structure with surface wave data of insufficiently high frequency. Therefore, reports of such layers in the literature based on inversions using receiver function data should be re-evaluated. We demonstrate that a simple resonance-removal filter can suppress these effects and yield reliable estimates of crustal structure, and advocate for its use in receiver-function based inversions.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We present a numerical method for simulating both single-event dynamic ruptures and earthquake sequences with full inertial effects in antiplane shear with rate-and-state fault friction. We use the second-order form of the wave equation, expressed in terms of displacements, discretized with high-order-accurate finite difference operators in space. Advantages of this method over other methods include reduced computational memory usage and reduced spurious high frequency oscillations. Our method handles complex geometries, such as non-planar fault interfaces and free surface topography. Boundary conditions are imposed weakly using penalties. We prove time stability by constructing discrete energy estimates. We present numerical experiments demonstrating the stability and convergence of the method, and showcasing applications of the method, including the transition in rupture style from crack-like ruptures to slip pulses for strongly rate-weakening friction and the simulation of earthquake sequences in a viscoelastic solid with a fully dynamic coseismic phase.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The 〈10, 〈4, 〈2.5 and 〈1 µm fractions of eight regolith samples have been extracted as aerosols, then analysed for more than 60 elements by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Precision and accuracy rivals that of conventional digestion and ICP-MS analysis for most elements, but the aerosol extraction and ablation approach can be completed within 30 min, avoids compromising the sample by screening, column settling, fusion and/or digestion, and includes data for elements such as Br, Cl, I and Se that are conventionally analysed by individual procedures. Major element chemistry and scanning electron microscope (SEM) imagery show that the aerosol fractions of regolith are dominated by kaolinite, with quartz in aeolian regolith. The aerosol fractions of Si- and Ca-rich regolith have higher trace element concentrations than the coarser fraction (0.45–2 mm), but chalcophile elements are depleted in the aerosols of Fe-rich regolith relative to the coarser fraction. Improvements in in-field analytical technology coupled with aerosol extraction mean that fine and ultrafine fraction chemistry can be used to guide mineral exploration programmes in close to real time.〈strong〉Supplementary material〈/strong〉〈strong〉:〈/strong〉 The results of the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of filter papers are available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4562807"〉https://doi.org/10.6084/m9.figshare.c.4562807〈/a〉〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉We derive a theoretical relationship between the cross correlation of ambient Rayleigh waves (seismic ambient noise) and the attenuation parameter α associated with Rayleigh-wave propagation. In particular, we derive a mathematical expression for the multiplicative factor relating normalized cross correlation to the Rayleigh-wave Green’s function. Based on this expression, we formulate an inverse problem to determine α from cross correlations of recorded ambient signal. We conduct a preliminary application of our algorithm to a relatively small instrument array, conveniently deployed on an island. In our setup, the mentioned multiplicative factor has values of about 2.5 to 3, which, if neglected, could result in a significant underestimate of α. We find that our inferred values of α are reasonable, in comparison with independently obtained estimates found in the literature. Allowing α to vary with respect to frequency results in a reduction of misfit between observed and predicted cross correlations.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Global phases, viz. seismic phases that travel through the Earth’s core, can be used to locally image the crust by means of seismic interferometry. This method is known as Global Phase Seismic Interferometry (GloPSI). Traditionally, GloPSI retrieves low-frequency information (up to 1 Hz). Recent studies, however, suggest that there is high-frequency signal present in the coda of strong, distant earthquakes. This research quantifies the potential of these high-frequency signals, by analysing recordings of a multitude of high-magnitude earthquakes (≥6.4 〈span〉M〈/span〉〈sub〉w〈/sub〉) and their coda on a selection of permanent USArray stations. Nearly half of the 〈span〉P, PKP〈/span〉 and PKIKP phases are recorded with a signal-to-noise ratio of at least 5 dB at 3 Hz. To assess the viability of using the high-frequency signal, the second half of the paper highlights two case studies. First, a known sedimentary structure is imaged in Malargüe, Argentina. Secondly, the method is used to reveal the structure of the Midcontinent Rift below the SPREE array in Minnesota, USA. Both studies demonstrate that structural information of the shallow crust (≤5 km) below the arrays can be retrieved. In particular, the interpreted thickness of the sedimentary layer below the Malargüe array is in agreement with earlier studies in the same area. Being able to use global phases and direct 〈span〉P〈/span〉-phases with large epicentral distances (〉80°) to recover the Earth’s sedimentary structure suggests that GloPSI can be applied in an industrial context.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The presence of large paleo-landslides can hinder the construction of railway lines if they cause an alteration of the natural balance of the slope, significantly increasing the cost of the project. During the construction works in a section of the Madrid-Asturias high-speed railway line (Spain), a large-scale hillside instability affected 460 m of the section. The ground movement began ten months after the start of the excavations and remained active throughout the observation period (2008-2010). Data provided by fieldwork, boreholes and instrumental monitoring has allowed the investigation of the geological units involved, together with the geometry and the kinematics of the mass movement. This landslide involves a Paleozoic basement with an estimated volume of 4,400,000 m〈sup〉3〈/sup〉. It shows low displacement rates (〈45 mm/year) and was accommodated on a single surface of rupture, that reaches more than 60 m in depth. The kinematics is favoured by structural and lithological conditions, there being a strong rheological contrast in the lower part of the mobilized ground. This movement is linked to a large paleo-landslide that was partially reactivated when excavations undermined the lower slopes of the hillside. Technical solutions for the stabilization have already caused an additional cost of 17% over the initial budget.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉The localization of passive seismic sources in form of microseismic tremors as well as large-scale earthquakes is a key issue in seismology. While most previous studies are assuming fairly good knowledge of the underlying velocity model, we propose an automatic spatial localization and joint velocity model building scheme that is independent of detailed 〈span〉a priori〈/span〉 information. The first step is a coherence analysis, estimating so-called wavefront attributes to locally describe the wavefield in terms of slopes and curvatures. In a similar fashion, we also obtain an initial guess of the source excitation times of the recorded events. The wavefront attributes constitute the input for wavefront tomography which represents the next step of the workflow and allows for a refinement of the previously evaluated source excitation times while simultaneously approximating the velocity distribution. In a last step, we use the final estimate of the velocity distribution and compute the respective image function by reverse time modelling to gain the source locations. This paper introduces the theoretical concept of our proposed approach for the general 3-D case. We analyse the feasibility of our strategy and the influences of different acquisition settings by means of a synthetic 2-D data example. In a final 3-D field data example we use the workflow to localize a deep earthquake without relying on a given velocity model. The approach can deal with high levels of noise and low signal amplitudes, respectively, as well as sparse geophone sampling. The workflow generally delivers good approximations of the long-wavelength velocity variations along with accurate source locations.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉The 2017 July 20, 〈span〉M〈/span〉〈sub〉w〈/sub〉6.6 Bodrum–Kos earthquake occurred in the Gulf of Gökova in the SE Aegean, a region characterized by N–S extension in the backarc of the easternmost Hellenic Trench. The dip direction of the fault that ruptured during the earthquake has been a matter of controversy where both north- and south-dipping fault planes were used to model the coseismic slip in previous studies. Here, we use seismic (seismicity, main shock modelling, aftershock relocations and aftershock mechanisms using regional body and surface waves), geodetic (GPS, InSAR) and structural observations to estimate the location, and the dip direction of the fault that ruptured during the 2017 earthquake, and the relationship of this event to regional tectonics. We consider both dip directions and systematically search for the best-fitting locations for the north- and south-dipping fault planes. Comparing the best-fitting planes for both dip directions in terms of their misfit to the geodetic data, proximity to the hypocenter location and Coulomb stress changes at the aftershock locations, we conclude that the 2017 earthquake ruptured a north-dipping fault. We find that the earthquake occurred on a 20–25 km long, ∼E–W striking, 40° north-dipping, pure normal fault with slip primarily confined between 3 and 15 km depth, and the largest slip exceeding 2 m between depths of 4 and 10 km. The coseismic fault, not mapped previously, projects to the surface within the western Gulf, and partly serves both to widen the Gulf and separate Kos Island from the Bodrum Peninsula of SW Anatolia. The coseismic fault may be an extension of a mapped, north-dipping normal fault along the south side of the Gulf of Gökova. While all of the larger aftershocks are consistent with N–S extension, their spatially dispersed pattern attests to the high degree of crustal fracturing within the basin, due to rapid trenchward extension and anticlockwise rotation within the southeastern Aegean.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉The harmonic electromagnetic noise produced by anthropic electrical structures is a critical component of the global noise affecting geophysical signals and increasing data uncertainty. It is composed of a series of harmonic signals whose frequencies are multiple integers of the fundamental frequency specific to the electrical noise source. To date, most model-based noise removal strategies assume that the fundamental frequency constraining the harmonic noise is single and constant over the duration of the geophysical record. In this paper, we demonstrate that classical harmonic processing methods lose efficacy when these assumptions are not valid. We present several surface nuclear magnetic resonance field data sets, which testify the increasing probability of recording the harmonic noise with such multiple or unstable frequency content. For each case (multiple frequencies or unstable frequency) we propose new processing strategies, namely, the 〈span〉2-D grid-search〈/span〉 and the 〈span〉segmentation〈/span〉 approach, respectively, which efficiently manage to remove the harmonic noise in these difficult conditions. In the process, we also apply a fast frequency estimator called the Nyman, Gaiser and Saucier estimation method, which shows equivalent performance as classical estimators while allowing a reduction of the computing time by a factor of 2.5.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Combinatorial methods are used to determine the spatial distribution of earthquake magnitudes on a fault whose slip rate varies along strike. Input to the problem is a finite sample of earthquake magnitudes that span 5 kyr drawn from a truncated Pareto distribution. The primary constraints to the problem are maximum and minimum values around the target slip-rate function indicating where feasible solutions can occur. Two methods are used to determine the spatial distribution of earthquakes: integer programming and the greedy-sequential algorithm. For the integer-programming method, the binary decision vector includes all possible locations along the fault where each earthquake can occur. Once a set of solutions that satisfy the constraints is found, the cumulative slip misfit on the fault is globally minimized relative to the target slip-rate function. The greedy algorithm sequentially places earthquakes to locally optimize slip accumulation. As a case study, we calculate how earthquakes are distributed along the megathrust of the Nankai subduction zone, in which the slip rate varies significantly along strike. For both methods, the spatial distribution of magnitudes depends on slip rate, except for the largest magnitude earthquakes that span multiple sections of the fault. The greedy-sequential algorithm, previously applied to this fault (Parsons et al., 2012), tends to produce smoother spatial distributions and fewer lower magnitude earthquakes in the low slip-rate section of the fault compared to the integer-programming method. Differences in results from the two methods relate to how much emphasis is placed on minimizing the misfit to the target slip rate (integer programming) compared to finding a solution within the slip-rate constraints (greedy sequential). Specifics of the spatial distribution of magnitudes also depend on the shape of the target slip-rate function: that is, stepped at the section boundaries versus a smooth function. This study isolates the effects of slip-rate variation along a single fault in determining the spatial distribution of earthquake magnitudes, helping to better interpret results from more complex, interconnected fault systems.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉The southcentral Hikurangi subduction margin (North Island, New Zealand) has a wide, low-taper accretionary wedge that is frontally accreting a 〉3-km-thick layer of sediments, with deformation currently focused near the toe of the wedge. We use a geological model based on a depth-converted seismic section, together with physically realistic parameters for fluid pressure, and sediment and décollement friction based on laboratory experiments, to investigate the present-day force balance in the wedge. Numerical models are used to establish the range of physical parameters compatible with the present-day wedge geometry and mechanics. Our analysis shows that the accretionary wedge stability and taper angle require either high to moderate fluid pressure on the plate interface, and/or weak frictional strength along the décollement. The décollement beneath the outer wedge requires a relatively weaker effective strength than beneath the inner (consolidated) wedge. Increasing density and cohesion with depth make it easier to attain a stable taper within the inner wedge, while anything that weakens the wedge—such as high fluid pressures and weak faults—make it harder. Our results allow a near-hydrostatic wedge fluid pressure, sublithostatic fluid overpressure at the subduction interface, and friction coefficients compatible with measurements from laboratory experiments on weak clay minerals.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We present a new methodology to compute the gravitational fields generated by tesseroids (spherical prisms) whose density varies with depth according to an arbitrary continuous function. It approximates the gravitational fields through the Gauss–Legendre Quadrature along with two discretization algorithms that automatically control its accuracy by adaptively dividing the tesseroid into smaller ones. The first one is a preexisting 2-D adaptive discretization algorithm that reduces the errors due to the distance between the tesseroid and the computation point. The second is a new density-based discretization algorithm that decreases the errors introduced by the variation of the density function with depth. The amount of divisions made by each algorithm is indirectly controlled by two parameters: the distance-size ratio and the delta ratio. We have obtained analytical solutions for a spherical shell with radially variable density and compared them to the results of the numerical model for linear, exponential, and sinusoidal density functions. The heavily oscillating density functions are intended only to test the algorithm to its limits and not to emulate a real world case. These comparisons allowed us to obtain optimal values for the distance-size and delta ratios that yield an accuracy of 0.1 per cent of the analytical solutions. The resulting optimal values of distance-size ratio for the gravitational potential and its gradient are 1 and 2.5, respectively. The density-based discretization algorithm produces no discretizations in the linear density case, but a delta ratio of 0.1 is needed for the exponential and most sinusoidal density functions. These values can be extrapolated to cover most common use cases, which are simpler than oscillating density profiles. However, the distance-size and delta ratios can be configured by the user to increase the accuracy of the results at the expense of computational speed. Finally, we apply this new methodology to model the Neuquén Basin, a foreland basin in Argentina with a maximum depth of over 5000 m, using an exponential density function.