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Lithospheric plates diverge at mid-ocean ridges and asthenospheric mantle material rises in response. The rising material decompresses, which can result in partial melting, potentially impacting the driving forces of the system. Yet the geometry and spatial distribution of the melt as it migrates to the ridge axis are debated. Organized melt fabrics can cause strong seismic anisotropy, which can be diagnostic of melt, although this is typically not found at ridges. We present anisotropic constraints from an array of 39 ocean-bottom seismometers deployed on 0-80 Ma lithosphere from March 2016 to March 2017 near the equatorial Mid-Atlantic Ridge (MAR). Local and SKS measurements show anisotropic fast directions away from the ridge axis, which are consistent with strain and associated fabric caused by plate motions with short delay times, dt (<1.1 s). Near the ridge axis, we find several ridge-parallel fast splitting directions, f, with SKS dt that are much longer (1.7-3.8 s). This is best explained by ridge-parallel sub-vertical orientations of sheet-like melt pockets. This observation is much different than anisotropic patterns observed at other ridges, which typically reflect fabric related to plate motions. One possibility is that thicker sub-ridge lithosphere with steep subridge topography beneath slower spreading centers focuses melt into vertical, ridge-parallel melt bands, which effectively weakens the plate. Associated buoyancy forces elevate the subridge plate, providing greater potential energy and enhancing the driving forces of the plates. [ABSTRACT FROM AUTHOR]

Mapping fluid accumulation in the crust is pertinent for numerous applications including volcanic hazard assessment, geothermal energy generation, and mineral exploration. Here, we use seismic attenuation tomography to map the distribution of fluids in the crust below Uturuncu volcano, Bolivia. Seismic P wave and S wave attenuation, as well as their ratio (QP/QS), constrain where the crust is partially and fully fluid‐saturated. Seismic anisotropy observations further constrain the mechanism by which the fluids accumulate, predominantly along aligned faults and fractures in this case. Furthermore, subsurface pressure‐temperature profiles and conductivity data allow us to identify the most likely fluid composition. We identify shallow regions of both dry and H2O/brine‐saturated crust, as well as a deeper supercritical H2O/brine column directly beneath Uturuncu. Our observations provide a greater understanding of Uturuncu's transcrustal hydrothermal system, and act as an example of how such methods could be applied to map crustal fluid pathways and hydrothermal/geothermal systems elsewhere. Plain Language Summary: Locating where water/brines, gas, and molten rock are in the crust is important various applications, including assessing volcanic hazard, generating geothermal energy, and exploring for critical metals. Here, we map how seismic energy is absorbed (or attenuated) in Earth's crust, in order to look for fluids in the subsurface. We do this at Uturuncu volcano, Bolivia. This allows us to imagine whether the crust is partly or fully saturated with fluids. We also use seismic anisotropy to help us understand how the seismic energy is absorbed. We then use other data, including pressure, temperature, and electrical conductivity data to identify what fluids can be found where. We find that we can map where water/brines are and whether they contain carbon dioxide (i.e., are "sparkling") or not (i.e., "still"). Key Points: Seismic attenuation tomography can map crustal fluids and elucidate whether fluids are compressible, incompressible, or supercriticalS wave velocity anisotropy suggests that crustal fluids at Uturuncu migrate and/or accumulate along fracturesSeismic attenuation combined with temperature, pressure, and conductivity measurements can provide constraint on fluid composition [ABSTRACT FROM AUTHOR]

This paper presents a weakly supervised machine learning method, which we call DAS-N2N, for suppressing strong random noise in distributed acoustic sensing (DAS) recordings. DAS-N2N requires no manually produced labels (i.e. pre-determined examples of clean event signals or sections of noise) for training and aims to map random noise processes to a chosen summary statistic, such as the distribution mean, median or mode, whilst retaining the true underlying signal. This is achieved by splicing (joining together) two fibres hosted within a single optical cable, recording two noisy copies of the same underlying signal corrupted by different independent realizations of random observational noise. A deep learning model can then be trained using only these two noisy copies of the data to produce a near fully denoised copy. Once the model is trained, only noisy data from a single fibre is required. Using a data set from a DAS array deployed on the surface of the Rutford Ice Stream in Antarctica, we demonstrate that DAS-N2N greatly suppresses incoherent noise and enhances the signal-to-noise ratios (SNR) of natural microseismic icequake events. We further show that this approach is inherently more efficient and effective than standard stop/pass band and white noise (e.g. Wiener) filtering routines, as well as a comparable self-supervised learning method based on masking individual DAS channels. Our preferred model for this task is lightweight, processing 30 s of data recorded at a sampling frequency of 1000 Hz over 985 channels (approximately 1 km of fibre) in <1 s. Due to the high noise levels in DAS recordings, efficient data-driven denoising methods, such as DAS-N2N, will prove essential to time-critical DAS earthquake detection, particularly in the case of microseismic monitoring. [ABSTRACT FROM AUTHOR]

ABSTRACT: We investigate fracture‐induced attenuation anisotropy in a cluster of events from a microseismic dataset acquired during hydraulic fracture stimulation. The dataset contains 888 events of magnitude −3.0 to 0.0. We use a log‐spectral‐amplitude‐ratio method to estimate change in t ∗ over a half‐hour time period where fluid is being injected and an increase in fracturing from S‐wave splitting analysis has been previously inferred. A Pearson's correlation analysis is used to assess whether or not changes in attenuation with time are statistically significant. P‐waves show no systematic change in t ∗ during this time. In contrast, S‐waves polarised perpendicular to the fractures show a clear and statistically significant increase with time, whereas S‐waves polarised parallel to the fractures show a weak negative trend. We also compare t ∗ between the two S‐waves, finding an increase in Δ t ∗ with time. A poroelastic rock physics model of fracture‐induced attenuation anisotropy is used to interpret the results. This model suggests that the observed changes in t* are related to an increase in fracture density of up to 0.04. This is much higher than previous estimates of 0.025 ± 0.002 based on S‐wave velocity anisotropy, but there is considerably more scatter in the attenuation measurements. This could be due to the added sensitivity of attenuation measurement to non‐aligned fractures, fracture shape, and fluid properties. Nevertheless, this pilot study shows that attenuation measurements are sensitive to fracture properties such as fracture density and aspect ratio. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate fracture-induced attenuation anisotropy in a cluster of events from a microseismic dataset acquired during hydraulic fracture stimulation. The dataset contains 888 events of magnitude −3.0 to 0.0. We use a log-spectral-amplitude-ratio method to estimate change in [ABSTRACT FROM AUTHOR]

Understanding the crustal structure and the storage and movement of fluids beneath a volcano is necessary for characterizing volcanic hazard, geothermal prospects and potential mineral resources. This study uses local earthquake traveltime tomography to image the seismic velocity structure beneath Nabro, an off‐rift volcano located within the central part of the Danakil microplate near the Ethiopia‐Eritrea border. Nabro underwent its first historically documented eruption in June 2011, thereby providing an opportunity to analyze its post‐eruptive state by mapping subsurface fluid distributions. We use a catalog of earthquakes detected on a temporary seismic array using machine learning methods to simultaneously relocate the seismicity and invert for the three‐dimensional P‐ and S‐wave velocity structures (VP, VS) and the ratio between them (VP/VS). Overall, our model shows higher than average P‐ and S‐wave velocities, suggesting the presence of high‐strength, solidified intrusive magmatic rocks in the crust. We identify an aseismic region of low VP, low VS, and high VP/VS ratio at depths of 6–10 km b.s.l., interpreted as the primary melt storage region that fed the 2011 eruption. Above this is a zone of high VS, low VP, and low VP/VS ratio, representing an intrusive complex of fractured rocks partially saturated with over‐pressurized gases. Our observations identify the persistence of magma in the subsurface following the eruption, and track the degassing of this melt through the crust to the surface. The presence of volatiles and high temperatures within the shallow crust indicate that Nabro is a viable candidate for geothermal exploration. Plain Language Summary: Understanding the structure of the crust and the distribution and movement of fluids beneath a volcano allows for the assessment of volcanic hazard, geothermal potential and possible mineral extraction. To identify different regions of the crust and differentiate between fluids, we use the fact that the speed of seismic waves depends on the material they are traveling through. For example, seismic waves will travel through magma (molten, or liquid, rock) at lower speeds than in the surrounding rock. The focus of this study is Nabro volcano in Eritrea, which erupted in 2011. We use earthquakes that have been automatically detected following the eruption to image the structure of the crust in the form of 3D variations in seismic wave speeds. This identifies a volume of magma stored at depths of 6–10 km below sea level, which fed the eruption. Above this, we observe a region of rocks that are likely remnants of earlier eruptions at Nabro, with fractures containing gases at high pressure. The source of this high pressure is the release of gas from the magma storage zone. The presence of hot fluids means Nabro could be used as a source of geothermal power in the future. Key Points: 3D seismic modeling reveals the structure of the magmatic and hydrothermal systems beneath Nabro volcano in EritreaThe primary melt storage region feeding the 2011 eruption is located at depths of 6–10 km below sea levelDegassing from the magma storage zone causes overpressure in partially saturated, fractured intrusive complex above [ABSTRACT FROM AUTHOR]

