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SUMMARY Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly informative due to their relatively simple structure in comparison to their continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there been few studies of transform faults in slow spreading ridges. Here, we present the first local seismicity catalogue based on event data recorded by a temporary broad-band network of 39 ocean–bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from 2016 to 2017 March. We locate 972 events in the area by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. We refine the depths and focal mechanisms of the larger events using deviatoric moment tensor inversion. Most of the earthquakes are located along the CTF (700) and Romanche transform fault (94) and the MAR (155); a smaller number (23) can be observed on the continuing fracture zones or in intraplate locations. The ridge events are characterized by normal faulting and most of the transform events are characterized by strike-slip faulting, but with several reverse mechanisms that are likely related to transpressional stresses in the region. CTF events range in magnitude from 1.1 to 5.6 with a magnitude of completeness around 2.3. Along the CTF we calculate a b-value of 0.81 ± 0.09. The event depths are mostly shallower than 15 km below sea level (523), but a small number of high-quality earthquakes (16) are located deeper, with some (8) located deeper than the brittle-ductile transition as predicted by the 600 °C-isotherm from a simple thermal model. The deeper events could be explained by the control of sea water infiltration on the brittle failure limit.

We investigate the physical processes that generate seismicity during hydraulic fracturing. Fluid processes (increases in pore pressure and poroelastic stress) are often considered to be the primary drivers. However, some recent studies have suggested that elastic stress interactions may significantly contribute to further seismicity. In this work we use a microseismic data set acquired during hydraulic fracturing to calculate elastic stress transfer during a period of fault activation and induced seismicity. We find that elastic stress changes may have weakly promoted initial failure, but at later times stress changes generally acted to inhibit further slip. Sources from within tight clusters are found to be the most significant contributor to the cumulative elastic stress changes. Given the estimated in situ stress field, relatively large increases in pore pressure are required to reach the failure envelope for these faults - on the order of 10 MPa. This threshold is far greater than the reliable cumulative elastic stress changes found in this study, with the vast majority of events receiving no more than 0.1 MPa of positive ΔCFS, further indicating that elastic stress changes were not a significant driver, and that interaction with the pressurized fluid was required to initiate failure. Thus, cumulative stress transfer from small events near the injection well does not appear to play a significant role in the reactivation of nearby faults.

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.

Seismicity at glaciers, ice sheets and ice shelves provides observational constraint of a number of glaciological processes. Detecting and locating this seismicity, specifically icequakes, is a necessary first step in studying processes such as basal slip, crevassing, and imaging ice fabric, for example. Most glacier deployments to date use conventional seismic networks, comprised of seismometers distributed over the entire area of interest. However, smaller aperture seismic arrays can also be used, which are typically sensitive to seismicity distal from the array footprint and require a smaller number of instruments. Here, we investigate the potential of arrays and array-processing methods to detect and locate seismicity in the cryosphere, benchmarking performance against conventional seismic network-based methods. We also provide an array-processing recipe for cryosphere applications. Results from an array and network deployed at Rutford Ice Stream, Antarctica, show that arrays and networks both have strengths and weaknesses. Arrays can detect icequakes from further distances whereas networks outperform arrays for more comprehensive studies of a process within the network extent, due to greater hypocentral constraint and a smaller magnitude of completeness. We also gain new insights into seismic behaviour at Rutford Ice Stream. The array detects basal icequakes in what was previously interpreted to be an aseismic region of the bed, as well as new icequake observations at the ice stream shear-margins, where it would be challenging to deploy instruments. Finally, we make some practical recommendations for future array deployments at glaciers.

We have used seismic refraction surveys of a wave-cut platform from a field site in South West England to characterize the impact of natural fracture networks on seismic velocities and anisotropy. Time-lapse surveys were performed as the high tide ebbed to investigate the seismic effects of the water draining from the rock. We also deployed a drone to map the fracture sets from the air. Azimuthal variations in the P- and S-wave velocities reflect the orientation of the main east–west-oriented joint set. Seismic velocities increased as the water drained, an effect attributed to a reduction in the effective density of the medium. The ratio of fracture normal ([Formula: see text]) to tangential ([Formula: see text]) compliance ([Formula: see text]), which can be used as a proxy for fracture saturation and permeability, was observed to increase from [Formula: see text] to [Formula: see text], primarily driven by a drop in [Formula: see text]. These variations are attributed to a decrease in the water content of the main fracture set as the tide retreats.