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We predict stresses and strains in the Tarfaya salt basin on the West African coast using a 3D static geomechanical model and compare the results against a simplified 2D plane-strain model. Both models are based on present-day basin geometries, are drained, and use a poroelastic description for the sediments and visco-plastic description for salt. We focus on a salt diapir, where an exploratory well has been drilled crossing a major fault. The 3D model shows a significant horizontal stress reduction in sediments at the top of the diapir, validated with measured data later obtained from the well. The 2D model predicts comparable stress reduction in sediments at the crest of the diapir. However, it shows a broader area affected by the stress reduction, overestimating its magnitude by as much as 1.5 MPa. Both models predict a similar pattern of differential displacement in sediments along both sides of the major fault, above the diapir. These displacements are the main cause of horizontal stress reduction detected at the crest of the diapir. Sensitivity analysis in both models shows that the elastic parameters of the sediments have a minimal effect on the stress–strain behaviour. In addition, the 2D sensitivity analysis concludes that the main factors controlling stress and strain changes are the geometry of the salt and the difference in rock properties between encasing sediments and salt. Overall, our study demonstrates that carefully built 2D models at the exploration stage can provide stress information and useful insights comparable to those from more complex 3D geometries.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Latest Oligocene-Pleistocene basic sheet intrusions in the Late Oligocene-Pliocene Wichianburi Sub-basin (onshore Thailand), and Eocene intrusions of the Ceduna Sub-basin (offshore southern Australia) provide examples of igneous intrusion architecture in rift and post-rift basin settings, respectively that have been imaged on 3D seismic reflection data. These examples indicate rift-related intrusions are overall smaller, have a higher aspect ratio, and tend to be more planar or transgressive in a single direction, than the saucer-shaped sills which dominate in the post-rift setting of the Ceduna Sub-basin. The long axis trend and dip-direction of most sills in the rift setting investigated tend to conform with rift structure (i.e. fault strike-directions and bedding dip and strike directions), and give a strong orientation bias, whereas orientations in the post-rift basin are more varied. The few saucer-shaped sills that formed in the Wichianburi Sub-basin are asymmetric and incompletely developed. A third area, the Kora region of the Taranaki Basin is a region where intrusions are related to a volcanic centre. This gives the intrusions a mixture of circumferential and radial intrusions related to magmatic processes (e.g. loading by the volcanic edifice, magma chamber pressure) and rift-related influences. Rift-related sills commonly form stacked arrays. Fundamental factors that affect the morphology of sills include: magma composition, volume, reservoir pressure, and the mechanical stratigraphy of the host rocks. In rifts extra complexities affecting some sills, or parts of sills include: rift structure (faults, rotated, folded bedding), basin size and morphology, stress distribution and sedimentary architecture.〈strong〉Supplementary material:〈/strong〉〈a href="https://doi.org/10.6084/m9.figshare.c.4620101"〉https://doi.org/10.6084/m9.figshare.c.4620101〈/a〉〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Widespread wrench tectonics has been described along the Northern Andes. The Bucaramanga fault, described as sinistral strike-slip, bounds the high Santander Massif. We combine structural and thermochronologic data at the central-southern portion of the fault, in order to investigate the vertical displacement.Structural survey data shows: 1) old phases of activity preserved in the host rocks along the fault trace, with superimposition of different slickenlines generations; and 2) both strike- and dip-slip kinematics indicators. New and previous thermochronologic data show that differential exhumation of the fault walls has been ongoing from 50 Ma. The hangingwall, the Santander Massif, records: 1) in the central portion, decreasing exhumation rates from the early Miocene to the middle-late Miocene; and 2) in the southern portion, constant rates through the Late Oligocene to the Pliocene.Combining such observations, the thermochronologic offset resulting from the relative motion of the two fault walls is comparable with the observed elevation drop across the fault, suggesting that the present topography of the Santander Massif is related to the vertical movement along the Bucaramanga fault. We infer that the fault has a significant Neogene reverse component, consistent with present-day horizontal GPS vector data, long-term exhumation rates and the structural assemblage.〈strong〉Supplementary material:〈/strong〉〈a href="https://doi.org/10.6084/m9.figshare.c.4620140"〉https://doi.org/10.6084/m9.figshare.c.4620140〈/a〉〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The dynamics of dyke emplacement are typically modeled by assuming an elastic rheology for the host rock. However, the resulting stress field predicts significant shear failure in the region surrounding the dyke tip. Here, we model the dyking process in an elastic-perfectly plastic host rock in order to simulate distributed shear fracturing and subsequent frictional slip on the fracture surfaces. The fluid mechanical aspects of the magma are neglected as we are interested only in the fracture mechanics of the process. Magma overpressure in dykes is typically of the same order of magnitude as the yield stress of the host rock in shear, especially when the pressure effect of volatiles exsolving from the magma is taken into account. Under these conditions, the plastic deformation zone has spatial dimensions that approach the length of the dyke itself, and concepts based on linear elastic fracture mechanics (LEFM) no longer apply. As incremental plasticity is path dependent, we describe two geologically meaningful endmember cases, namely dyke propagation at constant driving pressure, and gradual inflation of a pre-existing crack. For both models, we find that plastic deformation surrounding the fracture tip enhances dyke opening, and thus increases the energy input into the system due to pressure work integrated over the fracture wall. At the same time, energy is dissipated by plastic deformation. Dissipation in the propagation model is greater by about an order of magnitude than it is in the inflation model because the propagating dyke tip leaves behind it a broad halo of deformation due to plastic bending and unbending in the relict process zone. The net effect is that plastic deformation impedes dyke growth in the propagation model, while it enhances dyke growth in the inflation model. The results show that, when the plastic failure zone is large, a single parameter such as fracture toughness is unable to capture the physics that underpin the resistance of a fracture or dyke against propagation. In these cases, plastic failure has to be modeled explicitly for the given conditions. We provide analytical approximations for the propagation forces and the maximum dyke aperture for the two endmember cases, that is, the propagating dyke and the dyke formed by inflation of a crack. Furthermore, we show that the effect of plasticity on dyke energetics, together with an overestimate of magma pressure when interpreting dyke aspect ratios using elastic host rock models, offers a possible explanation for the long-standing paradox that laboratory measurements of fracture toughness of rocks consistently indicate values about two orders of magnitude lower than those derived from dyke observations.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Clay-with-flints Formation outcrops on the high chalk plateaux and interfluves of the chalk downs in southern England. Both current and historical definitions of the clay-with-flints are detailed and important distinctions are identified with other deposits which appear identical but are formed in different ways. Historically pedological or geomorphological studies have been carried out on the deposit. Engineering studies are only carried out where the deposit is crossed by infrastructure. The physical and chemical processes acting on the deposit and the resulting effects on the physical properties are discussed.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Building geomechanical models for induced seismicity in complex reservoirs poses a major challenge, in particular if many faults need to be included. We developed a novel way of calculating induced stress changes and associated seismic moment response for structurally complex reservoirs with tens to hundreds of faults. Our specific target was to improve the predictive capability of stress evolution along multiple faults, and to use the calculations to enhance physics-based understanding of the reservoir seismicity. Our methodology deploys a mesh-free numerical and analytical approach for both the stress calculation and the seismic moment calculation. We introduce a high-performance computational method for high-resolution induced Coulomb stress changes along faults, based on a Green's function for the stress response to a nucleus of strain. One key ingredient is the deployment of an octree representation and calculation scheme for the nuclei of strain, based on the topology and spatial variability of the mesh of the reservoir flow model. Once the induced stress changes are evaluated along multiple faults, we calculate potential seismic moment release in a fault system supposing an initial stress field. The capability of the approach, dubbed as MACRIS (〈strong〉M〈/strong〉echanical 〈strong〉A〈/strong〉nalysis of 〈strong〉C〈/strong〉omplex 〈strong〉R〈/strong〉eservoirs for 〈strong〉I〈/strong〉nduced 〈strong〉S〈/strong〉eismicity) is proven through comparisons with finite element models. Computational performance and suitability for probabilistic assessment of seismic hazards are demonstrated though the use of the complex, heavily faulted Gullfaks field.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We test the feasibility of GPS-based rapid centroid moment tensor (GPS CMT) methods for Taiwan, one of the most earthquake prone areas in the world. In recent years, Taiwan has become a leading developer of seismometer-based earthquake early warning systems, which have successfully been applied to several large events. The rapid determination of earthquake magnitude and focal mechanism, important for a number of rapid response applications, including tsunami warning, is still challenging because of the limitations of near-field inertial recordings. This instrumental issue can be solved by an entirely different observation system: a GPS network. Taiwan is well posed to take advantage of GPS because in the last decade it has developed a very dense network. Thus, in this research, we explore the suitability of the GPS CMT inversion for Taiwan. We retrospectively investigate six moderate to large (〈span〉M〈/span〉〈sub〉w〈/sub〉6.0 ∼ 7.0) earthquakes and propose a resolution test for our model, we find that the minimum resolvable earthquake magnitude of this system is ∼〈span〉M〈/span〉〈sub〉w〈/sub〉5.5 (at 5 km depth). Our tests also suggest that the finite fault complexity, often challenging for the near-field methodology, can be ignored under such good station coverage and thus, can provide a fast and robust solution for large earthquake directly from the near field. Our findings help to understand and quantify how the proposed methodology could be implemented in real time and what its contributions could be to the overall earthquake monitoring system.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Time-domain processing of seismic reflection data has always been an important engine that is routinely utilized to produce seismic images and to expeditiously construct subsurface models. The conventional procedure involves analysing parameters related to the derivatives of reflection traveltime with respect to offset including normal moveout (NMO) velocities (second-order derivatives) and quartic coefficients (fourth-order derivatives). In this study, we propose to go beyond the typical assumption of 1-D laterally homogeneous medium when relating those ‘processing’ parameters to the subsurface medium parameters and take into account the additional influences from lateral heterogeneity including curved interfaces and smoothly variable velocities. We fill in the theoretical gap from previous studies and develop a general framework for such connection in layered anisotropic media. We show that in general, the influences of lateral heterogeneity get accumulated from all layers via a recursive relationship according to the Fermat’s principle and can be approximately quantified in terms of the lateral derivatives of the layer interface surfaces and velocities. Based on the same general principle, we show that our approach can also be used to study the lateral heterogeneity effects on diffraction traveltime and its second-order derivative related to time-migration velocity. In this paper, we explicitly specify expressions for NMO and time-migration velocities with the influences from both types of heterogeneity suitable for 2-D data sets and also discuss possible extensions of the proposed theory to 3-D data sets and to parameters related to higher-order traveltime derivatives. Using numerical examples, we demonstrate that the proposed theory can lead to more accurate reflection and diffraction traveltime predictions in comparison with those obtained based on the 1-D assumption. Both the proposed theoretical framework and its numerical testing for forward traveltime computation presented in this study aid in understanding the effects from lateral heterogeneity on time-processing parameters and also serve as an important basis for designing an efficient technique to separate those influences in important processes such as Dix inversion for a more accurate subsurface model in the future.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Estimating shear wave velocity with depth from Rayleigh-wave dispersion data is limited by the accuracy of fundamental and higher mode identification and characterization. In many cases, the fundamental mode signal propagates exclusively in retrograde motion, while higher modes propagate in prograde motion. It has previously been shown that differences in particle motion can be identified with multicomponent recordings and used to separate prograde from retrograde signals. Here we explore the domain of existence of prograde motion of the fundamental mode, arising from a combination of two conditions: (1) a shallow, high-impedance contrast and (2) a high Poisson ratio material. We present solutions to isolate fundamental and higher mode signals using multicomponent recordings. Previously, a time-domain polarity mute was used with limited success due to the overlap in the time domain of fundamental and higher mode signals at low frequencies. We present several new approaches to overcome this low-frequency obstacle, all of which utilize the different particle motions of retrograde and prograde signals. First, the Hilbert transform is used to phase shift one component by 90° prior to summation or subtraction of the other component. This enhances either retrograde or prograde motion and can increase the mode amplitude. Secondly, we present a new time–frequency domain polarity mute to separate retrograde and prograde signals. We demonstrate these methods with synthetic and field data to highlight the improvements to dispersion images and the resulting dispersion curve extraction.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We use seismic noise cross-correlations to obtain a 3-D tomography model of 〈span〉SV〈/span〉-wave velocities beneath the western Indian Ocean, in the depth range of the oceanic crust and uppermost mantle. The study area covers 2000 × 2000 km〈sup〉2〈/sup〉 between Madagascar and the three spreading ridges of the Indian Ocean, centred on the volcanic hotspot of La Réunion. We use seismograms from 38 ocean bottom seismometers (OBSs) deployed by the RHUM-RUM project and 10 island stations on La Réunion, Madagascar, Mauritius, Rodrigues, and Tromelin. Phase cross-correlations are calculated for 1119 OBS-to-OBS, land-to-OBS, and land-to-land station pairs, and a phase-weighted stacking algorithm yields robust group velocity measurements in the period range of 3–50 s. We demonstrate that OBS correlations across large interstation distances of 〉2000 km are of sufficiently high quality for large-scale tomography of ocean basins. Many OBSs yielded similarly good group velocity measurements as land stations. Besides Rayleigh waves, the noise correlations contain a low-velocity wave type propagating at 0.8–1.5 km s〈sup〉−1〈/sup〉 over distances exceeding 1000 km, presumably Scholte waves travelling through seafloor sediments. The 100 highest-quality group velocity curves are selected for tomographic inversion at crustal and lithospheric depths. The inversion is executed jointly with a data set of longer-period, Rayleigh-wave phase and group velocity measurements from earthquakes, which had previously yielded a 3-D model of Indian Ocean lithosphere and asthenosphere. Robust resolution tests and plausible structural findings in the upper 30 km validate the use of noise-derived OBS correlations for adding crustal structure to earthquake-derived tomography of the oceanic mantle. Relative to crustal reference model CRUST1.0, our new shear-velocity model tends to enhance both slow and fast anomalies. It reveals slow anomalies at 20 km depth beneath La Réunion, Mauritius, Rodrigues Ridge, Madagascar Rise, and beneath the Central Indian spreading ridge. These structures can clearly be associated with increased crustal thickness and/or volcanic activity. Locally thickened crust beneath La Réunion and Mauritius is probably related to magmatic underplating by the hotspot. In addition, these islands are characterized by a thickened lithosphere that may reflect the depleted, dehydrated mantle regions from which the crustal melts where sourced. Our tomography model is available as electronic supplement.