ABSTRACT Fractures and faults within a reservoir can provide important pathways for the movement of reservoir fluids. Understanding the character and properties of these features on a range of length scales can be vital for the efficient exploitation of natural resources, whether it be enhanced oil and gas recovery, the safe storage of CO2, or better exploitation of geothermal heat. The monitoring of microseismicity within a reservoir illuminates active faults, but these events can be also used to characterize fracture networks through measurements of seismic anisotropy. In this study we use microseismic data acquired over an 18-day period in April 1997 at the Ekofisk oil field in the North Sea. Using the analysis of seismic multiplets we delineate a number of sub-vertical tectonic faults, which are consistent with previous core data analysis and seismic reflection work. We use shear wave splitting measurements, which are indicative of fracture-induced seismic anisotropy, to infer the orientation of aligned aseismic fracture sets within the reservoir. The estimated fracture dip and strike from the shear wave splitting analysis are consistent with the active tectonic fractures characterized by the multiplets, but this analysis also illuminates spatial variations in fracture properties. Such monitoring on a longer term and with multiple wells is a promising tool for better understanding fracture and fault-controlled flow within reservoirs. [ABSTRACT FROM AUTHOR]

ABSTRACT Constraining microseismic hypocentres in and around hydrocarbon reservoirs and their overburdens is essential for the monitoring of deformation related to hydraulic fracturing, production and injection and the assessment of reservoir security for CO2 and wastewater storage. Microseismic monitoring in hydrocarbon reservoirs can be achieved via a variety of surface and subsurface acquisition geometries. In this study we use data from a single, subsurface, vertical array of sensors. We test an existing technique that uses a 1D velocity model to constrain locations by minimizing differential S-to-P arrival times for individual sensors. We show that small errors in either arrival time picks or the velocity model can lead to large errors in depth, especially near velocity model discontinuities where events tend to cluster. To address this issue we develop two methods that use all available arrival times simultaneously in the inversion, thus maximizing the number of potential constraints from [ABSTRACT FROM AUTHOR]

Abstract: The granitic islands of the Seychelles Plateau have long been recognised to overlie continental crust, isolated from Madagascar and India during the formation of the Indian Ocean. However, to date the extent of continental crust beneath the Seychelles region remains unknown. This is particularly true beneath the Mascarene Basin between the Seychelles Plateau and Madagascar and beneath the Amirante Arc. Constraining the size and shape of the Seychelles continental fragment is needed for accurate plate reconstructions of the breakup of Gondwana and has implications for the processes of continental breakup in general. Here we present new estimates of crustal thickness and from H–κ stacking of receiver functions from a year long deployment of seismic stations across the Seychelles covering the topographic plateau, the Amirante Ridge and the northern Mascarene Basin. These results, combined with gravity modelling of historical ship track data, confirm that continental crust is present beneath the Seychelles Plateau. This is thick, but with a relatively high velocity lower crustal layer. This layer thins southwards from to over a distance of , which is consistent with the Seychelles being at the edge of the Deccan plume prior to its separation from India. In contrast, the majority of the Seychelles Islands away from the topographic plateau show no direct evidence for continental crust. The exception to this is the island of Desroche on the northern Amirante Ridge, where thicker low density crust, consistent with a block of continental material is present. We suggest that the northern Amirantes are likely continental in nature and that small fragments of continental material are a common feature of plume affected continental breakup. [Copyright &y& Elsevier]

Ice streams provide major drainage pathways for the Antarctic ice sheet. The stress distribution and style of flow in such ice streams produce elastic and rheological anisotropy, which informs ice-flow modelling as to how ice masses respond to external changes such as global warming. Here we analyse elastic anisotropy in Rutford Ice Stream, West Antarctica, using observations of shearwave splitting from three-component icequake seismograms to characterize ice deformation via crystalpreferred orientation. Over 110 high-quality measurements are made on 41 events recorded at five stations deployed temporarily near the ice-stream grounding line. To the best of our knowledge, this is the first well-documented observation of shear-wave splitting from Antarctic icequakes. The magnitude of the splitting ranges from 2 to 80ms and suggests a maximum of 6% shear-wave splitting. The fast shear-wave polarization direction is roughly perpendicular to ice-flow direction. We consider three mechanisms for ice anisotropy: a cluster model (vertical transversely isotropic (VTI) model); a girdle model (horizontal transversely isotropic (HTI) model); and crack-induced anisotropy (HTI model). Based on the data, we can rule out a VTI mechanism as the sole cause of anisotropy - an HTI component is needed, which may be due to ice crystal a-axis alignment in the direction of flow or the alignment of cracks or ice films in the plane perpendicular to the flow direction. The results suggest a combination of mechanisms may be at play, which represent vertical variations in the symmetry of ice crystal anisotropy in an ice stream, as predicted by ice fabric models. [ABSTRACT FROM AUTHOR]

The Afar Depression, at the northern end of the East African Rift, is the only place on land where the transition from a plume-induced continental breakup to seafloor spreading is active today. New images of seismic velocity structure, based on exceptional new data sets, show that the mantle plume that initiated rifting in Africa is absent beneath Afar today. The images are dominated by a major low-velocity feature at ~75 km depth closely mimicking the abrupt changes in rift axis orientation seen at the surface. This is likely associated with passive upwelling beneath the rift. Additional focused low-velocity anomalies show that small diapiric upwellings are present beneath major off-axis volcanoes. These multiple melting sources can explain the wide range of geochemical signatures seen in Afar. These images suggest that passive upwelling beneath Afar marks the initiation of rift segmentation as continental breakup progresses to seafloor spreading. [ABSTRACT FROM AUTHOR]

I D Bastow, J-M Kendall, A M Brisbourne, D B Snyder, D Thompson, D Hawthorn, G R Helffrich, J Wookey, A Horleston and D Eaton describe the motivation for-and successful operation of- a remote seismic survey in Arctic Canada. [ABSTRACT FROM AUTHOR]

Abstract: For carbon capture and storage (CCS) in geological formations to be scientifically viable, we must be able to model and monitor the effects of geomechanical deformation on the integrity of the caprock. Excess deformation may open fractures, providing pathways for CO2 leakage from the reservoir. An acceptable geomechanical model must provide a good match with field observations. Microseismic activity is a direct manifestation of mechanical deformation, so it can be used to constrain geomechanical models. The aim of this paper is to develop the concept of using observations of microseismic activity to help ground truth geomechanical models. Microseismic monitoring has been ongoing at the Weyburn CO2 Storage and Monitoring Project since 2003. We begin this paper by presenting these microseismic observations. Less than 100 events have been recorded, documenting a low rate of seismicity. Most of the events are located close to nearby producing wells rather than the injection well, a pattern that is difficult to interpret within the conventional framework for injection-induced seismicity. Many events are located in the overburden. Without geomechanical simulation it is difficult to assess what these observations mean for the integrity of the storage formation. To address these uncertainties we generate numerical geomechanical models to simulate the changes in stress induced by CO2 injection, and use these models to predict the generation of microseismic events and seismic anisotropy. The initial geomechanical model that we generate, using material properties based on laboratory core measurements, does not provide a good match with either event locations or S-wave splitting measurements made on the microseismic events. We find that an alternative model whose reservoir is an order of magnitude softer than lab core-sample measurements provides a much better match with observation, as it leads shear stresses to increase above the production wells, promoting microseismicity in these areas, and generates changes in effective horizontal stresses that match well with S-wave splitting observations. This agreement between geophysical observations and a softer-than-lab-measurements reservoir model highlights the difficulties encountered in upscaling lab scale results. There is a strong need to link geomechanical models with observable manifestations of deformation in the field, such as induced seismicity, for calibration. Only then can we accurately assess the risks of leakage generated by mechanical deformation. [Copyright &y& Elsevier]

The presence of fractures in hydrocarbon reservoirs can enhance porosity and permeability, and consequently increase production. The use of seismic anisotropy to characterize fracture systems has gained much interest in the last two decades. However, estimating fracture sizes from observations of seismic anisotropy has not been possible. Recent work has shown that frequency-dependent anisotropy (FDA) is very sensitive to the length-scale of the causative mechanism for the anisotropy. In this study, we observe FDA in a microseismic data set acquired from a carbonate gas field in Oman. The frequency-dependent shear wave anisotropy observations are modelled using a poroelastic model, which considers fluid communication between grain size pore spaces and larger scale fractures. A grid search is performed over fracture parameters (radius, density and strike) to find the model that best fits the real data. The results show that fracture size varies from the microscale within the shale cap rocks, to the metre-scale within the gas reservoir, to the centimetre-scale within the non-producing part of the carbonate formation. The lateral variation in fracture density agrees with previous conclusions from ordinary shear wave splitting (SWS) analysis. Cumulatively, the results show the potential for characterizing fracture systems using observations of FDA. [ABSTRACT FROM AUTHOR]