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Since the completion of the Gravity field and steady-state Ocean Circulation Explorer mission (GOCE), global gravity models of uniform quality and coverage are available. We investigate their potential of being useful tools for estimating the thermal structure of the continental lithosphere, through simulation and real-data test in Central-Eastern Europe across the Trans-European Suture Zone. Heat flow, measured near the Earth surface, is the result of the superposition of a complex set of contributions, one of them being the heat production occurring in the crust. The crust is enriched in radioactive elements respect to the underlying mantle and crustal thickness is an essential parameter in isolating the thermal contribution of the crust. Obtaining reliable estimates of crustal thickness through inversion of GOCE-derived gravity models has already proven feasible, especially when weak constraints from other observables are introduced. We test a way to integrate this in a geothermal framework, building a 3-D, steady state, solid Earth conductive heat transport model, from the lithosphere–asthenosphere boundary to the surface. This thermal model is coupled with a crust-mantle boundary depth resulting from inverse modelling, after correcting the gravity model for the effects of topography, far-field isostatic roots and sediments. We employ a mixed space- and spectral-domain based forward modelling strategy to ensure full spectral coherency between the limited spectral content of the gravity model and the reductions. Deviations from a direct crustal thickness to crustal heat production relationship are accommodated using a subsequent substitution scheme, constrained by surface heat flow measurements, where available. The result is a 3-D model of the lithosphere characterised in temperature, radiogenic heat and thermal conductivity. It provides added information respect to the lithospheric structure and sparse heat flow measurements alone, revealing a satisfactory coherence with the geological features in the area and their controlling effect on the conductive heat transport.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We investigate the possibility of passive monitoring of a salt-water disposal well in British Columbia, Canada, using continuously recorded ambient seismic noise. We find seismic velocity variations induced by a reduction of injection pressure in an effort to mitigate an elevated level of seismicity, most likely associated with the disposal of salt water. The relative velocity variations are derived from time-shifts measured between consecutive cross-correlation functions for each station pair in a surface array composed of five broad-band seismometers. The probable driving mechanisms responsible for the velocity changes are reduced pore pressures and/or lowered poroelastic stresses beyond the injection wellbore, respectively. Hydrologic data (e.g. snow and rainfall), noise energy trends and fluctuations in the incident direction of dominant noise sources do not correlate with the estimated relative velocity variations. Velocity variations are detected ahead of the zone of induced seismicity, thus indicating that seismic interferometry may aid in mitigation efforts to reduce the risk of induced seismicity by (1) providing verifiable and repeatable measurements of physical changes within the surrounding area and (2) providing hard constraints for modelling efforts to constrain how and where pore-pressure fronts change.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Electrical conductivity is one of the most commonly used geophysical method for reservoir and environmental studies. Its main interest lies in its sensitivity to key properties of storage and transport in porous media. Its quantitative use therefore depends on the efficiency of the petrophysical relationship to link them. In this work, we develop a new physically based model for estimating electrical conductivity of saturated porous media. The model is derived assuming that the porous media is represented by a bundle of tortuous capillary tubes with a fractal pore-size distribution. The model is expressed in terms of the porosity, electrical conductivity of the pore liquid and the microstructural parameters of porous media. It takes into account the interface properties between minerals and pore water by introducing a surface conductivity. Expressions for the formation factor and hydraulic tortuosity are also obtained from the model derivation. The model is then successfully compared with published data and performs better than previous models. The proposed approach also permits to relate the electrical conductivity to other transport properties such as the hydraulic conductivity.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Thinning of the lithosphere under continental collisional orogens is often attributed to delamination or convective thinning. Both processes remove part or all of the mantle lithosphere that has become denser and gravitationally unstable. Previous studies mostly focused on the different thermomagmatic consequences of these two processes; the dynamic links between them, and the critical conditions for one or the other process to dominate lithosphere thinning, remain uncertain. Here, we used high-resolution thermomechanical models with various rheology (linear viscous, power-law viscous and/or the extended Drucker–Prager plasticity) to systematically investigate the dynamics of delamination and convective thinning under collisional orogens. Our results show that convective thinning is favoured in models of linear (Newtonian) viscous rheology and low viscosity $({10^{19}}\!-\! {10^{20}}\,\,{\rm{Pa}} \, {\rm{s}})$. Power-law viscous rheology promotes strain localization, which reduces the effective viscosity and may lead to localized rising of the asthenosphere to the crustal base, thus triggering delamination. Further strain localization and stronger delamination are predicted with inclusion of plastic rheology in the model. These results indicate that convective thinning and delamination are dynamically linked and can occur in the same orogeny. Their relative dominance during orogenesis may be distinguished by the resulting spatiotemporal evolutions of thermal perturbation, magmatism and elevation changes. We applied the models to show that the evolution of the Central Anatolian Plateau is consistent with the dominance of convective thinning, whereas delamination played a major role in thinning the mantle lithosphere under central-northern Tibetan Plateau.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Developing a model for anthropogenic seismic hazard remains an open challenge whatever the geo-resource production. We analyse the (〈span〉M〈/span〉〈sub〉max〈/sub〉) largest reported magnitude on each site where (RTS) Reservoir Triggered Seismicity in documented (37 events, 1933–2008), for aftershocks of reservoir impoundment loading. We relate each reservoir impoundment to its magnitude-equivalent 〈span〉M〈/span〉*〈sub〉reservoir〈/sub〉 = 〈span〉M〈/span〉*(〈span〉L〈/span〉〈sub〉r〈/sub〉). We use (〈span〉L〈/span〉〈sub〉r〈/sub〉) the reservoir length as a proxy for a rupture length of the reservoir main shock-equivallent. This latter is derived from the empirical relationship that exists for tectonic earthquake among magnitude and rupture length. We resolve (i) 〈span〉M〈/span〉〈sub〉max〈/sub〉 for RTS are bounded by 〈span〉M〈/span〉*〈sub〉reservoir〈/sub〉 at a 95 per cent confidence level; (ii) in average 〈span〉M〈/span〉〈sub〉max〈/sub〉 are smaller than 〈span〉M〈/span〉*〈sub〉reservoir〈/sub〉 by 2.2 units (iii) 50 per cent of the 〈span〉M〈/span〉〈sub〉max〈/sub〉 occurrence is within 2 ± 1 yr from the reservoir impoundment. These triggering patterns support the signature of fluid driven seismicity during the slow reservoir impoundment emerges as a weaker efficiency (larger Δ〈span〉M〈/span〉 = 〈span〉M〈/span〉*〈sub〉reservoir〈/sub〉 – 〈span〉M〈/span〉〈sub〉max〈/sub〉) to trigger 〈span〉M〈/span〉〈sub〉max〈/sub〉 events than from earthquake interactions.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉The activities of frontal thrusts in the northern Qilian Shan are critical for understanding the deformation of the Qilian Shan and the northeastern Tibetan Plateau. In this study, we estimate the slip rate of the active Fodongmiao–Hongyazi thrust along the northern margin of the Qilian Shan. High-resolution satellite imagery interpretations and detailed field investigations suggest that the fault displaced late Pleistocene terraces and formed fresh prominent north-facing fault scarps. To quantify the slip rate of the fault, we measured the displacements along the fault scarps using an unmanned aerial vehicle system and dated the displaced geomorphic surfaces using optically stimulated luminescence (OSL) and 〈sup〉14〈/sup〉C methods. The vertical slip rate of the fault is estimated at 1.0 ± 0.3 mm yr〈sup〉−1〈/sup〉 for the western segment. The slip rates for two branches in the eastern segment are 0.3 ± 0.1 and 0.6 ± 0.1 mm yr〈sup〉−1〈/sup〉. Using a fault dip of 40 ± 10°, we constrain the corresponding shortening rates to 1.4 ± 0.5 and 1.2 ± 0.4 mm yr〈sup〉−1〈/sup〉, respectively. The rates are consistent with values over different timescales, which suggests steady rock uplift and northeastward growth of the western Qilian Shan. Crustal shortening occurs mainly on the range-bounding frontal thrust.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We present a numerical method for the simulation of earthquake cycles on a 1-D fault interface embedded in a 2-D homogeneous, anisotropic elastic solid. The fault is governed by an experimentally motivated friction law known as rate-and-state friction which furnishes a set of ordinary differential equations which couple the interface to the surrounding volume. Time enters the problem through the evolution of the ordinary differential equations along the fault and provides boundary conditions for the volume, which is governed by quasi-static elasticity. We develop a time-stepping method which accounts for the interface/volume coupling and requires solving an elliptic partial differential equation for the volume response at each time step. The 2-D volume is discretized with a second-order accurate finite difference method satisfying the summation-by-parts property, with boundary and fault interface conditions enforced weakly. This framework leads to a provably stable semi-discretization. To mimic slow tectonic loading, the remote side-boundaries are displaced at a slow rate, which eventually leads to earthquake nucleation at the fault. Time stepping is based on an adaptive, fourth-order Runge–Kutta method and captures the highly varying timescales present. The method is verified with convergence tests for both the orthotropic and fully anisotropic cases. An initial parameter study reveals regions of parameter space where the systems experience a bifurcation from period one to period two behaviour. Additionally, we find that anisotropy influences the recurrence interval between earthquakes, as well as the emergence of aseismic transients and the nucleation zone size and depth of earthquakes.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Palaeomagnetic constraints are essential factors in the reconstruction of the Mesozoic convergence of Eastern Asia blocks. As one of the key blocks, Indochina was constrained only by sedimentary-rocks-derived palaeomagnetic data. To evaluate whether the palaeomagnetic data used to restore the Late Triassic position of Indochina suffered inclination shallowing effects, we conducted a palaeomagnetic and geochronologic study on a coeval volcanic clastic rocks sequence in the western margin of the Khorat Basin, Thailand. The U-Pb SIMS dating on zircons indicates the age of the sampling section is between 205.1 ± 1.5 and 204.7 ± 1.4 Ma. Site mean directions are D〈sub〉g〈/sub〉/I〈sub〉g〈/sub〉 = 217.2°/−39.4° (κ〈sub〉g〈/sub〉 = 45.1, α〈sub〉95g〈/sub〉 = 10.1°) before and D〈sub〉s〈/sub〉/I〈sub〉s〈/sub〉 = 209.2°/−44.5° (κ〈sub〉s〈/sub〉 = 43.8, α〈sub〉95s〈/sub〉 = 10.2°) after tilt correction. The new data set indicates a positive reversal test result at ‘Category C’ level. The characteristic remanent magnetization recorded by the coexistent magnetite and hematite is interpreted to be primary remanence acquired during the initial cooling of the volcanic clastic rocks. The consistence of the corresponding palaeolatitudes derived from the volcanic clastic rocks and the former reported sedimentary rocks suggests that there is probably no significant inclination shallowing bias in the sedimentary-rocks-derived palaeomagnetic data. Therefore, the estimates of the Late Triassic position of Indochina are confirmed to be reliable. The Indochina Block had collided to the southern margin of Eurasia by the Late Triassic and played an important role in the Mesozoic convergence of the Eastern Asia blocks.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We present a numerically exact method for calculating the internal and external gravitational potential of aspherical and heterogeneous planets. Our approach is based on the transformation of Poisson’s equation into an equivalent equation posed on a spherical computational domain. This new problem is solved in an efficient iterative manner based on a hybrid pseudospectral/spectral element discretization. The main advantage of our method is that its computational cost reflects the planet’s geometric and structural complexity, being in many situations only marginally more expensive than boundary perturbation theory. Several numerical examples are presented to illustrate the method’s efficacy and potential range of applications.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉In the case of long-range propagation of forward scattering, due to the accumulation of phase changes caused by the velocity perturbations, the validity of the Born approximation will be violated. In contrast, the phase-change accumulation can be handled by the Rytov approximation, which has been widely used for long-distance propagation with only forward scattering or small-angle scattering involved. However, the weak scattering assumption (i.e. small velocity perturbation) in the Rytov approximation limits its scope of application. To address this problem, we analyse the integral kernel of the Rytov transform using the Wentzel-Kramers-Brillouin-Jeffreys (WKBJ) approximation and we demonstrate that the integral kernel is a function of velocity perturbation and scattering angle. By applying a small scattering angle approximation, we show that the phase variation has a linear relationship with the slowness perturbation, no matter how strong the magnitude of perturbation is. Therefore, the new integral equation is then referred to as the generalized Rytov approximation (GRA) because it overcomes the weak scattering assumption of the Rytov approximation. To show the limitations of the Rytov approximation and the advantages of the proposed GRA method, first we design a two-layer model and we analytically calculate the errors introduced by the small scattering angle assumption using plane wave incidence. We show that the phase (traveltime) variations predicted by the GRA are always more accurate than the Rytov approximation. Particularly, the GRA produces accurate phase variations for the normal incident plane wave regardless of the magnitude of velocity perturbation. Numerical examples using Gaussian anomaly models demonstrate that the scattering angle has a crucial impact on the accuracy of the GRA. If the small scattering angle assumption holds, the GRA can produce an accurate phase approximation even if the velocity perturbation is very strong. On the contrary, both the first-order Rytov approximation and the GRA fail to get satisfying results when the scattering angle is large enough. The proposed GRA method has the potential to be used for traveltime modelling and inversion for large-scale strong perturbation media.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Estimating the location of geologic and tectonic features on a subducting plate is important for interpreting their spatial relationships with other observables including seismicity, seismic velocity and attenuation anomalies, and the location of ore deposits and arc volcanism in the over-riding plate. Here we present two methods for estimating the location of predictable features such as seamounts, ridges and fracture zones on the slab. One uses kinematic reconstructions of plate motions, and the other uses multidimensional scaling to flatten the slab onto the surface of the Earth. We demonstrate the methods using synthetic examples and also using the test case of fracture zones entering the Lesser Antilles subduction zone. The two methods produce results that are in good agreement with each other in both the synthetic and real examples. In the Lesser Antilles, the subducted fracture zones trend northwards of the surface projections. The two methods begin to diverge in regions where the multidimensional scaling method has its greatest likely error. Wider application of these methods may help to establish spatial correlations globally.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Asbestos nucleation and concentration in rocks are mostly associated with mechanisms of fibre formation, combined with the water-dependent mineralogical alteration produced during serpentinisation of ultramafic masses. Very little is known about the structural settings and tectonic histories that influence and control asbestos occurrence in non-serpentinised rocks, which are diffusely embedded within tectonised ophiolitic suites. Focussing on a case history provided by a tectonised metagabbro from the Ligurian Alps (northern Italy), a multiscale structural-petrographical approach is used to investigate the relationships between rock fabric and fibrous amphibole growth within the metagabbro. Meso- to micro-structural observations are used to document the role of structurally controlled fluid-rock interactions in localising the fibrous amphibole growth during ductile-to-brittle shearing (mylonitic foliation to shear veins). A qualitative structural scenario is here provided for illustrating the growth of asbestos amphiboles in shear veins during the progression of shear deformation towards semi-brittle rheological conditions. The mechanisms of structurally controlled growth of fibrous amphibole in non-serpentinised rocks imply an examination of the tectonic boundary conditions that are at the origin of the asbestos concentration in ophiolitic rocks involved in orogenic belt construction.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Microseismic monitoring is a primary tool for understanding and tracking the progress of mechanical processes occurring in active rock fracture systems. In geothermal or hydrocarbon fields or along seismogenic fault systems, the detection and location of microseismicity facilitates resolution of the fracture system geometry and the investigation of the interaction between fluids and rocks, in response of stress field perturbations. Seismic monitoring aims to detect locate and characterize seismic sources. The detection of weak signals is often achieved at the cost of increasing the number of false detections, related to transient signals generated by a range of noise sources, or related to instrumental problems, ambient conditions or human activity that often affect seismic records. A variety of fast and automated methods has been recently proposed to detect and locate microseismicity based on the coherent detection of signal anomalies, such as increase in amplitude or coherent polarization, at dense seismic networks. While these methods proved to be very powerful to detect weak events and to reduce the magnitude of completeness, a major problem remains to discriminate among weak seismic signals produced by microseismicity and false detections. In this work, the microseimic data recorded along the Irpinia fault zone (Southern Apennines, Italy) are analysed to detect weak, natural earthquakes using one of such automated, migration-based, method. We propose a new method for the automatic discrimination of real vs false detections, which is based on empirical data and information about the detectability (i.e. detection capability) of the seismic network. Our approach allows obtaining high performances in detecting earthquakes without requiring a visual inspection of the seismic signals and minimizing analyst intervention. The proposed methodology is automated, self-updating and can be tuned at different success rates.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉A multitaper estimator is proposed that accommodates time-series containing gaps without using any form of interpolation. In contrast with prior missing-data multitaper estimators that force standard Slepian sequences to be zero at gaps, the proposed missing-data Slepian sequences are defined only where data are present. The missing-data Slepian sequences are frequency independent, as are the eigenvalues that define the energy concentration within the resolution bandwidth, when the process bandwidth is $[ { - 1/2,\,\,\,1/2} )$ for unit sampling and the sampling scheme comprises integer multiples of unity. As a consequence, one need only compute the ensuing missing-data Slepian sequences for a given sampling scheme once, and then the spectrum at an arbitrary set of frequencies can be computed using them. It is also shown that the resulting missing-data multitaper estimator can incorporate all of the optimality features (i.e. adaptive-weighting, 〈span〉F〈/span〉-test and reshaping) of the standard multitaper estimator, and can be applied to bivariate or multivariate situations in similar ways. Performance of the missing-data multitaper estimator is illustrated using length of day, seafloor pressure and Nile River low stand time-series.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉The analysis of surface wave dispersion curves (DCs) is widely used for near-surface 〈span〉S〈/span〉-wave velocity (VS) reconstruction. However, a comprehensive characterization of the near-surface requires also the estimation of 〈span〉P〈/span〉-wave velocity (VP). We focus on the estimation of both VS and VP models from surface waves using a direct data transform approach. We estimate a relationship between the wavelength of the fundamental mode of surface waves and the investigation depth and we use it to directly transform the DCs into VS and VP models in laterally varying sites. We apply the workflow to a real data set acquired on a known test site. The accuracy of such reconstruction is validated by a waveform comparison between field data and synthetic data obtained by performing elastic numerical simulations on the estimated VP and VS models. The uncertainties on the estimated velocity models are also computed.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The decline or drying up of groundwater sources near a tunnel route is damaging to groundwater users. Therefore, forecasting the impact of a tunnel on nearby groundwater sources is a challenging task in tunnel design. In this study, numerical and analytical approaches were applied to the Qomroud water conveyance tunnel (located in Lorestan province, Iran) to assess the impact of tunnelling on the nearby extraction water wells. Using simulation of groundwater-level fluctuation owing to tunnelling, the drawdown at the well locations was determined. From the drawdowns and using Dupuit's equation, the depletion of well flow rates after tunnelling was estimated. To evaluate the results, observed well flow rates before and after tunnelling were compared with the predicted flow rates. The observed and estimated water well flows (before and after tunnelling) showed a regression factor of 0.64, pointing to satisfactory results〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Mullardoch Tunnel is a hydro-electric tunnel in the Highlands of Scotland, driven through metamorphic rocks of the Moine Series between 1947 and 1952. As with all large-scale hydro-electric projects, the geology exerted significant control and influence on the overall design and construction. However, while there was implicit and explicit consideration of geological aspects during the planning, design and construction of the schemes, the works largely pre-dated the systematic application of engineering geology and rock engineering in the UK. Nonetheless, the geologists involved with the project were able to identify and effectively communicate the main geological issues relevant to the engineers. Given the ongoing national import of hydro-electric schemes, this forms an interesting case study through which to retrospectively consider the geological input to hydro-electric tunnels. The geology, design, construction and maintenance of the tunnel are discussed together with the findings of recent inspections. Examples of good and bad practice are identified and implications for improved practice are drawn, including the ongoing importance of high quality geological input to engineering projects and the need for improved training in geological skills—particularly when applying modern practice to historic structures.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Seismic signal recognition can serve as a powerful auxiliary tool for analysing and processing ever-larger volumes of seismic data. It can facilitate many subsequent procedures such as first-break picking, statics correction, denoising, signal detection, events tracking, structural interpretation, inversion and imaging. In this study, I propose an automatic technique of seismic signal recognition taking advantage of unsupervised machine learning. In the proposed technique, seismic signal recognition is considered as a problem of clustering data points. All the seismic sampling points in time domain are clustered into two clusters, that is, signal or non-signal. The hierarchical clustering algorithm is used to group these sampling points. Four attributes, that is, two short-term-average-to-long-term-average ratios, variance and envelope are investigated in the clustering process. In addition, to quantitatively evaluate the performance of seismic signal recognition properly, I propose two new statistical indicators, namely, the rate between the total energies of original and recognized signals (RTE), and the rate between the average energies of original and recognized signals (RAE). A large number of numerical experiments show that when the signal is slightly corrupted by noise, the proposed technique performs very well, with recognizing accuracy, precision and RTE of nearly 1 (i.e. 100 per cent), recall greater than 0.8 and RAE about 1–1.3. When the signal is moderately corrupted by noise, the proposed technique can hold recognizing accuracy about 0.9, recognizing precision nearly to 1, RTE about 0.9, recall around 0.6 and RAE about 1.5. Applications of the proposed technique to real microseismic data induced from hydraulic fracturing and reflection seismic data demonstrate its feasibility and encouraging prospect.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Measurements of seismo-acoustic events by collocated seismic and infrasound arrays allow for studying the two wavefields that were produced by the same event. However, some of the scientific and technical constraints on the building of the two technologies are different and may be contradicting. For the case of a new station, an optimal design that will satisfy the constraints of the two technologies can be found. However, in the case of upgrading an existing array by adding the complementing technology, the situation is different. The site location, the array configuration and physical constraints are fixed and may not be optimal for the complementing technology, which may lead to rejection of the upgrade. The International Monitoring System (IMS) for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes 37 seismic arrays and 51 infrasound arrays. Although the CTBT verification regime is fixed in the treaty, an upgrade of the existing arrays by adding more technologies is possible.The Mount Meron seismic array (MMAI), which is part of the IMS, is composed of 16 sites. Microbarometers were installed at five MMAI sites to form the Mount Meron infrasound array. Due to regulation and physical constraints, it was not possible to relocate the sites nor to install analogue noise reduction filters (i.e. a pipe array). In this study, it is demonstrated that the installation of the MMAI infrasound array is beneficial despite the non-optimal conditions. It is shown that the noise levels of the individual array sites are between the high and median global noise levels. However, we claim that the more indicative measures are the noise levels of the beams of interest, as demonstrated by analysing the microbaroms originated from the Mediterranean Sea. Moreover, the ability to detect events relevant to the CTBT is demonstrated by analysing man-made events during 2011 from the Libya region.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Static and quasi-static Coulomb stress changes produced by large earthquakes can modify the probability of occurrence of subsequent events on neighbouring faults. This approach is based on physical (Coulomb stress changes) and statistical (probability calculations) models, which are influenced by the quality and quantity of data available in the study region. Here, we focus on the Wasatch fault zone (WFZ), a well-studied active normal fault system having abundant geological and palaeoseismological data. Palaeoseismological trench investigations of the WFZ indicate that at least 24 large, surface-faulting earthquakes have ruptured the fault's five central, 35–59-km long segments since ∼7 ka. Our goal is to determine if the stress changes due to the youngest palaeoevents have significantly modified the present-day probability of occurrence of large earthquakes on each of the segments. For each segment, we modelled the cumulative (coseismic + post-seismic) Coulomb stress changes (∆CFS〈sub〉cum〈/sub〉) due to earthquakes younger than the most recent event on the segment in question and applied the resulting values to the time-dependent probability calculations. Results from the Coulomb stress modelling suggest that the Brigham City, Salt Lake City, and Provo segments have accumulated ∆CFS〈sub〉cum〈/sub〉 larger than 10 bar, whereas the Weber segment has experienced a stress decrease of 5 bar, in the scenario of recent rupture of the Great Salt Lake fault to the west. Probability calculations predict high probability of occurrence for the Brigham City and Salt Lake City segments, due to their long elapsed times (〉1–2 ka) when compared to the Weber, Provo and Nephi segments (〈1 ka). The range of calculated coefficients of variation has a large influence on the final probabilities, mostly in the case of the Brigham City segment. Finally, when the Coulomb stress and the probability models are combined, our results indicate that the ∆CFS〈sub〉cum〈/sub〉 resulting from earthquakes post-dating the youngest events on each of the five segments substantially affects the probability calculations for three of the segments: Brigham City, Salt Lake City and Provo. The probability of occurrence of a large earthquake in the next 50 yr on these three segments may, therefore, be underestimated if a time-independent approach, or a time-dependent approach that does not consider ∆CFS, is adopted.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Quantifying landslide activity in remote regions is difficult because of the numerous complications that prevent direct landslide observations. However, building exhaustive landslide catalogues is critical to document and assess the impacts of climate change on landslide activity such as increasing precipitation, glacial retreat and permafrost thawing, which are thought to be strong drivers of the destabilization of large parts of the high-latitude/altitude regions of the Earth. In this study, we take advantage of the capability offered by seismological observations to continuously and remotely record landslide occurrences at regional scales. We developed a new automated machine learning processing chain, based on the Random Forest classifier, able to automatically detect and identify landslide seismic signals in continuous seismic records. We processed two decades of continuous seismological observations acquired by the Alaskan seismic networks. This allowed detection of 5087 potential landslides over a period of 22 yr (1995–2017). We observe an increase in the number of landslides for the period and discuss the possible causes.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉To describe the energy transport in the seismic coda, we introduce a system of radiative transfer equations for coupled surface and body waves in a scalar approximation. Our model is based on the Helmholtz equation in a half-space geometry with mixed boundary conditions. In this model, Green’s function can be represented as a sum of body waves and surface waves, which mimics the situation on Earth. In a first step, we study the single-scattering problem for point-like objects in the Born approximation. Using the assumption that the phase of body waves is randomized by surface reflection or by interaction with the scatterers, we show that it becomes possible to define, in the usual manner, the cross-sections for surface-to-body and body-to-surface scattering. Adopting the independent scattering approximation, we then define the scattering mean free paths of body and surface waves including the coupling between the two types of waves. Using a phenomenological approach, we then derive a set of coupled transport equations satisfied by the specific energy density of surface and body waves in a medium containing a homogeneous distribution of point scatterers. In our model, the scattering mean free path of body waves is depth dependent as a consequence of the body-to-surface coupling. We demonstrate that an equipartition between surface and body waves is established at long lapse-time, with a ratio which is predicted by usual mode counting arguments. We derive a diffusion approximation from the set of transport equations and show that the diffusivity is both anisotropic and depth dependent. The physical origin of the two properties is discussed. Finally, we present Monte Carlo solutions of the transport equations which illustrate the convergence towards equipartition at long lapse-time as well as the importance of the coupling between surface and body waves in the generation of coda waves.