In this paper, we investigate production induced microseismicity based on modelling material failure from coupled fluid-flow and geomechanical simulation. The model is a graben style reservoir characterized by two normal faults subdividing a sandstone reservoir into three compartments. The results are analysed in terms of spatial and temporal variations in distribution of material failure. We observe that material failure and hence potentially microseismicity is sensitive to not only fault movement but also fluid movement across faults. For sealing faults, failure is confined to the volume in and around the well compartment, with shear failure localized along the boundaries of the compartment and shear-enhanced compaction failure widespread throughout the reservoir compartment. For non-sealing faults, failure is observed within and surrounding all three reservoir compartments as well as a significant distribution located near the surface of the overburden. All shear-enhanced compaction failures are localized within the reservoir compartments. Fault movement leads to an increase in shear-enhanced compaction events within the reservoir as well as shear events located within the side-burden adjacent to the fault. We also evaluate the associated moment tensor mechanisms to estimate the pseudo scalar seismic moment of failure based on the assumption that failure is not aseismic. The shear-enhanced compaction events display a relatively normal and tight pseudo scalar seismic moment distribution centred about 106 Pa, whereas the shear events have pseudo scalar seismic moments that vary over three orders of magnitude. Overall, the results from the study indicate that it may be possible to identify compartment boundaries based on the results of microseismic monitoring. [ABSTRACT FROM AUTHOR]

Seismic anisotropy is a common feature in the upper mantle and measuring shear wave splitting in core phases is a common approach in estimating its characteristics. Large lateral variations in estimated splitting parameters are observed over small spatial distances in many differing tectonic regions, including areas of continental break-up such as the Main Ethiopian Rift (MER). We investigate the ability of shear wave splitting analysis to constrain spatial variations in anisotropy using a one-way wave equation modelling scheme to generate bandlimited waveforms for a suite of models representing regions with rapidly changing anisotropy. We show that shear wave splitting can identify lateral variation in anisotropy on the order of 20–50 km, where a change in fast direction demarcates the transition in anisotropy. In addition, variation in the amount of splitting is complicated close to the transition, and is sensitive to the vertical thickness of anisotropy. We have used these modelling results to interpret shear wave splitting measurements for the MER. The model that best fits the observations has a 100- km wide rift zone with a fast direction of 30° outside and 20° inside the rift. The model has 9 per cent anisotropy close to the western margin, with 7 per cent anisotropy elsewhere. In all regions of the model we constrain the anisotropy to begin at a depth of 90 km. The depth of anisotropy is consistent with geochemical estimates of the depth of melt initiation beneath the region. Also the elevated splitting beneath the western margin supports evidence of low velocities and highly conductive zones from seismic tomography and magneto-tellurics, suggesting melt is more focused along the western margin. This study shows how observations of SKS-wave splitting from dense seismic networks can be used to map sharp lateral changes and constrain the depth of the anisotropy. [ABSTRACT FROM AUTHOR]

Abstract: The Archean western Superior Province in Canada represents the nucleus of the North American continent whose origin has been speculated to be the result of widespread crustal accretion some 2.7Ga ago. In this paper, crustal and upper-mantle seismic discontinuities beneath the western Superior Province of the Canadian shield are imaged with teleseismic P-to-S and S-to-P converted phases using the receiver function method. Three crustal discontinuities are observed: the Moho, ranging in depth between 38 and 47km and dipping to the south; and two intra-crustal discontinuities having depths of approximately 15 and 30km. The crustal discontinuities undulate laterally and often lose continuity, possibly indicating an imbricated structure and/or regions of velocity gradients. In the shallow lithosphere, a positive discontinuity is imaged at approximately 65km depth and is consistent with earlier refraction and wide-angle reflection results. Additionally, two zones of negative receiver function amplitudes at 55km depth are observed and are coincident with a region of anomalous tomographic low P- and S-wave velocities as well as a zone of high electrical conductivity. The images for the crust and shallow upper-mantle, when integrated with previous geophysical studies, are consistent with ideas of continental root formation due to imbrication of Archean subducted material and accretion of island arcs observed in surface geology. [Copyright &y& Elsevier]

The ability to detect aligned fractures using seismic anisotropy provides a valuable tool for exploiting hydrocarbon reservoirs better. Perhaps the most direct way of identifying anisotropy is by observing shear wave splitting. However, the interaction of shear waves with subsurface structure is often complicated. Although fractures in hydrocarbon reservoirs are usually subvertical, shear waves recorded on downhole receivers from microseismic events in or near the reservoir are not likely to have travelled vertically. As such, interpreting splitting measurements made on such waves is a non-trivial problem. Here we develop an approach to model the effects of subsurface structure on non-vertically propagating shear waves. Rock physics theory is used to model the effects of sedimentary fabrics as well as fractures, allowing us to use shear wave splitting measurements to invert for aligned fractures. We use synthetic examples to demonstrate how it is possible to assess in advance how well splitting measurements will image structures, and how this is highly dependent on the available range of ray coverage. Finally, we demonstrate the inversion technique on a passive seismic data set collected during hydraulic fracture stimulation. Despite an unfavourable source–receiver geometry, the strike of an aligned fracture set is accurately identified. [ABSTRACT FROM AUTHOR]

Abstract: Upper-mantle deformation near the margins of the Caribbean plate is investigated using observations of shear-wave splitting in teleseismic and local shear phases. The Caribbean plate is almost stationary in the hot-spot reference frame and is wedged between the North America, South America, Nazca and Cocos plates; collisional belts and major shear zones encircle the plate. Data from seismic stations operated by IRIS, GEOSCOPE, the Venezuelan Seismological Network, and the British Geological Survey have provided nearly 2000 seismic records for analysis. Analysis of shear-wave splitting in teleseismic core phases (e.g., SKS) at stations reveals fast shear-wave polarisations that are conformal to the plate boundary, paralleling the major structural features. The magnitude of the splitting is in general quite large (1.2–2.1 s). In northern Venezuela, the magnitude of splitting increases towards the Caribbean–South American collisional front. Local shear phases from earthquakes up to 200 km deep beneath northeastern Venezuela show very small amounts of splitting (0.1–0.3 s). Analysis of the depth dependence in the magnitude of the splitting suggests that most of the upper-mantle wedge is isotropic and the splitting in the local phases is mostly accrued in the crust and uppermost mantle. In NE Venezuela fast shear-wave polarisations in both the local and teleseismic phases closely parallel the fault systems in the region, suggesting that the crust and mantle are coupled in this area. Stations on the Island of Montserrat and near Bucaramanga, Columbia, also show very small magnitudes in local S-wave splitting, but large amounts of SKS splitting. In general, the small magnitude of the local S-phase splitting suggests that the teleseismic phases accrue considerable splitting beneath the top of the slab. We interpret the bulk of the splitting in terms of sub-slab flow that is forced around the Caribbean plate due to the convergence of the surrounding plates. [Copyright &y& Elsevier]

Wave front construction (WFC) methods are a useful tool for tracking wave fronts and are a natural extension to standard ray shooting methods. Here we describe and implement a simple WFC method that is used to interpolate wavefield properties throughout a 3-D heterogeneous medium. Our approach differs from previous 3-D WFC procedures primarily in the use of a ray interpolation scheme, based on approximating the wave front as a ‘locally spherical’ surface and a ‘first arrival mode’, which reduces computation times, where only first arrivals are required. Both of these features have previously been included in 2-D WFC algorithms; however, until now they have not been extended to 3-D systems. The wave front interpolation scheme allows for rays to be traced from a nearly arbitrary distribution of take-off angles, and the calculation of derivatives with respect to take-off angles is not required for wave front interpolation. However, in regions of steep velocity gradient, the locally spherical approximation is not valid, and it is necessary to backpropagate rays to a sufficiently homogenous region before interpolation of the new ray. Our WFC technique is illustrated using a realistic velocity model, based on a North Sea oil reservoir. We examine wavefield quantities such as traveltimes, ray angles, source take-off angles and geometrical spreading factors, all of which are interpolated on to a regular grid. We compare geometrical spreading factors calculated using two methods: using the ray Jacobian and by taking the ratio of a triangular area of wave front to the corresponding solid angle at the source. The results show that care must be taken when using ray Jacobians to calculate geometrical spreading factors, as the poles of the source coordinate system produce unreliable values, which can be spread over a large area, as only a few initial rays are traced in WFC. We also show that the use of the first arrival mode can reduce computation time by ∼65 per cent, with the accuracy of the interpolated traveltimes, ray angles and source take-off angles largely unchanged. However, the first arrival mode does lead to inaccuracies in interpolated angles near caustic surfaces, as well as small variations in geometrical spreading factors for ray tubes that have passed through caustic surfaces. [ABSTRACT FROM AUTHOR]