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉In the profile analysis of faults, the distribution of GNSS sites directly affects the accuracy of the results of slip rate and locking depth. This paper discusses strategies for designing the layout of GNSS stations perpendicular to strike-slip faults in terms of site spacing and the Minimum Effective Distance, which is 20 times the locking depth of the fault. Three layout models are proposed considering the complexity of strike-slip faults: (1) Equal spacing layout, in which many stations are deployed in the far field, only a few are deployed in the near field. (2) Equal deformation layout, in which stations are densely arranged in the near field and sparsely arranged in the far field according to the frequency of deformation curve. (3) Equal slope spacing layout, in which stations are arranged according to the nonlinear degree of the deformation curve, with dense distribution in regions with high nonlinearity and sparse distribution in approximately linear regions. The three models were used to redistribute the sites in the Qiaojia to Dongchuan segment of the Xiaojiang fault profile, and their performances were compared with that of the current sites distribution of the segment. The results showed that model 1 is optimal for fitting the accuracy of slip rate and model 3 is optimal for the accuracy of locking depth. Overall, model 3 appears to be the best choice, considering that the accuracy of the locking depth is more difficult to control. One of the main purposes of deployment is to identify the seismogenic depth of the fault. With the locking depth of the fault gradually approaching the depth of the seismogenic layer during an interseismic period, the accuracy of observations of sites deployed at a preset value of historical seismogenic depth of the fault would improve.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉HY-2A is China's first satellite altimeter mission, launched in Aug. 2011. Its geodetic mission (GM) started from 2016 March 30 till present, collecting sea surface heights for about five 168-d cycles. To test how the HY-2A altimeter performs in marine gravity derivation, we use the least-squares collocation method to determine marine gravity anomalies on 1′ × 1′ grids around the South China Sea (covering 0°–30°N, 105°E–125°E) from the HY-2A/GM-measured geoid gradients. We assess the qualities of the HY-2A/GM-derived gravity over different depths and areas using the bias and tilt-adjusted ship-borne gravity anomalies from the U.S. National Centers for Environmental Information (NCEI) and the Second Institute of Oceanography, Ministry of Natural Resources (MNR) of P. R. China. The RMS difference between the HY-2A/GM-derived and the NCEI ship-borne gravity is 5.91 mGal, and is 5.33 mGal when replacing the HY-2A value from the Scripps Institution of Oceanography (SIO) V23.1 value. The RMS difference between the HY-2A/GM-derived and the MNR ship-borne gravity is 2.90 mGal, and is 2.76 mGal when replacing the HY-2A value from the SIO V23.1 value. The RMS difference between the HY-2A and SIO V23.1 value is 3.57 mGal in open sea areas at least 20 km far away from the coast. In general, the difference between the HY-2A/GM-derived gravity and ship-borne gravity decreases with decreasing gravity field roughness and increasing depth. HY-2A results in the lowest gravity accuracy in areas with islands or reefs. Our assessment result suggests that HY-2A can compete with other Ku-band altimeter missions in marine gravity derivation.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉In hot drylands the main causes of concrete deterioration are physical salt attack by aggressive evaporite salts, reinforcement corrosion (e.g. carbonation, chloride ingress) and aggregate unsoundness. The manufacture of concrete presents more problems than in temperate countries due to the higher ambient temperatures and drying winds, plus windblown salts (mainly chlorides and sulphates). However, it is not just aridity that is significant to concrete in hot drylands, but also the availability of moisture and the duration of moist conditions. This paper draws upon the lifetime's experience gained by the lead author (PGF) and aims to provide a basic introduction to the ‘engineering geology of concrete' for hot drylands. The focus is on the Middle East and North Africa, but the general messages have broader applicability.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Unlike sedimentary clays, many tropical residual soils do not exhibit clear mechanical–empirical relationships to assist in their engineering characterization. In contrast, this paper discusses one residual clay in which such relationships may be determined, and further examines whether the effects of structure in this clay may be assessed using a framework previously developed for sedimentary clays. The Northland Allochthon tropical residual clay of New Zealand is a problematic soil of the fersiallitic type, prone to slope instability. Atterberg limit tests on soils from five field sites in the same geological unit show considerable variation, but that they are mechanically related. Laboratory element tests were performed on reconstituted and intact soil specimens from one field site. Normalization of the strength envelope using the equivalent stress on the intrinsic compression line suggests that soil structure, which is destroyed in reconstituted specimens, plays a role in the shear strength of this soil in its intact state. Overconsolidated behaviour, in the absence of geological preloading, points to the existence of a pseudo-preconsolidation pressure associated with weathering processes. The results further show that the saturated mechanical behaviour of this residual soil is in line with that of sedimentary clays and that mechanical–empirical relationships developed for such clays may be applied in this case.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉A structural domain represents a volume of rock masses with similar mechanical and hydrological properties. This paper presents a new multivariate method, built upon the classical statistical method, for identifying structural domains by considering a combined effect of multiple joint characteristics. First, joints are placed into corresponding cells according to a comprehensive categorization of joint characteristics. Second, based on the frequencies of joints in each cell, a contingency table analysis and appropriate χ〈sup〉2〈/sup〉 test are used to identify structural domains. In this method, the joint frequency in each cell is determined not only by joint orientation but also by joint trace length and aperture. A group of simulated joint populations is used to check the validity of the proposed method. Seven joint populations with a total of 731 joints collected in Southeast Dalian Port, China are evaluated using the proposed method. Calculations show that the differences in both distribution forms and sequences of joint characteristics produce apparently different identification results. A combined effect of multiple joint characteristics is found to play an important role in the identification of structural domains. Reliable results for the multivariate method are obtained, given the objective of analysis and practical application.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The geotechnical parameters and permeability of the Bazoft dam site were investigated. The bedrock of the Bazoft dam site belongs to the Asmari and Jahrum (Eocene, Oligocene and Miocene) formations. Rock mass rating (RMR) and geological strength index (GSI) classification systems were used to evaluate the dam site. Physical, mechanical and dynamic tests were performed on samples in a rock mechanics laboratory. Shear strength parameters and geotechnical parameters of the rock mass were determined. The results of Lugeon tests for determining rock mass permeability were in the range of 10–30. Based on the RMR, the dam site is located in the ‘good’ category. Finally, the relationship between Lugeon, the number of joints and rock quality designation 〈strong〉(〈/strong〉RQD) of the rock mass was evaluated using statistical analysis for 24 exploratory boreholes with a total length of 3400 m. The values of these parameters were determined and analysed using equations, tables and reports in sections of 5 m along the borehole. The results showed that the relationship between Lugeon with RQD and the joint number is an exponential equation with a good correlation coefficient. Multiple linear analysis also showed high accuracy of the estimated Lugeon based on RQD and joint number.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Gerede is the longest and most problematic water transmission tunnel in Turkey, at 31.6 km long with an overburden of 600 m. The tunnel will transmit water from the Ulusu river to the Camlidere reservoir. High-pressure water ingress and collapses have resulted in many delays. Although construction began in 2012, and was intended to be completed in 2014, completion is now estimated to be in 2019. The tunnel passes through permeable Tertiary sedimentary and volcano-sedimentary rocks, and Jurassic–Cretaceous limestone. Water ingress has occurred on 45 occasions along the 26 km excavated. The initial total discharge was 1.7 m〈sup〉3〈/sup〉 s〈sup〉−1〈/sup〉 and decreased to 0.5 m〈sup〉3〈/sup〉 s〈sup〉−1〈/sup〉 over time. The recharge conditions, tectonic structure, progress of the excavation and sealing by grouting affect the quantity, locations and temporal changes of the inflow of water. The major faults and formation contacts are the most problematic sites to be considered.With the many delays in the tunnel, the driving plan and tunnel boring machine (TBM) were changed while the construction continued. The tunnel shows that hydrogeological conditions, in particular, should be taken into consideration when choosing the TBM and driving course. However, the problems we encountered provided valuable experience for future projects.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉This paper examines the procedures developed and employed by BEAR Scotland Limited to efficiently deal with geotechnical emergencies on the trunk road network in NW Scotland, and specifically how BEAR can call upon expert geotechnical consultancy and construction services to promptly resolve failures which impact on road infrastructure. The paper considers two case studies involving the failure of a significant embankment on the A9 and a natural terrain hazard (landslide/debris flow) failure affecting the A83, and how BEAR's approach allowed these geotechnical emergencies to be dealt with in an efficient manner.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Fiji's infrastructure is regularly affected by rainfall-induced slope failures, but the engineering properties of failed soils are rarely described. We report mineralogical, geotechnical and index properties of soils from headscarp exposures of 18 slope failures from tropical residual soils of differing parent rocks in Viti Levu, Fiji. Scanning electron microscopy and X-ray diffraction revealed that kaolinite and smectite are the dominant clay minerals in the soils. Index properties included 〈span〉in situ〈/span〉 moisture content (28–114%), dry bulk density (0.7–1.5 g cm〈sup〉−3〈/sup〉), Atterberg limits (25–56% plastic limit; 38–79% liquid limit), effective particle size (0.4–12.6 μm) and clay fraction (0.6–19%). Geotechnical measurements included field compressive strength (127–461 kPa), hydraulic conductivity (〈span〉c.〈/span〉 10〈sup〉−7〈/sup〉 m s〈sup〉−1〈/sup〉), shear vane (16–128 kPa), ring shear (9.3–17.4°) and Emerson dispersion. Collectively, results indicated that most of the soils were cohesive, stiff, sensitive and in a plastic state in the field. Soils plotted below the A-line on the plasticity chart as fine silts of intermediate to high plasticity, and can theoretically sustain 〉50° slopes. Failure of these soils following high rainfall events is influenced by low permeability and the presence of expanding clays (e.g. smectite), causing temporary porewater pressure increases. No explicit relationships between soil properties and parent lithology were evident.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Trace elements in soils might cause contamination and ecological risks to the environment with increasing anthropogenic disturbance. This study investigated the distribution, speciation, risks, and possible sources of 28 target trace elements in soils of the northeastern Qinghai-Tibet Plateau. The average concentrations of trace elements in soils of the study area ranged from 0.25 (Hg) to 697.38 (Cr) mg/kg. The residual fraction was the dominant host of V, Cr, Cu, Sn, Sb, Hg and REEs while Co, Ni, Zn, Mo, Cd and Pb had large proportions in the non-residual fractions. Risk assessment code analysis showed that Cd should be recognized as a priority pollutant in the study area. Correlation analysis indicated that Pb, Cu, Zn, Cd and Sn might originate from the same or similar source. The 〈sup〉206〈/sup〉Pb/〈sup〉207〈/sup〉Pb and 〈sup〉208〈/sup〉Pb/〈sup〉206〈/sup〉Pb ratios of the soils were in the range of 1.166-1.224 and 2.031-2.122, respectively. The anthropogenic Pb contribution proportion was in the range of 0-53.92% for the study area. The anthropogenic Pb contribution was higher in the areas with more intensive anthropogenic activities. Heavy metals and excessive anthropogenic disturbance should be effectively controlled in the northeastern Qinghai-Tibet Plateau to maintain the ecological sustainability and human health of this fragile area.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Virtual Deep Seismic Sounding (VDSS) has emerged as a novel method to image the crust-mantle-boundary (CMB) and potentially other lithospheric boundaries. In Part 1 (Liu et al., 2018), we showed that the arrival time and waveform of post-critical 〈span〉SsPmp〈/span〉, the post-critical reflection phase at the CMB used in VDSS, is sensitive to several different attributes of the crust and upper mantle. Here, we synthesize our methodology of deriving Moho depth, average crustal 〈span〉Vp〈/span〉 and uppermost-mantle 〈span〉Vp〈/span〉 from single-station observations of post-critical 〈span〉SsPmp〈/span〉 under a 1D assumption. We first verify our method with synthetics and then substantiate it with a case study using the Yellowknife and POLARIS arrays in the Slave Craton, Canada. We show good agreement of crustal and upper-mantle properties derived with VDSS with those given by previous active-source experiments and our own P receiver functions (PRF) in our study area. Finally, we propose a PRF-VDSS joint analysis method to constrain average crustal 〈span〉Vp〈/span〉/〈span〉Vs〈/span〉 ratio and composition. Our PRF-VDSS joint analysis shows that the southwest Slave Craton has an intermediate crustal composition, most consistent with a Mesoarchean age.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The inherent heterogeneity of carbonate rocks suggests that carbonate-hosted fault zones are also likely to be heterogeneous. Coupled with a lack of host-fault petrophysical relationships, this makes the hydraulic behaviour of carbonate-hosted fault zones difficult to predict. Here we investigate the link between host and fault rock porosity, permeability and texture, by presenting data from series of host rock, damage zone and fault rock samples from normally faulted, shallowly buried limestones from Malta. Core plug X-ray tomography indicates that texturally heterogeneous host rocks lead to greater variability in the porosity and permeability of fault rocks. Fault rocks derived from moderate to high porosity (〉20%) formations experience permeability reductions of up to six orders of magnitude relative to the host; 〉50% of these fault rocks could act as barriers to fluid flow over production timescales. Fault rocks derived from lower porosity (〈20%) algal packstones have permeabilities that are lower than their hosts by up to three orders of magnitude, which is unlikely to impact fluid flow on production timescales. The variability of fault rock permeability is controlled by a number of factors, including the initial host rock texture and porosity, the magnitude of strain localisation, and the extent of post-deformation diagenetic alteration. Fault displacement has no obvious control over fault rock permeability. The results enable better predictions of fault rock permeability in similar lithotypes and tectonic regimes. This may enable predictions of fault zone sealing potential when combined with data on fault zone architecture.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The variation of temperature in the crust is difficult to quantify due to the sparsity of surface heat flow observations and lack of measurements on the thermal properties of rocks at depth. We examine the degree to which the thermal structure of the crust can be constrained from Curie depth and surface heat flow data in Southeastern Australia. We cast the inverse problem of heat conduction within a Bayesian framework and derive its adjoint so we can efficiently find the optimal model that best reproduces the data and prior information on the thermal properties of the crust. Efficiency gains obtained from the adjoint method facilitates a detailed exploration of thermal structure in SE Australia, where we predict high temperatures within Precambrian rocks of 650 〈sup〉○〈/sup〉C due to relatively high rates of heat production (0.9–1.4 μW m〈sup〉−3〈/sup〉). In contrast, temperatures within dominantly Phanerozoic crust reach only 520 〈sup〉○〈/sup〉C at the Moho due to the low rates of heat production in Cambrian mafic volcanics. A combination of Curie depth and heat flow data are required to constrain the uncertainty of lower crustal temperatures to ± 73 〈sup〉○〈/sup〉C. We also show that parts of the crust are unconstrained if either dataset is omitted from the inversion.