We have developed an automated method for measuring shear wave splitting and applied it to SKS phases recorded at permanent broad-band stations in Canada. Our method performs two measurements, one seeking to minimize the energy on the transverse component, the other minimizing the smaller of two eigenvalues calculated from the covariance matrix of particle motion. A short-term-average/long-term-average (STA/LTA) phase picker is used to identify SKS arrivals and successive splitting measurements are made on incrementally larger windows around the SKS arrival. The final result is derived from the longest series of windows over which the splitting parameters remain constant. In this study we have processed over 20 000 station/event combinations from 34 stations and 1540 events. Roughly 2 per cent of the data produce reliable estimates of splitting, due to the stringent quality controls applied. We find good correlation between our results and previously published results from the same stations. At some stations we find significant differences in results between the two methods, an observation that has been verified using manual measurements. In each case, eigenvalue derived measurements are consistent with a single layer of anisotropy but transverse-energy derived results require a more complex interpretation. This discrepancy can be explained by misalignment of horizontal components. Tests with synthetic seismograms show that component misalignment can lead to false interpretations of multilayer or dipping anisotropy using the transverse-energy method. The quality of results at individual stations is variable, being influenced both by the volume of available data (determined by station deployment date and the distribution of natural seismicity), and the performance of the STA/LTA picker used to define the start of the SKS window. We compare the orientation of the inferred anisotropy at each station with local tectonic features and directions of absolute plate motion (APM); there is no consistent correlation. Many stations give results that agree with both, while some agree better with local geology and others agree better with APM directions. Results are available through a dedicated website and provide a potential aid to other studies of anisotropy in the deep Earth. [ABSTRACT FROM AUTHOR]

Anisotropy in the lowermost few 100 km of mantle, or D″ region, is indicative of deformation-induced alignment of crystals and/or inclusions of material, and as such offers insights into the dynamic nature of this region. Observations of shear-wave splitting in phases that transit this region provide constraints on such anisotropy. We investigate the effects of lower-mantle seismic anisotropy on SKS and SKKS phases through linked effective-medium modelling and ray-based waveform modelling. A mantle with vertical-transverse-isotropy (VTI) will not produce any splitting in such core phases. Instead we consider the effects of azimuthal-anisotropy due to aligned disk-shaped and tubular inclusions and aligned perovskite, periclase and columbite. Models are constructed subject to constraints imposed by observed anisotropy (<3%) and plausible variations in aggregate isotropic velocities (<± 2.5%). Melt-filled inclusions are much more effective in generating anisotropy than solid-filled inclusions and disk-shaped inclusions produce more anisotropy than tubular inclusions. In general the degree of splitting produced by most of the models is small, similar to that produced by the crust (<0.5 s). The exceptions are melt-filled vertically-aligned disk-shaped inclusions and horizontally aligned periclase, the former most likely in low-velocity regions, the latter in high-velocity regions. Both models produce splitting significant enough to mask the effects of upper-mantle anisotropy. Strong azimuthal variations in splitting and discrepancies in SKS and SKKS splitting are diagnostic of these anisotropic models. [Copyright &y& Elsevier]

In situ stress monitoring of crustal rocks is desirable as it yields insights into earthquake mechanisms, volcanic eruptions and changes in hydrocarbon reservoirs. Shear wave splitting, induced by stress-controlled cracks in the shallow crust, provides a way to infer this stress. Temporal variations in these observations can be difficult to quantify due to scatter in the data and discontinuous observations. Here we present evidence of temporal variations in shear wave splitting from a continuous time-series with a high occurrence of microseismic events recorded in a borehole. We interpret these observations in terms of stress-controlled cracks and are able to infer changes in stress and, via modelling, suggest the cause of the anisotropy. Possible origins of the temporal variation in per cent anisotropy are tidal or oil-field production processes. Our results suggest that shear wave splitting is a viable probe for inferring changes in crustal stress in cracked rock. [ABSTRACT FROM AUTHOR]

We have analysed prediffracted S-waves with turning points beneath northern Siberia in a study of anisotropy in the lowermost mantle. Deep-focus earthquakes beneath the Marianas, Izu Bonin and the Sea of Japan recorded at stations in western Europe are used. A correction for upper-mantle anisotropy is applied to the data. Comparisons of the data with synthetic data for models with and without a high velocity D' layer suggest that there is a velocity discontinuity at the top of the D' region and that the style of anisotropy is transversely isotropic in this region. Time separations between S-waves on the radial and transverse component show a weak trend where the separation increases with epicentral distance. A normalization of this separation with the travel distance within D' (300 km thick in this region) suggests that the anisotropy is uniformly distributed within this layer and has an average value of 0.5 per cent. A combination of different studies which investigate the structure of the lowermost mantle beneath Europe and northern Siberia reveals a complicated picture. Tomographic models from this area and evidence of D' anisotropy, lower mantle scatterers, reflections from a D' discontinuity and ultralow-velocity zones suggest two distinct regions. One exhibits high velocities, D' anisotropy, a D' discontinuity and no evidence of scatterers or ultra-low-velocity zones. These features are likely associated with the palaeosubduction of the Izanagi plate well into the lowermost mantle. The other region has a lower overall velocity and shows evidence of scatterers and ultra-lowvelocity zones, perhaps suggesting the presence of partial melt. These results suggest dramatic lateral variations in the nature of the lowermost mantle beneath northern Asia over a length scale of roughly 30 degrees. [ABSTRACT FROM AUTHOR]

In most areas of the globe it is only possible to sample the lowermost mantle or D' region with a limited range of azimuths. This is due to limitations in source-receiver geometries. An exception is the region beneath northern Siberia where we are able to sample D' along orthogonal great-circle paths using northern-Pacific earthquakes recorded in Europe and Hindu-Kush earthquakes recorded in Canada. Receiver vespagrams (slant stacks of one source recorded at many receivers) for a Kurile event recorded at the German Regional Seismic Network (GRSN) and the Gräfenberg-array and source vespagrams (many sources, one receiver) of Hindu-Kush events recorded at stations of the Canadian National Seismograph Network (CNSN) show reflections from a discontinuity at the top of D'. Vespagrams for real data are compared to vespagrams for synthetic data in order to guide their interpretation. Additional support for our interpretation comes from frequency-wavenumber (f - k) analysis which provides a measure of the slowness and azimuth of a given phase. The traveltimes of the D' reflected phases are then used to provide estimates of D' thickness. In regions where the roughly orthogonal paths overlap we estimate the thickness to be 310 (±15) km, which is in agreement with existing models for this region. We also estimate a thickness of 210 (±15) km in a previously unsampled region east of this area. Using the slowness, backazimuth and traveltime values for a third source-receiver combination, we find a reflection point at a depth of 2800 km (thickness of D' of 91 km) in a region several degrees to the west. These results and those of previous studies suggest considerable topographic relief (100 km) over distances of less than 5° (approximately 300 km at the CMB). Comparing the results from this study with results from other seismic studies of D' structure in this area, we find this to be a complicated region which exhibits heterogeneity on many scales. This... [ABSTRACT FROM AUTHOR]

Presents seismic wave evidence for a zone of seismic anisotropy below the D'' layer discontinuity lying 250 kilometers above the core-mantle boundary (CMB). Seismic S-wave phases analyzed; Seismic tomography; Cassini equal-area projection of the region of study; Travel time separations.