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The mantle transition zone is the region between the globally observed major seismic velocity discontinuities around depths of 410 and 660 km and is important for determining the style of convection and mixing between the upper and the lower mantle. In this study, P-to-S converted waves, or receiver functions, are used to study these discontinuities beneath the Alaskan subduction zone, where the Pacific plate subducts underneath the North American plate. Previous tomographic models do not agree on the depth extent of the subducting slab, therefore improved imaging of the Earth structure underneath Alaska is required. We use 27,800 high quality radial receiver functions to make common conversion point stacks. Upper mantle velocity anomalies are accounted for by two recently published regional tomographic S-wave velocity models. Using these two tomographic models, we show that the discontinuity depths within our CCP stacks are highly dependent on the choice of velocity model, between which velocity anomaly magnitudes vary greatly. We design a quantitative test to show whether the anomalies in the velocity models are too strong or too weak, leading to over- or under-corrected discontinuity depths. We also show how this test can be used to rescale the 3D velocity corrections in order to improve the discontinuity topography maps. After applying the appropriate corrections, we find a localised thicker mantle transition zone and an uplifted 410 discontinuity, which show that the slab has clearly penetrated into the mantle transition zone. Little topography is seen on the 660 discontinuity, indicating that the slab has probably not reached the lower mantle. In the southwest, P410s arrivals have very small amplitudes or no significant arrival at all. This could be caused by water or basalt in the subducting slab, reducing the strength at the 410, or by topography on the 410 discontinuity, preventing coherent stacking. In the southeast of Alaska, a thinner mantle transition zone is observed. This area corresponds to the location of a slab window, and thinning of the mantle transition zone may be caused by hot mantle upwellings.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉A conference was held at the Geological Society of London on the theme of ‘Ground-related Risk to Transportation Infrastructure’. This conference attracted nearly 200 delegates from around the world and, while there was a UK focus, presentations and posters of a very high quality were given on a wide range of topics illustrated by global examples. The thematic set of papers presented in this issue of the journal give a good indication of the range and quality of the subject matter presented to the conference. Transportation infrastructure is of vital strategic importance to countries, and is often referred to as the backbone of a modern economy. Risks arising from hazards in the ground, either natural or anthropogenic, pose a significant threat to the safety and performance of such transport networks and, hence, the subject of geotechnical asset management is of great importance. This set of papers covers examples drawn from the four themes of the conference: strategic geotechnical asset management; current and future resilience and monitoring; ground-related hazards; and operational response to hazards and events. It is hoped that the spirit of knowledge sharing and discussion engendered by the conference continues with future events and publications.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉We present an iterative classification scheme using inter-event cross-correlation to update an existing earthquake catalogue with similar events from a list of automatic seismic event detections. The algorithm automatically produces catalogue quality events, with improved hypocentres and reliable P and S arrival time information. Detected events are classified into four event categories with the purpose of using the top category, with the highest assessed event quality and highest true-to-false ratio, directly for local earthquake tomography without additional manual analysis. The remaining categories have varying proportions of lower quality events, quality being defined primarily by the number of observed phase onsets, and can be viewed as different priority groups for manual inspection to reduce the time spent by a seismic analyst. A list of 3348 event detections from the geothermally active volcanic region around Hengill, southwest Iceland, produced by our migration and stack detector (Wagner et al. 2017), was processed using a reference catalogue of 1108 manually picked events from the same area. P and S phase onset times were automatically determined for the detected events using correlation time lags with respect to manually picked phase arrivals from different multiple reference events at the same station. A significant improvement of the initial hypocentre estimates was achieved after re-locating the detected events using the computed phase onset times. The differential time dataset resulting from the correlation was successfully used for a double-difference re-location of the final updated catalogue. The routine can potentially be implemented in real-time seismic monitoring environments in combination with a variety of seismic event/phase detectors.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉To describe the energy transport in the seismic coda, we introduce a system of radiative transfer equations for coupled surface and body waves in a scalar approximation. Our model is based on the Helmholtz equation in a half-space geometry with mixed boundary conditions. In this model, Green’s function can be represented as a sum of body waves and surface waves, which mimics the situation on Earth. In a first step, we study the single-scattering problem for point-like objects in the Born approximation. Using the assumption that the phase of body waves is randomized by surface reflection or by interaction with the scatterers, we show that it becomes possible to define, in the usual manner, the cross-sections for surface-to-body and body-to-surface scattering. Adopting the independent scattering approximation, we then define the scattering mean free paths of body and surface waves including the coupling between the two types of waves. Using a phenomenological approach, we then derive a set of coupled transport equations satisfied by the specific energy density of surface and body waves in a medium containing a homogeneous distribution of point scatterers. In our model, the scattering mean free path of body waves is depth dependent as a consequence of the body-to-surface coupling. We demonstrate that an equipartition between surface and body waves is established at long lapse-time, with a ratio which is predicted by usual mode counting arguments. We derive a diffusion approximation from the set of transport equations and show that the diffusivity is both anisotropic and depth dependent. The physical origin of the two properties is discussed. Finally, we present Monte-Carlo solutions of the transport equations which illustrate the convergence towards equipartition at long lapse-time as well as the importance of the coupling between surface and body waves in the generation of coda waves.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉In planetary fluid cores, the density depends on temperature and chemical composition, which diffuse at very different rates. This leads to various instabilities, bearing the name of double-diffusive convection. We investigate rotating double-diffusive convection (RDDC) in fluid spheres. We use the Boussinesq approximation with homogeneous internal thermal and compositional source terms. We focus on the finger regime, in which the thermal gradient is stabilising whereas the compositional one is destabilising. First, we perform a global linear stability analysis in spheres. The critical Rayleigh numbers drastically drop for stably stratified fluids, yielding large-scale convective motions where local analyses predict stability. We evidence the inviscid nature of this large-scale double-diffusive instability, enabling the determination of the marginal stability curve at realistic planetary regimes. In particular, we show that in stably stratified spheres, the Rayleigh numbers 〈span〉Ra〈/span〉 at the onset evolve like 〈span〉Ra〈/span〉 ∼ 〈span〉Ek〈/span〉〈sup〉−1〈/sup〉, where 〈span〉Ek〈/span〉 is the Ekman number. This differs from rotating convection in unstably stratified spheres, for which 〈span〉Ra〈/span〉 ∼ 〈span〉Ek〈/span〉〈sup〉−4/3〈/sup〉. The domain of existence of inviscid convection thus increases as 〈span〉Ek〈/span〉〈sup〉−1/3〈/sup〉. Second, we perform nonlinear simulations. We find a transition between two regimes of RDDC, controlled by the strength of the stratification. Furthermore, far from the RDDC onset, we find a dominating equatorially anti-symmetric, large-scale zonal flow slightly above the associated linear onset. Unexpectedly, a purely linear mechanism can explain this phenomenon, even far from the instability onset, yielding a symmetry breaking of the nonlinear flow at saturation. For even stronger stable stratification, the flow becomes mainly equatorially-symmetric and intense zonal jets develop. Finally, we apply our results to the early Earth core. Double diffusion can reduce the critical Rayleigh number by four decades for realistic core conditions. We suggest that the early Earth core was prone to turbulent RDDC, with large-scale zonal flows.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Time-domain processing of seismic reflection data has always been an important engine that is routinely utilized to produce seismic images and to expeditiously construct subsurface models. The conventional procedure involves analyzing parameters related to the derivatives of reflection traveltime with respect to offset including normal moveout (NMO) velocities (second-order derivatives) and quartic coefficients (fourth-order derivatives). In this study, we propose to go beyond the typical assumption of 1D laterally homogeneous medium when relating those ‘processing’ parameters to the subsurface medium parameters and take into account the additional influences from lateral heterogeneity including curved interfaces and smoothly variable velocities. We fill in the theoretical gap from previous studies and develop a general framework for such connection in layered anisotropic media. We show that in general, the influences of lateral heterogeneity get accumulated from all layers via a recursive relationship according to the Fermat’s principle and can be approximately quantified in terms of the lateral derivatives of the layer interface surfaces and velocities. Based on the same general principle, we show that our approach can also be used to study the lateral heterogeneity effects on diffraction traveltime and its second-order derivative related to time-migration velocity. In this paper, we explicitly specify expressions for NMO and time-migration velocities with the influences from both types of heterogeneity suitable for 2D datasets and also discuss possible extensions of the proposed theory to 3D datasets and to parameters related to higher-order traveltime derivatives. Using numerical examples, we demonstrate that the proposed theory can lead to more accurate reflection and diffraction traveltime predictions in comparison with those obtained based on the 1D assumption. Both the proposed theoretical framework and its numerical testing for forward traveltime computation presented in this study aid in understanding the effects from lateral heterogeneity on time-processing parameters and also serve as an important basis for designing an efficient technique to separate those influences in important processes such as Dix inversion for a more accurate subsurface model in the future.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉H-κ stacking is used routinely to infer crustal thickness and bulk-crustal V〈sub〉〈span〉P〈/span〉〈/sub〉/V〈sub〉〈span〉S〈/span〉〈/sub〉 ratio from teleseismic receiver functions. The method assumes that the largest amplitude P-to-S conversions beneath the seismograph station are generated at the Moho. This is reasonable where the crust is simple and the Moho marks a relatively abrupt transition from crust to mantle, but not if the crust-mantle transition is gradational and/or complex intra-crustal structure exists. We demonstrate via synthetic seismogram analysis that H-κ results can be strongly dependent on the choice of stacking parameters (the relative weights assigned to the Moho P-to-S conversion and its subsequent reverberations, the choice of linear or phase-weighted stacking, input crustal P-wave velocity) and associated data parameters (receiver function frequency content and the sample of receiver functions analyzed). To address this parameter sensitivity issue, we develop an H-κ approach in which cluster analysis selects a final solution from 1000 individual H-κ results, each calculated using randomly-selected receiver functions, and H-κ input parameters. Ten quality control criteria that variously assess the final numerical result, the receiver function dataset, and the extent to which the results are tightly clustered, are used to assess the reliability of H-κ stacking at a station. Analysis of synthetic datasets indicates H-κ works reliably when the Moho is sharp and intra-crustal structure is lacking but is less successful when the Moho is gradational. Limiting the frequency content of receiver functions can improve the H-κ solutions in such settings, provided intra-crustal structure is simple. In cratonic Canada, India and Australia, H-κ solutions generally cluster tightly, indicative of simple crust and a sharp Moho. In contrast, on the Ethiopian plateau, where Paleogene flood-basalts overlie marine sediments, H-κ results are unstable and erroneous. For stations that lie on thinner flood-basalt outcrops, and/or in regions where Blue Nile river incision has eroded through to the sediments below, limiting the receiver function frequency content to longer periods improves the H-κ solution and reveals a 6–10 km gradational Moho, readily interpreted as a lower-crustal intrusion layer at the base of a mafic (V〈sub〉〈span〉P〈/span〉〈/sub〉/V〈sub〉〈span〉S〈/span〉〈/sub〉=1.77–1.87) crust. Moving off the flood-basalt province, H-κ results are reliable and the crust is thinner and more felsic (V〈sub〉〈span〉P〈/span〉〈/sub〉/V〈sub〉〈span〉S〈/span〉〈/sub〉=1.70–1.77), indicating the lower crustal intrusion layer is confined to the region covered by flood-basaltic volcanism. Analysis of data from other tectonically-complex settings (e.g., Japan, Cyprus) shows H-κ stacking results should be treated cautiously. Only in regions of relatively simple crust can H-κ stacking analysis be considered truly reliable.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The 280-km-long San Cristobal Trough (SCT), created by the tearing of the Australia plate as it subducts under the Pacific plate near the Solomon and Vanuatu subduction zones, has hosted strike-slip earthquake sequences in 1993 and 2015. Both sequences, which likely represent a complete seismic cycle, began along the oldest section of the SCT—the portion farthest from the tear that has experienced the most cumulative displacement—and migrated to the younger sections closer to the tear. The SCT's abundant seismicity allows us to study transform boundary development—a process rarely observed along a single fault system—through observations of earthquake rupture properties. Using the spectral ratio method based on empirical Green's functions (EGFs), we calculate the corner frequencies of three M〈sub〉w〈/sub〉 ∼7 2015 earthquakes and co-located smaller earthquakes. We utilize two different spectral ratio stacking methods and fit both Brune and Boatwright models to the stacked spectral ratios. Regardless of stacking methods and spectral models, we find that the corner frequencies of the 2015 M〈sub〉w〈/sub〉 ∼7 earthquakes decrease slightly with distance from the tear. Assuming a constant rupture velocity and an omega-square spectral model, this corner frequency decrease may be due to an increase in rupture length with distance from the tear. The spectrum of the 2015 earthquake farthest from the tear also deviates from the omega-square model, which may indicate rupture complexity. Stress drop estimates from the corner frequencies of the 2015 M〈sub〉w〈/sub〉 ∼7 earthquakes range between 1 and 7 MPa, whereas stress drop estimates of their EGFs range from ∼0.05 to 10 MPa with most values between 0.1 to 1 MPa. Independent evidence from a second moments analysis of the 2015 earthquake sequence also indicates a possible increase in rupture length with distance from the tear, confirming the results from the spectral ratio analysis. We also observe an increase in normalized centroid time-delay values, a first-order proxy for rupture behavior, with distance from the tear for the 2015 sequence. A similar trend for the 1993 sequence suggests that earthquake rupture varies systematically along the SCT. Since distance from the tear corresponds to cumulative fault displacement, these along-strike rupture variations may be due to a displacement-driven fault maturation process.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We use a 2D forward finite-element model to explore how a laterally continuous permeable bed impacts the geological evolution and the geomechanical properties of a salt basin. We show that a permeable bed tilted by rise of a salt diapir substantially increases pore pressure in sediments near the diapir through hydraulically connecting these sediments to deep, high-overpressure sediments far from the diapir. The pore-pressure increase near the diapir has the following significant consequences: it causes a faster rise of the diapir; brings sediments near the diapir close to shear failure 〈span〉in situ〈/span〉; causes unloading of sediments around the crest of the permeable bed; and reduces the margin of appropriate mud weights for drilling near the diapir. The rise of the salt diapir induces concentrated lateral deformation and thereby overpressure in mudrocks encasing the permeable bed in an area near the bottom of the basin. This anomalously high overpressure is in marked contrast with the overpressure in the permeable bed, resulting in a large pore-pressure gradient between the permeable bed and encasing mudrocks. Our study provides insight into the importance of permeable beds to the structural evolution of a salt basin and to the exploration and production of hydrocarbon near salt diapirs.