The Maslov asymptotic method addresses some of the problems with standard ray theory, such as caustics and shadows. However, it has been applied relatively little, perhaps because its accuracy remains untested. In this study we examine Maslov integral (MI) seismograms by comparing them with finite-difference (FD) seismograms for several cases of interest, such as (1) velocity gradients generating traveltime triplications and shadows, (2) wave-front bending, kinking and folding in a low-velocity waveguide, and (3) wavefield propagation perturbed by a high-velocity slab. The results show that many features of high- and intermediate-frequency waveforms are reliably predicted by Maslov's technique, but also that it is far less reliable and even fails for low frequencies. The terms 'high' and 'low' are model-dependent, but we mean the range over which it is sensible to discuss signals associated with identifiable wave fronts and local (if complicated) effects that potentially can be unravelled in interpretation. Of the high- and intermediate-frequency wave components, those wavefront anomalies due to wave-front bending, kinking, folding or rapid ray divergence can be accurately given by MI. True diffractions due to secondary wave-front sections are theoretically not included in Maslov theory (as they require true diffracted rays), but in practice they can often be satisfactorily predicted. This occurs roughly within a wavelength of the truncated geometrical wave front, where such diffractions are most important since their amplitudes may still be as large as half that on the geometrical wave front itself. Outside this region MI is inaccurate (although then the diffractions are usually small). Thus waveforms of high and intermediate frequencies are essentially controlled by classical wave-front geometry. Our results also show that the accuracy of MI can be improved by rotating the Maslov integration axis so that the nearest wave-front anomaly is adequately 'sampled'.... [ABSTRACT FROM AUTHOR]

The rifting of continents and evolution of ocean basins is a fundamental component of plate tectonics, yet the process of continental break-up remains controversial. Plate driving forces have been estimated to be as much as an order of magnitude smaller than those required to rupture thick continental lithosphere. However, Buck has proposed that lithospheric heating by mantle upwelling and related magma production could promote lithospheric rupture at much lower stresses. Such models of mechanical versus magma-assisted extension can be tested, because they predict different temporal and spatial patterns of crustal and upper-mantle structure. Changes in plate deformation produce strain-enhanced crystal alignment and increased melt production within the upper mantle, both of which can cause seismic anisotropy. The Northern Ethiopian Rift is an ideal place to test break-up models because it formed in cratonic lithosphere with minor far-field plate stresses. Here we present evidence of seismic anisotropy in the upper mantle of this rift zone using observations of shear-wave splitting. Our observations, together with recent geological data, indicate a strong component of melt-induced anisotropy with only minor crustal stretching, supporting the magma-assisted rifting model in this area of initially cold, thick continental lithosphere. [ABSTRACT FROM AUTHOR]

Icequakes, microseismic earthquakes at glaciers, offer insights into the dynamics of ice sheets. For the first time in the Antarctic, we explore the use of fiber optic cables as Distributed Acoustic Sensors (DAS) as a new approach for monitoring basal icequakes. We present the use of DAS for studying icequakes as a case study for the application of DAS to microseismic datasets in other geological settings. Fiber was deployed on the ice surface at Rutford Ice Stream in two different configurations. We compare the performance of DAS with a conventional geophone network for: microseismic detection and location; resolving source and noise spectra; source mechanism inversion; and measuring anisotropic shear‐wave splitting parameters. Both DAS array geometries detect fewer events than the geophone array. However, DAS is superior to geophones for recording the microseism signal, suggesting the applicability of DAS for ambient noise interferometry. We also present the first full‐waveform source mechanism inversions using DAS anywhere, successfully showing the horizontal stick‐slip nature of the icequakes. In addition, we develop an approach to use a 2D DAS array geometry as an effective multi‐component sensor capable of accurately characterizing shear‐wave splitting due to the anisotropic ice fabric. Although our observations originate from a glacial environment, the methodology and implications of this work are relevant for employing DAS in other microseismic environments. Plain Language Summary: Icequakes are like small earthquakes but are caused by the movement of ice rather than two plates sliding past one another. They allow us to investigate glacier processes. For the first time in the Antarctic, we use lasers fired down fiber optic cables to detect and analyze icequake signals. This technique is called Distributed Acoustic Sensing (DAS). These fiber optic cables were laid on the surface of Rutford Ice Stream, Antarctica, in two different shapes. We compare the performance of DAS to conventional geophones for icequake detection and location, investigating the frequency of the earthquake source, investigating the physics that generates the icequake, and the effect of the ice fabric on the travel of seismic waves through ice. For our experiment, DAS is not as good as conventional geophones for detecting icequakes. However, DAS is better than geophones for looking at the frequency of an icequake and the physics that causes an icequake. It also allows us to investigate ice fabric properties in a similar way to geophones. Although our results are for icequakes at a glacier, the methods we use and our findings are relevant for using DAS in many other environments where small earthquakes occur. Key Points: Distributed acoustic sensing can outperform geophones for source spectra and full‐waveform source mechanism inversion2D distributed acoustic sensing array geometries can be used as a multi‐component sensor capable of measuring shear‐wave splittingLarger surface distributed acoustic sensing deployments and/or hybrid networks are required for accurate microseismic detection/location [ABSTRACT FROM AUTHOR]

Oceanic transform faults are intriguing in that they do not produce earthquakes as large as might be expected given their dimensions. We use 1‐year of local seismicity (370 events above MC = 2.3) recorded on an array of ocean bottom seismometers (OBSs) and geophysical data to study the seismotectonic properties of the Chain transform, located in the equatorial Mid‐Atlantic. We extend our analysis back in time by considering stronger earthquakes (MW ≥ 5.0) from global catalogs. We divide Chain into three areas (east, central, and west) based on historical event distribution, morphology, and multidimensional OBS seismicity cluster analysis. Seismic activity recorded by the OBS is the highest at the eastern area of Chain where there is a lozenge‐shaped topographic high, a negative rMBA gravity anomaly, and only a few historical MW ≥ 5.5 events. OBS seismicity rates are lower in the western and central areas. However, these areas accommodate the majority of seismic moment release, as inferred from both OBS and historical data. Higher b‐values are significantly correlated with lower rMBA and with shallower bathymetry, potentially related to thickened crust. Our results suggest high lateral heterogeneity along Chain. Patches with moderate to low OBS seismicity rates that occasionally host MW ≥ 6.0 earthquakes are interrupted by segments with abundant OBS activity but few historical events with 5.5 ≤ MW < 6.0. This segmentation is possibly due to variable fluid circulation and alteration, which may also change in time. Plain Language Summary: Oceanic transform faults (TFs) typically host earthquakes much smaller than expected based on their total seismogenic area. We study the seismotectonic properties of the Chain TF by combining 1‐year of seismicity recorded by an ocean bottom seismometer (OBS) array with geophysical data (bathymetry, tidal height, gravity anomalies). We supplement our analysis with strong historical earthquakes (M ≥ 5.0) from global catalogs. Our analysis divides Chain into three areas: The east area is characterized by the highest OBS seismicity rates, negative gravity anomalies, large topographic highs, and few historical events with 5.5 ≤ M < 6.0. The west and central areas demonstrate lower OBS seismicity rates. However, they occasionally produce M ≥ 6.0, being responsible for most of the total seismic energy release. Higher numbers of stronger events occur in areas with negative gravity anomalies and shallower water depths. Our results suggest high lateral heterogeneity along Chain, with alternating seismic and aseismic patches, variable crustal thickness, different degrees of hydrothermal circulation/alteration, and potentially time‐dependent behavior. Key Points: Segmentation along Chain transform fault (Mid‐Atlantic Ocean) is studied by means of seismicity cluster analysis and geophysical dataChain demonstrates lateral heterogeneity with high moment‐release patches interrupted by segments with abundant but small magnitude eventsSpatiotemporal variations of seismicity are possibly manifested by variable hydrothermal circulation and alteration [ABSTRACT FROM AUTHOR]

ABSTRACT We present an assessment of how microseismic moment magnitude [ABSTRACT FROM AUTHOR]

Abstract: Earthquakes recorded at 36 stations in Nunavut between 1992 and 2011 and analyzed jointly using receiver-function and SKS-splitting techniques indicate the presence of multiple, dipping anisotropic layers in the lithosphere beneath the northern Hudson Bay and Baffin Island region. Prominent discontinuities indicate layers interpreted to dip at 6–30° at depths between 50 and 150km beneath ten of the stations. One set of regionally continuous discontinuities in anisotropic fabric in the southeastern Rae craton dip gently southeast consistent with underthrusting beneath the Hearne and Meta Incognita-Sugluk blocks. SKS splitting recorded at twelve stations indicate patterns characteristic of two layers of anisotropic fabric. The deeper fabric as well as the single fast polarization direction recorded at other stations appears aligned with recent plate motion. The shallower fabric orientations at stations on and south of Southampton Island in northern Hudson Bay are consistent with known crustal fabric orientations formed during early (pre-1.85Ga) stages of the Hudsonian orogeny. The shallower fabric north and west of Southampton Island has a more northerly orientation and is associated with west-dipping discontinuities that may correspond to Neoarchean accretion of the Mesoarchean north Rae domain to the central Rae domain. [Copyright &y& Elsevier]

Abstract: Continental flood basalts are commonly thought to form by direct melting of lithospheric mantle, either during rifting episodes or when a mantle plume impinges on the base of the lithosphere. If correct, then significant alteration of the chemical and physical properties of the lithospheric mantle should be observable beneath such provinces. To date, however, this hypothesis remains unproved through lack of direct evidence. Here we present the results from a seismic experiment conducted in the Seychelles, a region that separated from India at the time of the eruption of the Deccan Traps. The analysis shows a stratified upper mantle with exceptionally low S-wave velocities at depths between 50 and 190km. It is difficult to explain these anomalies without invoking the presence of melt. However, the depth extent of the low velocities rules out the presence of silicate melt. We argue that the most probable interpretation is that these layers contain sulphide melt trapped from earlier interactions of the lithosphere with episodes of continental-flood basalt generation. The presence of this melt over a depth interval of suggests that it is stagnant, and has remained coupled with the Seychelles lithosphere. [Copyright &y& Elsevier]