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Traditional methods for imaging salt bodies seldom consider near-salt stress perturbations caused by salt, and the associated velocity perturbations of seismic waves in the sediments near the salt. To demonstrate the importance of stress changes caused by the salt on accurately imaging salt bodies, in this study we develop and apply a combined method of geomechanical stress modelling and salt imaging. We simulate the stress perturbations in sediments induced by a salt sphere using a static geomechanical model, and calculate the associated velocity changes of seismic waves in the sediments by using our model stress perturbations. We use the reverse time migration and imaging method to image the salt sphere, and then analyse the imaging results of two cases including and excluding the effects of stress perturbations by the salt sphere on velocity changes of seismic waves. The results show that the near-salt velocity changes of seismic waves induced by stress perturbations near salt bodies can have a significant impact on the salt imaging. We find that when the effects of near-salt stress perturbations are ignored, the imaging of the salt sphere is clearly distorted: the salt sphere is extended vertically and becomes a salt ellipse with a vertical major axis. In contrast, when we include the effects of near-salt stress perturbations, the imaging of this salt sphere accurately matches the salt geometry and position. Thus, the near-salt stress perturbations should not be ignored in salt imaging. This study provides scientific insights for petroleum geologists and exploration geophysicists on the relationship between near-salt stress perturbations and accurate imaging of salt structures.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The post-rift history of the North Viking Graben has been backstripped in 3D, producing a sequence of palaeobathymetric maps that culminates at the Late Jurassic synrift stage. The backstripping takes into account the three main processes which drive post-rift basin development: thermal subsidence, flexural-isostatic loading and sediment compaction. Before backstripping was performed, the Norwegian Trench, a bathymetric feature within the present-day seabed, was smoothed in order to remove associated decompaction artefacts within the backstripping results.Palaeobathymetric restorations at the top and base of the Paleocene take into account regional transient dynamic uplift, probably related to the Iceland Plume. 350 m of uplift is incorporated at the Base Tertiary (65 Ma) and 300 m at the Top Balder Formation (54 Ma), followed by rapid collapse of this same uplift.At the top of the Lower Cretaceous (98.9 Ma), very localized fault-block topography, inherited from the Jurassic rift, is predicted to have remained emergent within the basin. At the Base Cretaceous (140 Ma), the fault-block topography is much more prominent and numerous isolated footwall islands are shown to have been present. At the Late Jurassic synrift stage (155 Ma), these islands are linked to form emergent island chains along the footwalls of all of the major faults. This is the Jurassic archipelago, the islands of which were the products of synrift footwall uplift. The predicted magnitude and distribution of footwall emergence calibrates well against available well data and published stratigraphic information, providing important constraints on the reliability of the results.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Thickness of cover over crystalline basement is an important consideration for mineral exploration in covered regions. It can be estimated from a variety of geophysical data types using a variety of inference methods. A robust method for combining such estimates to map the cover-basement interface over a region of interest is needed. Due to the large uncertainties involved, these need to be a probabilistic maps. Predominantly, interpolation methods are used for this purpose, but these are built on simplifying assumptions about the inputs which are often inappropriate. Bayesian estimate fusion is an alternative capable of addressing that issue by enabling more extensive use of domain knowledge about all inputs. This study is intended as a first step towards making Bayesian estimate fusion a practical tool for cover thickness uncertainty mapping. The main contribution is to identify the types of data assumptions that are important for this problem, to demonstrate their importance using synthetic tests, and to design a method that enables their use without introducing excessive tedium. We argue that interpolation methods like kriging often cannot achieve this goal and demonstrate that Markov chain Monte Carlo sampling can. This paper focuses on development of statistical methodology and presents synthetic data tests designed to reflect realistic exploration scenarios on an abstract level. Intended application is for the early stages of exploration where some geophysical data is available while drill hole coverage is poor.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The uneven distribution of earthquakes and stations in seismic tomography leads to slower convergence of nonlinear inversions and spatial bias in inversion results. Including dense regional arrays, such as USArray or Hi-Net, in global tomography causes severe convergence and spatial bias problems, against which conventional preconditioning schemes are ineffective. To save computational cost and reduce model bias, we propose a new strategy based on a geographical weighting of sources and receivers. Unlike approaches based on ray density or Voronoi tessellation, this method scales to large full-waveform inversion problems and avoids instabilities at the edges of dense receiver or source clusters. We validate our strategy using a 2D global waveform inversion test and show that the new weighting scheme leads to a nearly two-fold reduction in model error and much faster convergence relative to a conventionally-preconditioned inversion. We implement this geographical weighting strategy for global adjoint tomography.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉There is a pressing need for new exploration tools to target and vector towards mineralization in covered terrains. Groundwater provides a valuable and under-utilized geochemical sampling medium, and represents an important and cost-effective tool to expose covered terrains to systematic exploration. For Au exploration, researchers agree the best hydrogeochemistry pathfinder is dissolved Au itself, with additional potential from other pathfinders (albeit non-unique) such as As, Ag, W and Mo. Despite Au's relatively low solubility, with rigorous field protocols and appropriate analytical methods, explorers can respond to dissolved Au directly with robust parts per trillion (ppt) level analyses.Even with ppt-level analyses, a practical implication of Au's low solubility is that a deposit's dissolved Au signature is generally weaker than seen in other more mobile pathfinders, producing a smaller detectable footprint, which must be considered when designing exploration programs. Using purpose-drilled groundwater sampling bores, explorers can collect groundwater samples at the density required to respond to dissolved Au where existing bore hole coverage is otherwise insufficient. In addition to its use at the regional scale, with even tighter sample density, hydrogeochemistry also shows promise at the project scale, allowing for 3D modelling of pathfinder dispersion.For hydrogeochemistry to be widely adopted for Au exploration, explorers need confidence in ppt-level dissolved Au analyses, and the context to understand their significance. This paper aims to address these topics and provide a straightforward starting point for Au explorers interested in applying hydrogeochemistry by: (i) summarizing examples of regional sampling programs and more focused case studies to illustrate how covered Au deposits create measurable dissolved Au footprints distinguishable from background; and (ii) sharing examples of dissolved Au analyses being integrated into exploration at the regional and project scales.As seen in the results, the distributions of dissolved Au in the regional- and project-scale programs show remarkably similar and easy to interpret high-contrast, low-frequency anomalies against relatively low backgrounds. These are desirable attributes of any geochemical pathfinder. When combined with the benefits of hydrogeochemistry versus other geochemical exploration tools (e.g. groundwater can create larger footprints requiring fewer samples to detect, and groundwater can recharge from depth to reflect deeper mineralization), dissolved Au is a powerful pathfinder ideally suited for Au exploration in covered terrains.While this paper focuses on the use of dissolved Au, additional pathfinders can provide valuable information, including indications of lithological changes, hydrothermal alteration, and different styles of mineralization, as well as opportunities to use secondary pathfinders when sample density or local conditions may not result in detectable dissolved Au signatures.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉S-wave spectral amplitudes from 312 crustal earthquakes recorded at the Iranian National Broadband Seismic Network in the Alborz region between 2005 and 2017 are analysed in order to evaluate earthquake source parameters, path attenuation and site amplification functions using the nonparametric generalized inversion technique (GIT). We exploit a total number of 1117 seismograms with M〈sub〉L〈/sub〉 3–5.6 in the frequency range 0.3–20 Hz. The evaluated nonparametric attenuation functions decay uniformly with distance for the entire frequency range and the estimated S-wave quality factor shows low Q values with relatively strong frequency dependence. We assume the omega-square source model to retrieve earthquake source parameters from the inverted source spectra. The obtained stress drops range from 0.02 to 16 MPa with a mean value of 1.1 MPa. Stress drop and radiated energy show fairly self-similar scaling with seismic moment over the available magnitude range; however, the magnitude range of this study is too narrow to draw a definite conclusion on source scaling characteristics. The obtained moment magnitude M〈sub〉W〈/sub〉 and the local magnitude M〈sub〉L〈/sub〉 are linearly correlated and approximately equivalent in the range of M〈sub〉W〈/sub〉 3–4. For larger events, M〈sub〉W〈/sub〉 generally underestimates M〈sub〉L〈/sub〉 by about 0.1–0.5 magnitude units. The estimated site amplification functions for horizontal component (GIT H) are nearly flat with no obvious predominant frequency peaks for most stations, as expected for the sites of permanent broadband seismic stations located on rock, though a few stations show amplification peaks from 1 to 8 Hz, with a maximum amplification of about a factor of 7 with respect to the reference site. The evaluated site responses for the vertical components present remarkable amplification or deamplification, leading to differences of the H/V amplitude levels in comparison with the GIT H amplification curves. The results of this study provide a valuable basis for predicting appropriate ground motions in a context of seismic hazard assessment.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Upper Lutetian–Bartonian sedimentary mélanges, corresponding to ancient mud-rich submarine mass transport deposits, are widely distributed over an area 〈span〉c.〈/span〉 300 km long and tens of kilometres wide along the exhumed outer part of the External Ligurian accretionary wedge in the Northern Apennines. The occurrence of methane-derived carbonate concretions (septarians) in a specific tectonostratigraphic position below these sedimentary mélanges allows us to document the relationships among a significant period of regional-scale slope failure, climate change (the Early and Mid-Eocene Optimum stages), the dissociation of gas hydrates and accretionary tectonics during the Ligurian Tectonic Phase (early–mid-Lutetian). The distribution of septarians at the core of thrust-related anticlines suggests that the dissociation of gas hydrates was triggered by accretionary tectonics rather than climate change. The different ages of slope failure emplacement and the formation of the septarians support the view that the dissociation of gas hydrates was not the most important trigger for slope failure. The latter occurred during a tectonic quiescence stage associated with a regressive depositional trend, and probably minor residual tectonic pulses, which followed the Ligurian Tectonic Phase, favouring the dynamic re-equilibrium of the External Ligurian accretionary wedge. Our findings provide useful information for a better understanding of the factors controlling giant slope failure events in modern accretionary settings, where they may cause tsunamis.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Geochronological, elemental and Sr-Nd-Hf isotopic data for intermediate- acid volcanic sequences from the western North Huaiyang Belt of the Dabie Orogen, China, provide new constraints for the nature of the lithosphere and the potential involvement of the subducted Yangtze crust. These rocks are calc-alkaline andesite, trachyandesite and dacite. They were dated at 129∼127 Ma and show the geochemical affinity to arc volcanics. (87Sr/86Sr)i ratios and εNd(t) values range from 0.7065 to 0.7080 and −16.9 to −13.2, respectively. Zircon in-situ εHf(t) values range from –29.4 to –20.2 and Hf model ages from 3.03 Ga to 2.46 Ga. These volcanics might be derived from a hybridized source of NCC enriched lithosphere with the deep-recycled Yangtze continental crust. Our data, along with geological observations of the early Cretaceous igneous rocks, suggest an east-west extensional magmatic pulse at 130-120 Ma along the whole North Huaiyang Belt. From east to west, the recycled Yangtze continental crust has been decreasingly involved in the NCC enriched lithosphere in response to its Triassic northwardly deep subduction and subsequent exhumation. The lithospheric extension might induce the unrooting of previously-thickened lithosphere at early Cretaceous, resulting in the generation of the synchronous Cretaceous volcanics along the North Huaiyang Belt.〈strong〉Supplementary material:〈/strong〉〈a href="https://doi.org/10.6084/m9.figshare.c.4611911"〉https://doi.org/10.6084/m9.figshare.c.4611911〈/a〉〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Palaeozoic sedimentary successions in northern Ethiopia contain evidence for two Gondwana glaciations during the Late Ordovician and Carboniferous–Permian. We compare sediments of the two glaciations regarding their detrital zircon U–Pb ages. The main age group for both formations is Pan-African (〈span〉c〈/span〉. 550–700 Ma). However, the remaining spectra are different: The Upper Ordovician–Lower Silurian Enticho Sandstone is characterised by a Stenian–Tonian (〈span〉c.〈/span〉 1 Ga) zircon population. The Carboniferous–Permian Edaga Arbi Glacials contain a prominent 〈span〉c.〈/span〉 800 Ma population. The Stenian–Tonian zircons are likely derived from the centre of the East African Orogen and were supplied via the Gondwana super-fan system. This material was transported by the Late Ordovician glaciers and formed the Enticho Sandstone. Tonian (〈span〉c.〈/span〉 800 Ma) zircons are abundant in the Ethiopian basement and represent the earliest formation stage of the southern Arabian–Nubian Shield. Glaciers of the Late Palaeozoic Ice Age must have cut deeply into the basement for efficient erosion. No recycling of the Enticho Sandstone by the Edaga Arbi Glacials took place on a grand scale — probably because sedimentation of the former was limited to northern Ethiopia, whereas the source area for the latter was to the south.〈strong〉Supplementary material:〈/strong〉 A detailed description of the analytical parameters and supplementary data are available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4605548"〉https://doi.org/10.6084/m9.figshare.c.4605548〈/a〉.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Studies of the submerged Pleistocene conglomerate cored and dredged in the English Channel show that it resulted from the cementation of stony beaches under a loess blanket. Although the loess cover was later washed out by younger transgressions, the northern limit of the conglomerate corresponds to the original offshore extension of the loess deposits. Compilation of offshore and onshore altitudes of the limits of these deposits shows that loess was deposited by low-level wind fields never thicker than 200 m. Mapping surveys show that the present limits of loess are probably close to their original boundaries. The past proximity of the British Ice Sheet, the accumulation of dusty sediments coating north-facing cliffs of Brittany originating in the outwash of the British–Irish Ice Sheet and in the palaeo-rivers of the English Channel, as well as the southeastward orientation of the palaeo-winds deduced from particle size analysis and heavy minerals distribution, suggest that large volumes of loess were transported by strong katabatic winds blowing from the northern ice-covered regions towards Brittany and Normandy. The existence of katabatic winds is also consistent with the drift of ice rafts that transported Icelandic basalts now grounded in Brittany and possibly with the orientation of 〈span〉Homo neandertalensis〈/span〉 shelters.