The gravitational pulls from the moon and the sun result in tidal forces which influence both Earth's solid and water mass. These stresses are periodically added to the tectonic ones and may become sufficient for initiating rupture in fault systems critically close to failure. Previous research indicates correlations between increased seismicity rates and low tides for fast‐ and intermediate‐spreading mid‐ocean ridges in the Pacific Ocean. Here, we present a microseismicity data set (4,719 events) recorded by an ocean bottom seismometer deployment at the equatorial Mid‐Atlantic Ridge. We show that low, as well as decreasing ocean water level, result in relatively elevated seismicity rates at higher magnitudes (lower b‐values), translated into increased probabilities of stronger event occurrence at or towards low tides. Moreover, seismic bursts (enhanced activity rate clusters), occurring at rates well above the reference seismicity, are exclusively present during values of either high tidally induced extensional stresses or high extensional stress rates. Although the b‐value differences are not significant enough to be conclusive, the seismicity rate variations exhibit statistical significance, supporting the previous findings for tidal triggering at low tides within normal‐faulting regimes and extending the range of observations to slow‐spreading ridges. Observed triggering of slip on low angle faults at low tides is predicted by Coulomb stress modeling. The triggering of slip on high angle faults observed here, is not easily explained without another factor. It may be related to the presence of a shallow magma body beneath the ridge, as supported by previous seismic imaging in the region. Plain Language Summary: The Earth is periodically subjected to tidal forces due to the combined influence of solar and lunar gravity. Solid Earth tides dominate in the continents, whereas in particular marine sites ocean tides are strong enough to generate considerable stress. Although theoretically plausible, tidal triggering of earthquakes has been only occasionally observed in the Pacific Ocean, with the results often being controversial. The deployment of ocean bottom seismometers (OBS) in combination with modern earthquake detection techniques, offer better chances to study the remote oceanic environments, making it possible to recognize a tidal footprint on small earthquakes. Here we present a dense data set of small earthquakes from the equatorial Mid‐Atlantic Ridge, recorded by a temporary OBS network operated from March 2016 to February 2017. We find that the majority of the earthquakes occurred during or towards low tides, corresponding to high extensional stressing. We show that seismicity bursts (very high activity rates) exclusively occur at low tides. There is also evidence that a larger proportion of stronger events occur during or just before low tides. We interpret our results in terms of stress change modeling, considering faulting properties and the potential existence of a magma chamber beneath the study area. Key Points: Tidal triggering is investigated for a microseismicity data set from the equatorial Mid‐Atlantic RidgeSeismicity rates are increased and b‐values are decreased during and towards low tidesCoulomb stress modeling suggests additional factors are required, for example, a shallow magma chamber beneath the ridge [ABSTRACT FROM AUTHOR]

Knowledge of the velocity structure of the upper mantle beneath the Western Superior Province (WSP) is key to understanding better the accretionary processes active during the Archean. To that end, teleseismic P- and S-wave travel times recorded as part of the Teleseismic Western Superior Transect (TWST) were inverted for their respective seismic velocities. This experiment involved 17 portable broadband stations arrayed in northern Ontario, Canada, so as to cross-cut the strike of many subprovinces as well as the boundary with the Proterozoic Trans-Hudson Orogen to the north. The 5-month deployment yielded 1423 P-wave and 651 S-wave high-quality residuals for inversion. The resulting tomographic images reveal three apparently-robust velocity anomalies represented by: (i) a dipping tabular high-velocity anomaly; (ii) a relatively shallow low-velocity anomaly directly above the positive anomaly; and (iii) a deep low-velocity body. The first anomaly may be interpreted in the Western Superior context as a 30–50 km thick eclogite/dunite layer representing remnant subducted oceanic lithosphere. The presence of such a body within the cratonic root would suggest its origin at around 2.7 Ga and the apparent SE–NW strike is noticeably oblique to the main EW trend of the subprovince boundaries. The low-velocity anomalies may be related to processes that occurred at the edges of the descending slab or they may be expressions of later upwelling material. The presence of a thick cratonic root (&ap;300 km) may also be revealed by the tomographic images. Overall, these travel time results are considered compatible with late Archean structures at depth resembling those of modern subduction tectonics, though other possibilities (e.g. small-scale convection cells) exist. [Copyright &y& Elsevier]

When synthetic seismograms are computed for complicated velocity models using asymptotic methods based on ray theory, the caustic and/or pseudo-caustic geometrical catastrophes that often occur in the ray manifolds can cause many difficulties, because ray amplitudes can become infinite, inaccurate or vanish in such situations. The longer the propagation path relative to the scale of the velocity inhomogeneities and the more the propagation tends to be oblique rather than normal to velocity variations, the more troublesome the catastrophes become. Although the Maslov integral method (MI) largely cures the problems of the classical geometric ray theory (GRT) at caustics (where nearby rays in geometrical space cross) and in geometric shadow zones, it suffers from serious artefacts at pseudo-caustics (the crossing points of rays drawn in the mixed geometrical and slowness space of the Maslov integral). Some methods have been devised to overcome this limitation, but none is satisfactory when it is necessary to compute automatically large suites of seismograms for complex models without continuous intervention from a skilled seismologist. In this paper, we demonstrate how combining the Maslov integral method with a stage of Kirchhoff integration can greatly reduce MI's pseudocaustic problems while retaining MI's ability to overcome the caustic and shadow problems of GRT. In many cases, the additional computational burden increases by one order of magnitude but is still much less than that required in a finite-difference modelling, and is not great for a common workstation. The Maslov-Kirchhoff (MK) technique is shown to be substationally more accurate than ordinary Maslov integration when pseudo-caustics are present. Furthermore, it is more robust and automated than the phase-partitioning technique in curing the problem of joint caustics and pseudo-caustics. We also show that the MK scheme can be more efficient than other Kirchhoff modelling techniques where the... [ABSTRACT FROM AUTHOR]

Hydraulically forced crevassing is thought to reduce the stability of ice shelves and ice sheets, affecting structural integrity and providing pathways for surface meltwater to the bed. It can cause ice shelves to collapse and ice sheets to accelerate into the ocean. However, direct observations of the hydraulically forced crevassing process remain elusive. Here we report a novel method and observations that use icequakes to directly observe crevassing and determine the role of hydrofracture. Crevasse icequake depths from seismic observations are compared to a theoretically derived maximum dry crevasse depth. We observe icequakes below this depth, suggesting hydrofracture. Furthermore, icequake source mechanisms provide insight into the fracture process, with predominantly opening cracks observed, which have opening volumes of hundredths of a cubic meter. Our method and findings provide a framework for studying a critical process that is key for the stability of ice shelves and ice sheets and, therefore, future sea level rise projections. Key Points: We demonstrate a novel method for using crevasse icequake depths to discriminate between dry and hydrofracture driven surface crevassingIcequakes can be used to directly observe and elucidate the crevasse hydrofracture processIcequakes show tensile crack failure with opening volumes calculated from moment magnitudes [ABSTRACT FROM AUTHOR]

Well-constrained marine sediment characteristics (sediment thickness and shear wave velocity) are important not only for the study of climate over geologic times scales but also for correcting and accounting for its presence in seismic data used to investigate deeper structures. We use data from the PI-LAB (Passive Imaging of the Lithosphere Asthenosphere Boundary) experiment, which consisted of 39 broadband ocean bottom seismometers deployed at the Equatorial Mid-Atlantic Ridge near the Chain fracture zone covering 0–80 Myr old seafloor. We compute admittance between the pressure to the vertical displacement at the seafloor at frequencies between 0.1 and 0.2 Hz for microseism-generated Rayleigh waves for 18 stations where data quality is good to determine the sediment thickness and shear wave velocity. We find a general trend of increasing sediment thickness with the seafloor ages, as expected with sediment thicknesses that range from 10–450 m and, shear wave velocities that range from 0.05–0.34 km/s. We find sediment thickness varies almost uniformly across both sides of the ridge, and it indicates that both sides experienced a similar sedimentation process. Our results are in good agreement with the global sediment model that is based on drilling cores and active source experiments, but thinner by up to 50 m at several stations on seafloor older than 25 My. Overlap of the 95% confidence regions between admittance and Ps estimates for thickness and shear velocity is found at 15 stations where we have both Ps and admittance estimates. It suggests that both methods yield accurate estimates for sediment thickness. In addition, our admittance result extends the lateral resolution of sediment characteristics to stations that were not previously resolved by Ps. [ABSTRACT FROM AUTHOR]