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉We introduce a new relative moment tensor (MT) inversion method for clusters of nearby earthquakes. The method extends previous work by introducing constraints from S-waves that do not require modal decomposition and by employing principal component analysis to produce robust estimates of excitation. At each receiver, P- and S-waves from each event are independently aligned and decomposed into principal components. P-wave constraints on MTs are obtained from a ratio of coefficients corresponding to the first principal component, equivalent to a relative amplitude. For S-waves we produce constraints on MTs involving three events, where one event is described as a linear combination of the other two, and coefficients are derived from the first two principal components. Non-linear optimization is applied to efficiently find best-fit tensile-earthquake and double-couple solutions for relative MT systems. Using synthetic data, we demonstrate the effectiveness of the P and S constraints both individually and in combination. We then apply the relative MT inversion to a set of 16 earthquakes from southern Alaska, at ∼125 km depth within the subducted Yakutat terrane. Most events are compatible with a stress tensor dominated by down-dip tension, however, we observe several pairs of earthquakes with nearly antiparallel slip implying that the stress regime is heterogeneous and/or faults are extremely weak. The location of these events near the abrupt down-dip termination of seismicity and the low-velocity zone suggest that they are caused by weakening via grain-size and volume reduction associated with eclogitization of the lower-crustal gabbro layer.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The complete part of the earthquake frequency-magnitude distribution, above the completeness magnitude 〈span〉mc〈/span〉, is well described by the Gutenberg-Richter law. On the other hand, incomplete data does not follow any specific law, since the shape of the frequency-magnitude distribution below max(〈span〉mc〈/span〉) is function of 〈span〉mc〈/span〉 heterogeneities that depend on the seismic network spatiotemporal configuration. This paper attempts to solve this problem by presenting an asymmetric Laplace mixture model, defined as the weighted sum of Laplace (or double exponential) distribution components of constant 〈span〉mc〈/span〉, where the inverse scale parameter of the exponential function is the detection parameter κ below 〈span〉mc〈/span〉, and the Gutenberg-Richter β-value above 〈span〉mc〈/span〉. Using a variant of the expectation maximization algorithm, the mixture model confirms the ontology proposed by Mignan [2012, 〈a href="https://doi.org/10.1029/2012JB009347"〉https://doi.org/10.1029/2012JB009347〈/a〉], which states that the shape of the earthquake frequency-magnitude distribution shifts from angular (in log-linear space) in a homogeneous space-time volume of constant 〈span〉mc〈/span〉 to rounded in a heterogeneous volume corresponding to the union of smaller homogeneous volumes. The performance of the proposed mixture model is analysed, with encouraging results obtained in simulations and in 8 real earthquake catalogues that represent different seismic network spatial configurations. We find that 〈span〉k〈/span〉 = κ/ln(10) ≈ 3 in most earthquake catalogues (compared to 〈span〉b〈/span〉 = β/ln(10) ≈ 1), suggesting a common detection capability of different seismic networks. Although simpler algorithms may be preferred on pragmatic grounds to estimate 〈span〉mc〈/span〉 and the 〈span〉b〈/span〉-value, other methods so far fail to model the angular distributions observed in homogeneous space-time volumes. Mixture modelling is a promising strategy to model the full earthquake magnitude range, hence potentially increasing seismicity data availability tenfold, since c. 90 per cent of earthquake catalogue events are below max(〈span〉mc〈/span〉).〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Full waveform inversion (FWI) is a nonlinear waveform matching procedure, which suffers from cycle skipping when the initial model is not kinematically-accurate enough. To mitigate cycle skipping, wavefield reconstruction inversion (WRI) extends the inversion search space by computing wavefields with a relaxation of the wave equation in order to fit the data from the first iteration. Then, the subsurface parameters are updated by minimizing the source residuals the relaxation generated. Capitalizing on the wave-equation bilinearity, performing wavefield reconstruction and parameter estimation in alternating mode decomposes WRI into two linear subproblems, which can solved efficiently with the alternating-direction method of multiplier (ADMM), leading to the so-called iteratively refined wavefield reconstruction inversion (IR-WRI). Moreover, ADMM provides a suitable framework to implement bound constraints and different types of regularizations and their mixture in IR-WRI. Here, IR-WRI is extended to multiparameter reconstruction for VTI acoustic media. To achieve this goal, we first propose different forms of bilinear VTI acoustic wave equation. We develop more specifically IR-WRI for the one that relies on a parametrisation involving vertical wavespeed and Thomsen’s parameters δ and ε. With a toy numerical example, we first show that the radiation patterns of the virtual sources generate similar wavenumber filtering and parameter cross-talks in classical FWI and IR-WRI. Bound constraints and TV regularization in IR-WRI fully remove these undesired effects for an idealized piecewise constant target. We show with a more realistic long-offset case study representative of the North Sea that anisotropic IR-WRI successfully reconstruct the vertical wavespeed starting from a laterally homogeneous model and update the long-wavelengths of the starting ε model, while a smooth δ model is used as a passive background model. VTI acoustic IR-WRI can be alternatively performed with subsurface parametrisations involving stiffness or compliance coefficients or normal moveout velocities and η parameter (or horizontal velocity).〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Although the oxidation of pyrite in geological materials has been linked to various problems in civil engineering works for many years, the risk of further problems has not been eliminated. Pyrite occurs widely as a minor constituent of geo-materials and many construction processes result in the exposure of pyrite bearing materials to air and water, which then result in oxidation. The paper presents an historical review of construction scenarios in which pyrite has had significant adverse impact. The cases considered include rapid deterioration of rock materials, chemical attack on concrete and metallic structures in the ground, and the expansion of fills and stabilised soils. These cases have been important turning points in understanding deleterious pyrite processes and the development of guidance for their avoidance. By following the guidance presented in the relevant standards and technical documentation, destructive processes due to pyrite should be avoided. However, as many of the problems are influenced by variables, such as site location factors and construction processes, adhering to published guidance may not eliminate the possibility of adverse impacts. It is important that expert technical advice is provided at all stages of projects and that any deviations from the expected performance should be properly evaluated.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Seismic signal recognition can serve as a powerful auxiliary tool for analyzing and processing ever-larger volumes of seismic data. It can facilitate many subsequent procedures such as first-break picking, statics correction, denoising, signal detection, events tracking, structural interpretation, inversion and imaging. In this study, I propose an automatic technique of seismic signal recognition taking advantage of unsupervised machine learning (ML). In the proposed technique, seismic signal recognition is considered as a problem of clustering data points. All the seismic sampling points in time domain are clustered into two clusters, i.e. signal or non-signal. The hierarchical clustering (HC) algorithm is used to group these sampling points. Four attributes, i.e. two short-term-average-to-long-term-average ratios (STA/LTAs), variance and envelope are investigated in the clustering process. In addition, to quantitatively evaluate the performance of seismic signal recognition properly, I propose two new statistical indicators, namely the rate between the total energies of original and recognized signals (RTE), and the rate between the average energies of original and recognized signals (RAE). A large number of numerical experiments show that, when the signal is slightly corrupted by noise, the proposed technique performs very well, with recognizing accuracy, precision and RTE of nearly 1 (i.e., 100%), recall greater than 0.8 and RAE about 1-1.3. When the signal is moderately corrupted by noise, the proposed technique can hold recognizing accuracy about 0.9, recognizing precision nearly to 1, RTE about 0.9, recall around 0.6 and RAE about 1.5. Applications of the proposed technique to real micro-seismic data induced from hydraulic fracturing and reflection seismic data demonstrate its feasibility and encouraging prospect.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Bitumen-bearing fractures and vugs were investigated in the highly organic-rich Jordan Oil Shale (JOS) of Late Cretaceous to Eocene age, which has potential as a highly fractured, unconventional hydrocarbon play. Bitumen is present as macroscopically visible deposits and as inclusions in the cement of abundant natural fractures and adjacent vugs. The frequency of bitumen occurrence in fractures closely correlates with Total Organic Carbon (TOC) and burial depth. Petrographic and organic-geochemical analyses on bitumen samples extracted from fractures and their host rock matrix show that the fracture-filling bitumen comprises indigenous low maturity hydrocarbons derived from the surrounding organic-rich Oil Shale and has not migrated from a deeper source. Maturity indicators imply that the oil shale is in the pre-oil generation stage of early catagenesis throughout the investigated area, but with a regional increase in thermal maturity from west to east as the result of greater maximum burial depth. Bitumen mobilization in the host rock was mainly controlled by vertical loading stress acting on the non-Newtonian bitumen phase in load bearing configurations in the organic-rich matrix. Bitumen fractures were developed by hydraulic fracturing as the result of fluid overpressure in the organic matter. Overpressured bitumen has acted as a fracture driver, generating bitumen veins in both the organic-rich mudstones and the adjacent chert and silicified intervals.〈strong〉Supplementary material:〈/strong〉〈a href="https://doi.org/10.6084/m9.figshare.c.4602290"〉https://doi.org/10.6084/m9.figshare.c.4602290〈/a〉〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Mantle plumes may play a major role in the transport of heat and mass through the Earth, but establishing their existence and structure using seismology has proven challenging and controversial. Previous studies have mainly focused on imaging plumes using waveform modelling and inversion (i.e. tomography). In this study we investigate the potential visibility of mantle plumes using array methods, and in particular whether we can detect seismic scattering from the plumes. By combining geodynamic modelling with mineral physics data we compute ‘seismic’ plumes whose shape and structure correspond to dynamically-plausible thermochemical plumes. We use these seismic models to perform a full-waveform simulation, sending seismic waves through the plumes, in order to generate synthetic seismograms. Using velocity spectral analysis and slowness-backazimuth plots, we are unable to detect scattering. However at longer dominant periods (25 seconds) we see several arrivals from outside the plane of the great circle path, that are consistent with an apparent bending of the wavefront around the plume conduit. At shorter periods (15 seconds), these arrivals are less obvious and less strong, consistent with the expected changes in the waves' behaviour at higher frequencies. We also detect reflections off the iron-rich chemical pile which serves as the plume source in the D'' region, indicating that D'' reflections may not always be due to a phase transformation. We suggest that slowness-backazimuth analysis may be a useful tool to locate mantle plumes in real array datasets. However, it is important to analyse the data at different dominant periods since, depending on the width of the plume, there is probably an optimum frequency band at which the plume is most visible. Our results also show the importance of studying the incoming energy in all directions, so that any apparently out-of-plane arrivals can be correctly interpreted.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉We test the feasibility of GPS-based rapid centroid moment tensor (GPS CMT) methods for Taiwan, one of the most earthquake prone areas in the world. In recent years Taiwan has become a leading developer of seismometer-based earthquake early warning systems which have successfully been applied to several large events. The rapid determination of earthquake magnitude and focal mechanism, important for a number of rapid response applications, including tsunami warning, is still challenging because of the limitations of near-field inertial recordings. This instrumental issue can be solved by an entirely different observation system: a GPS network. Taiwan is well-posed to take advantage of GPS because in the last decade it has developed a very dense network. Thus, in this research, we explore the suitability of the GPS CMT inversion for Taiwan. We retrospectively investigate six moderate to large (M〈sub〉w〈/sub〉6.0∼7.0) earthquakes and propose a resolution test for our model, we find that the minimum resolvable earthquake magnitude of this system is ∼M〈sub〉w〈/sub〉5.5 (at 5 km depth). Our tests also suggest that the finite fault complexity, often challenging for the near-field methodology, can be ignored under such good station coverage and thus, can provide a fast and robust solution for large earthquake directly from the near-field. Our findings help to understand and quantify how the proposed methodology could be implemented in real-time and what its contributions could be to the overall earthquake monitoring system.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Salt structures present numerous challenges for targeting reservoirs. Salt movement within the subsurface can follow complex pathways, producing deformation patterns in surrounding strata which are often difficult to decipher. Consequently, the relative role of key salt-flow drivers and geological sensitivities on salt-structure evolution are often poorly understood. To address this, we have developed 2D geomechanical models using the finite-element method to simulate salt diapir and pillow development in two extensional tectonic settings. We conducted model sensitivity analyses to examine the influence of geological parameters on field-scale salt structures and their corresponding deformation pattern. Modelled diapirs developing in thin-skinned extensional settings closely resemble published analogue experiments; however, active and passive stages of diapir growth are seldom or never reached, respectively, thus challenging existing ideas that diapir evolution is dominated by passive growth. In all modelled cases, highly strained domains bound the diapir flanks where extensive small-scale faulting and fracturing can be expected. Asymmetrical diapirs are prone to flank collapse and are observed in models with fast extension or sedimentation rates, thin roof sections or salt layers, or initially short or triangular-shaped diapirs. In modelled thick-skinned extensional settings, salt pillows and suprasalt overburden faults can be laterally offset (decoupled) from a reactivating basement fault. This decoupling increases with increased salt-layer thickness, overburden thickness, sedimentation rate and fault angle, and decreased fault slip rates. Contrary to existing consensus, overburden grounding onto the basement fault scarp does not appear to halt development of salt structures above the footwall basement block.〈strong〉Supplementary material:〈/strong〉 Animations for all model runs are available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4446272"〉https://doi.org/10.6084/m9.figshare.c.4446272〈/a〉〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Aquifer thermal energy storage (ATES) as a complement to fluctuating renewable energy systems is a reliable technology to guarantee continuous energy supply for heating and air conditioning. We investigated a high-temperature (HT) mono-well system (〈span〉c〈/span〉. 100°C), where the well screens are separated vertically within the aquifer, as an alternative to conventional doublet ATES systems for an underground storage in northern Oman. We analysed the impact of thermal inference between injection and extraction well screens on the heat recovery factor (HRF) in order to define the optimal screen-to-screen distance for best possible systems efficiency. Two controlling interference parameters were considered: the vertical screen-to-screen distance and aquifer heterogeneities. The sensitivity study shows that with decreasing screen-to-screen distances, thermal interference increases storage performance. A turning point is reached if the screen distance is too close, causing either water breakthrough or negative thermal interference between the screens. Our simulations show that a combined heat plume with spherical geometry results in the highest heat recovery factors due to the lowest surface area to volume ratios. Thick aquifers for mono-well HT-ATES are thus not mandatory. Our study shows that a HT-ATES mono-well system is a feasible storage design with high heat recovery factors for continuous cooling or heating purposes.〈/span〉