Geophysical monitoring of landslides can provide insights into spatial and temporal variations of subsurface properties associated with slope failure. Recent improvements in equipment, data analysis, and field operations have led to a significant increase in the use of such techniques in monitoring. Geophysical methods complement intrusive approaches, which sample only a very small proportion of the subsurface, and walk‐over or remotely sensed data, which principally provide information only at the ground surface. In particular, recent studies show that advances in geophysical instrumentation, data processing, modeling, and interpretation in the context of landslide monitoring are significantly improving the characterization of hillslope hydrology and soil and rock hydrology and strength and their dynamics over time. This review appraises the state of the art of geophysical monitoring, as applied to moisture‐induced landslides. Here we focus on technical and practical uses of time‐lapse methods in geophysics applied to monitoring moisture‐induced landslide. The case studies identified in this review show that several geophysical techniques are currently used in the monitoring of subsurface landslide processes. These geophysical contributions to monitoring and predicting the evolution of landslide processes are currently underrealized. Hence, the further integration of multiple‐parametric and geotechnically coupled geophysical monitoring systems has considerable potential. The complementary nature of certain methods to map the distribution of subsurface moisture and elastic moduli will greatly increase the predictive and monitoring capacity of early warning systems in moisture‐induced landslide settings. Key Points: Geophysical monitoring reveals high resolution spatial and temporal information from the subsurface of landslide systemsDevelopments in geophysical monitoring have led to substantial increases in applications to landslides and frequencies of data acquisitionLinking geophysical data with geotechnical measurements can monitor hydrological and geomechanical processes in time and space [ABSTRACT FROM AUTHOR]

We present a workflow linking coupled fluid-flow and geomechanical simulation with seismic modelling to predict seismic anisotropy induced by non-hydrostatic stress changes. We generate seismic models from coupled simulations to examine the relationship between reservoir geometry, stress path and seismic anisotropy. The results indicate that geometry influences the evolution of stress, which leads to stress-induced seismic anisotropy. Although stress anisotropy is high for the small reservoir, the effect of stress arching and the ability of the side-burden to support the excess load limit the overall change in effective stress and hence seismic anisotropy. For the extensive reservoir, stress anisotropy and induced seismic anisotropy are high. The extensive and elongate reservoirs experience significant compaction, where the inefficiency of the developed stress arching in the side-burden cannot support the excess load. The elongate reservoir displays significant stress asymmetry, with seismic anisotropy developing predominantly along the long-edge of the reservoir. We show that the link between stress path parameters and seismic anisotropy is complex, where the anisotropic symmetry is controlled not only by model geometry but also the nonlinear rock physics model used. Nevertheless, a workflow has been developed to model seismic anisotropy induced by non-hydrostatic stress changes, allowing field observations of anisotropy to be linked with geomechanical models. [ABSTRACT FROM AUTHOR]

An automatic analysis code, Anisomat+, was developed, tested and improved to calculate anisotropic parameters: fast polarization direction and delay time. Anisomat+ was applied on data coming from three zones of the Apennines in Italy. For each area, anisotropic parameters have been interpreted to determine the fracture and stress field taking into account the geological and structural settings. It was recognized that the averages of fast directions are NW-SE-oriented at all sites, in agreement with the orientation of maximum horizontal stress as well as with the strike of the main fault structures. The mean values of normalized delay time range from 0.005 s/km to 0.007 s/km and to 0.009 s/km, respectively for L'Aquila region, Alto Tiberina Fault area and Val d'Agri basin, suggesting a 3-4% of crustal anisotropy. Moreover, for each area, the spatial distribution of anisotropic parameters is examined, and for L'Aquila 2009 seismic sequence also temporal distribution is discussed. [ABSTRACT FROM AUTHOR]

Abstract: Subduction of oceanic lithosphere is the most direct feedback between the Earth’s surface and deep interior. However, the detail of its interaction with the broader convecting mantle is still unclear. Mantle flow around subduction zones can be constrained using seismic anisotropy, but despite many such studies, a simple global picture is lacking. The Sangihe subduction zone (where the Molucca Sea microplate is subducting westward beneath the Eurasian plate) is part of the tectonically complex Sulawesi–Philippine region, and an ideal natural laboratory to study complex subduction processes. We investigate the anisotropic structure of the Sangihe subduction zone with shear wave splitting measurements of local S and SKS phases at two stations (MNI in Sulawesi, DAV in the Philippines), as well as downgoing S phases at five stations at teleseismic distances. Combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase. The broad depth distribution of local events (∼60–630km) allows us to observe a change in splitting behaviour at ∼380km depth: above, fast directions () are trench-parallel and delay times () are ∼0.34–0.53s with no increase with depth. We suggest this anisotropy is caused by aligned cracks, possibly melt-filled beneath the volcanic arc, and fossil anisotropy in the overriding plate. Below ∼380km, is predominantly trench-normal and are slightly higher (∼0.53–0.65s). As no correlation is observed with inferred distance travelled inside the slab, we attribute this anisotropy to shear layers atop the slab, which are coherent from ∼200 to 400km depth and perhaps extend into the transition zone. SKS and source-side measurements show larger (∼1.53 and 1.33s, respectively) and trench-parallel . Since these phases predominantly sample sub-slab mantle, we consider along-strike lateral flow associated with the double-sided subduction of the Molucca Sea microplate to be the most likely explanation. We thus infer three dominant regions of anisotropy at the Sangihe subduction zone: one within the overriding lithosphere, one along the slab–wedge interface, and one below the subducting Molucca Sea slab. The mantle wedge above 200km depth and the slab itself do not seem to contribute notably to the measured anisotropy. This study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems. [Copyright &y& Elsevier]

Abstract: Many areas of old continental interiors have thick roots that extend to depths of 250km or more, in contrast to the oceans and younger continents, whose lithospheric thickness is less than 100km. These cratonic roots might perturb temperatures in their surrounding mantle, though the net result could be either cooling or heating; both of which may lead to small-scale convective flow around the root. We show here, using new data from a study of the seismic structure of the Canadian Shield, that the relative positions of the 410km and 660km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the Earth. Differential arrival times and internal discontinuity structure are remarkably uniform and simple, varying by little more than ±0.5s over the root''s 3500km lateral extent. This implies that the root has no significant thermal effect on the underlying mantle (<50K), and any small-scale convection or cold mantle downwelling associated with the large free-air gravity anomaly beneath the shield must be confined to the upper mantle. Our observations are also consistent with phase changes solely in the olivine system ((Mg,Fe)2SiO4), with the caveat that our method is dominantly sensitive to S-wave velocity jumps alone. [Copyright &y& Elsevier]

The study of seismic anisotropy in exploration seismology is gaining interest as it provides valuable information about reservoir properties and stress directions. In this study we estimate anisotropy in a petroleum field in Oman using observations of shear-wave splitting from microseismic data. The data set was recorded by arrays of borehole geophones deployed in five wells. We analyse nearly 3400 microearthquakes, yielding around 8500 shear-wave splitting measurements. Stringent quality control reduces the number of reliable measurements to 325. Shear-wave splitting modelling in a range of rock models is then used to guide the interpretation. The difference between the fast and slow shear-wave velocities along the raypath in the field ranges between 0-10% and it is controlled both by lithology and proximity to the NE-SW trending graben fault system that cuts the field formations. The anisotropy is interpreted in terms of aligned fractures or cracks superimposed on an intrinsic vertical transversely isotropic (VTI) rock fabric. The highest magnitudes of anisotropy are within the highly fractured uppermost unit of the Natih carbonate reservoir. Anisotropy decreases with depth, with the lowest magnitudes found in the deep part of the Natih carbonate formation. Moderate amounts of anisotropy are found in the shale cap rock. Anisotropy also varies laterally with the highest anisotropy occurring either side of the south-eastern graben fault. The predominant fracture strikes, inferred from the fast shear-wave polarizations, are consistent with the trends of the main faults (NE-SW and NW-SE). The majority of observations indicate subvertical fracture dip (). Cumulatively, these observations show how studies of shear-wave splitting using microseismic data can be used to characterize fractures, important information for the exploitation of many reservoirs. [ABSTRACT FROM AUTHOR]

Abstract: Whether or not plate tectonic processes operated on a younger, hotter Earth remains ambiguous. Seismic data from new networks in the Hudson Bay region of the Canadian Shield, where the Precambrian geological record spans more than 2 billion years, offer fresh scope to address this problem. Using receiver function analyses we show that the crust of the Rae domain, which exhibits ages of Paleo- to Neoarchean (3.9–2.7Ga), is likely felsic-to-intermediate in composition (average Vp/Vs <1.73) and seismically transparent with a sharp Moho. There is little evidence for modern-style plate tectonics, and based on the simplicity and spatial extent of the felsic crust, models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results. Data from the Hearne domain, which exhibits widespread ~2.7Ga granite-and-greenstone geology, show a more complex crust with higher Vp/Vs ratios, consistent with a greater mafic component. The Trans-Hudson Orogen (THO), proposed to be a Himalayan-scale mountain belt during the Paleoproterozoic, is thought to have formed during the ~1.8Ga collision of the Superior and Churchill plates. Results from the Quebec–Baffin Island segment of the THO appear to map out the first-order shape of the underthrusting Superior plate, with elevated Vp/Vs ratios likely representing the rifted margin of the Superior craton. Consistently thicker crust is observed beneath central and southern Baffin Island (~43km), coincident with widespread high-grade metamorphic surface geology. These features can be explained by crustal thickening due to stacking of accreted terranes during continent–continent collision, analogous to the present-day Tibetan Plateau, followed by erosion. When reviewed in light of age and compositional constraints from the geological record, our seismic observations point towards secular crustal evolution from non-plate tectonic during the Paleo- to Mesoarchean evolving towards fully-developed modern-style plate tectonics during the Paleoproterozoic. [Copyright &y& Elsevier]

Microseismic monitoring in petroleum settings provides insights into induced and naturally occurring stress changes. Such data are commonly acquired using an array of sensors in a borehole, providing measures of arrival times and polarizations. Events are located using 1D velocity models, P- and S-wave arrival times and the azimuths of P-wave particle motions. However in the case of all the sensors being deployed in a vertical or near-vertical borehole, such analysis leads to an inherent 180° ambiguity in the source location. Here we present a location procedure that removes this ambiguity by using the dip of the particle motion as an a priori information to constrain the initial source location. The new procedure is demonstrated with a dataset acquired during hydraulic fracture stimulation, where we know which side of the monitoring well the events are located. Using a -step location procedure, we then reinvestigate a microseismic data set acquired in April 1997 at the Ekofisk oilfield in the North Sea. Traveltimes for 2683 candidate events are manually picked. A noise-weighted analytic-signal polarization analysis is used to estimate the dip and azimuth of P-wave particle motions. A modified t-test is used to statistically assess the reliability of event location. As a result, 1462 events are located but 627 are deemed to be statistically reliable. The application of a hierarchal cluster analysis highlights coherent structures that cluster around wells and inferred faults. Most events cluster at a depth of roughly 3km in the Ekofisk chalk formation but very little seismicity is observed from the underlying Tor chalk formation, which is separated from the Ekofisk formation by an impermeable layer. We see no evidence for seismicity in the overburden but such events may be too distant to detect. The resulting picture of microseismicity at Ekofisk is very different from those presented in previous studies. [ABSTRACT FROM AUTHOR]

Although it is well known that sedimentary rocks can be seismically anisotropic, there have been few detailed investigations of the underlying cause of such anisotropy. Here, we investigate anisotropy due to the preferred orientation of minerals, or lattice preferred orientation (LPO), in a suite of sedimentary rocks. Seismic properties are predicted by averaging single-crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate. Both Electron Backscattered Diffraction (EBSD) and X-ray Texture Goniometry (XTG) are tested as quantitative techniques for measuring the LPO of sedimentary rocks. Although EBSD has promise for future LPO measurements in polymineralic sedimentary rocks, problems currently remain in measuring low-symmetry phases (e.g. feldspars) and very small clay or mica particles. However, the LPO of very fine-grained phyllosilicates can be measured using XTG and the LPO of low-symmetry minerals can be measured using manual EBSD pattern analysis. Here, we use such a hybrid approach to estimate LPO in a suite of sedimentary samples. The seismic properties calculated from LPO data show anisotropy values for P-waves ranging from 1.5 per cent in sandstones, to over 3.5 per cent in a siltstone, to 12 per cent in a shale. The effect of thin multilayering on long-wavelength propagation in a siltstone is predicted by applying Backus-type averaging. The layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types. The LPO of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate-rich rocks (siltstones and shales). The weak LPO of quartz in sandstones causes a few per cent anisotropy. Cumulatively, our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology. [ABSTRACT FROM AUTHOR]

Abstract: Using elastic constants determined by ab initio molecular dynamics over a range of temperatures, we show that the seismic anisotropy of MgSiO3 perovskite is significantly temperature dependent. At 90 GPa, the direction of greatest shear wave splitting changes from [001] at 0 K to [010] at 3500 K. In addition, there is no shear wave splitting in the [100] direction at 0 K, but a significant amount of splitting is exhibited at 3500 K. We have also calculated the first set of high-temperature elastic constants of the new post-perovskite phase by ab initio molecular dynamics methods. We find that, in contrast to MgSiO3 perovskite, temperature has little effect on the acoustic anisotropy of this phase. We determine ∂ln V S/∂ln V P for the post-perovskite phase and find it to be very low, about 0.9. This may be an important parameter for mapping out the post-perovskite phase in the lowermost mantle. [Copyright &y& Elsevier]

Understanding the dominant physical processes that cause fault reactivation due to fluid injection is vital to develop strategies to avoid and mitigate injection‐induced seismicity. Injection‐induced seismicity is a risk for several industries, including hydraulic fracturing, geothermal stimulation, oilfield waste disposal and carbon capture and storage, with hydraulic fracturing having been associated with some of the highest magnitude induced earthquakes (M>5). As such, strict regulatory schemes have been implemented globally to limit the felt seismicity associated with operations. In the UK, a very strict "traffic light" system is currently in place. These procedures were employed several times during injection at the PNR‐1z well at Preston New Road, Lancashire, UK, from October to December 2018. As injection proceeded, it became apparent to the operator that stages were interacting with a seismogenic planar structure, interpreted as a fault zone, with several ML>0.5 events occurring. Microseismicity was clustered along this planar structure in a fashion that could not readily be explained through pore pressure diffusion or hydraulic fracture growth. Instead, we investigate the role of static elastic stress transfer created by the tensile opening of hydraulic fractures. We find that the spatial distributions of microseismicity are strongly correlated with areas that receive positive Mohr‐Coulomb stress changes from the tensile fracture opening, while areas that receive negative Mohr‐Coulomb stress change are quiescent. We conclude that the stressing due to tensile hydraulic fracture opening plays a significant role in controlling the spatiotemporal distribution of induced seismicity. Key Points: The spatiotemporal distribution of microseismic events during hydraulic fracturing‐induced fault activation at Preston New Road, UK, could not be simply explained by pore pressure diffusion or fracture growthA stochastic approach for modeling elastic stress transfer from the opening of hydraulic fractures is developed to test if this mechanism could explain observationsDistribution of microseismic events are well correlated with fracture opening elastostatic stress changes, implying this mechanism significantly affected the behavior in the adjacent fault zone [ABSTRACT FROM AUTHOR]

Accurate marine sediment characteristics, for example, thickness and seismic velocity, are important for constraining sedimentation rates with implications for climate variations and for seismic imaging of deeper structures using ocean bottom seismic deployments. We analyze P‐to‐S seismic phase conversions from the sediment‐crust boundary recorded by the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment to infer the sediment thickness across the Mid‐Atlantic Ridge covering 0‐ to 80‐Myr‐old seafloor. We find Pds‐P delay times of 0.04–0.37 s, or 5‐ to 82‐m thickness. Sediment thickness increases with age. Thickness agrees with global estimates for young (<15–20 Myr) seafloor but is thinner on older lithosphere. Our result may represent a lower limit on sediment thickness, given that several of our stations are on topographic highs. The sedimentation rate decrease observed from 5 to 1.2 mm/kyr at ∼10 Myr suggests a recent increase in productivity related to climate change, eolian dust fluxes, and/or biogenic marine activity. Plain Language Summary: Studies on marine sediment production rate provide historical information on the past climates and sedimentation processes. Unfortunately, such data from the seafloor is sporadic across the oceans, limited to localized drilled sites and seismic profiles. Here we analyze the sediment characteristics along the equatorial Mid‐Atlantic Ocean, starting from age 0 at the ridge to sediments 80 million years old. We use newly acquired seismic data from the recent state‐of‐the‐art Passive Imaging of the Lithosphere‐Asthenosphere Boundary experiment. Thirty‐nine seismometers were put at the ocean bottom down to about 5,000 m below sea level and left recording seismic data for one year between 2016 and 2017. Conversion of seismic phases from the boundary between the crust and the sediment were analyzed to determine the sediment seismic velocity and thickness below each station. It was found that the thickness increases with age at a slower rate than previously thought and that an abrupt increase in the sedimentation rate occurred in the last 10 million years, from 1.2 to 5 mm per thousand years. The recent increase in productivity may be due to factors such as changes in climate, changes in the eolian dust fluxes from Africa to the Atlantic, and/or changes in biogenic marine activity. Key Points: P‐to‐S delay times from the sediment‐crust boundary suggest 5‐82 m of sediment beneath PI‐LABThe thickness generally agrees with global estimates but is less than expected on older lithosphereHigher deposition rates at <10 Myr suggest higher productivity perhaps related to stronger monsoons [ABSTRACT FROM AUTHOR]