Measurements of the seismic properties of Antarctic ice streams are critical for constraining glacier dynamics and future sea-level rise contributions. In 2020, passive seismic data were acquired at the Rutford Ice Stream, West Antarctica, with the aim of imaging the near-surface firn layer. A DAS (distributed acoustic sensing) interrogator and 1 km of optic fibre were supplemented by 3-component geophones. Taking advantage of transient seismic energy from a petrol generator and seismicity near the ice stream shear margin (10s of km away from the DAS array), which dominated the ambient seismic noise field, we retrieve Rayleigh wave signals from 3 to 50 Hz. The extracted dispersion curve for a linear fibre array shows excellent agreement with an active seismic surface wave survey (Multichannel Analysis of Surface Waves) but with lower frequency content. We invert the dispersion curves for a 1D S-wave velocity profile through the firn layer, which shows good agreement with the previously acquired seismic refraction survey. Using a triangular-array geometry we repeat the procedure and find no evidence of seismic anisotropy at our study site. Our study presents challenges and solutions for processing noisy but densely sampled DAS data, for noise interferometry and imaging.

Flood embankments (levees or dykes) are used worldwide to protect homes, industry and farmland from flooding caused by extreme weather events and tidal surges. Their role is becoming increasingly important for two key reasons: climate change is causing larger and more frequent floods, and the number of people living on floodplains is increasing globally. Both these factors necessitate that flood defences are well maintained to minimise failure during flood events, and reduce disruption, damage, and even loss of life. There are more than 10,000 km of flood embankments in UK alone, so condition monitoring must be rapid. Current monitoring relies on qualitative walkover surveys every 6-12 months, but this can only detect the surface features that form in response to subsurface processes or characteristics. If we could detect subsurface properties and deterioration features directly it would enable us to identify areas at risk significantly earlier, minimising both risk and mitigation costs. Two complementary geophysical methods stand out: Electrical Resistivity Tomography (ERT) and Multi Channel Analysis of Surface Waves (MASW). These are sensitive to different hydro-mechanical properties of the materials that make up flood embankments and their foundations. ERT is sensitive to moisture content, clay content and porosity, whereas MASW is sensitive to elastic properties controlled by material strength, density, porosity and saturation. In this work we combine co-located ERT and MASW surveys with time-lapse airborne lidar on three contrasting embankments on the River Thames, River South Tyne, and the Humber Estuary. Each site was selected based on known anomalies or the availability of existing geotechnical information to ground truth the geophysical measurements. The three embankments represent a range of different soil types, ages and varying foundation materials, making an ideal suite of targets to test the different geophysical methods. In total c. 1 km of embankment was surveyed. Preliminary analysis shows good spatial agreement between units imaged by the ERT and those identified in the borehole data for each site. Areas of greatest settlement identified using time-lapse lidar also correlate with low resistivity anomalies indicating areas of soft clay and peat. Further data analysis will incorporate the MASW results and use clustering to quantitatively divide the subsurface into units with similar electrical and seismic properties. Geotechnical properties will then be attributed to each of the clusters, allowing more accurate fragility analysis of the embankment during flood conditions to be conducted.

Carbon capture and storage involves removing CO2 from industrial emissions or the atmosphere and sequestering it in a pressurised form. Onshore or offshore injection sites pump the liquefied CO2 below ground, preventing its release into the atmosphere. Earthquakes can occur when fluid is injected into a formation, inducing stress changes that can act on nearby faults, resulting in a rupture. The physics of these ruptures and the effect that fault lubrication processes have in triggering them remain key topics of research. Real-time microseismic monitoring at injection sites is the most readily available tool for painting a better picture. Injection sites on land are easier to monitor, with instruments requiring relatively little maintenance. Offshore sites, however, are more costly and less convenient because local monitoring could require expensive ocean-bottom instruments and complex deployment procedures. Induced microseismicity at CO2 injection sites is a critical measure of the reservoir and cap rock's response to injection. Thus, there is a need to locate events with low uncertainty, particularly in-depth.There are plans for several megatonne-scale offshore CO2 injection projects around the UK. Microseismicity at these sites must be well monitored over decades to ensure long term storage security, requiring novel, cost-effective monitoring strategies. We attempt to constrain the effectiveness of small, land-based arrays that could be used to deliver relatively low-cost monitoring and map out the areas of highest risk from induced seismicity. This study compares the data of such an array installed in northwest England with the national seismic network operated by the British Geological Survey (BGS). We analyse the capability of the array to detect and locate low magnitude (M2 storage monitoring by modelling the optimum size and spatial distribution of small seismic arrays.

This study presents the first demonstration of the transferability of a convolutional neural network (CNN) trained to detect microseismic events in one fiber-optic distributed acoustic sensing (DAS) data set to other data sets. DAS increasingly is being used for microseismic monitoring in industrial settings, and the dense spatial and temporal sampling provided by these systems produces large data volumes (approximately 650 GB/day for a 2 km long cable sampling at 2000 Hz with a spatial sampling of 1 m), requiring new processing techniques for near-real-time microseismic analysis. We have trained the CNN known as YOLOv3, an object detection algorithm, to detect microseismic events using synthetically generated waveforms with real noise superimposed. The performance of the CNN network is compared to the number of events detected using filtering and amplitude threshold (short-term average/long-term average) detection techniques. In the data set from which the real noise is taken, the network is able to detect >80% of the events identified by manual inspection and 14% more than detected by standard frequency-wavenumber filtering techniques. The false detection rate is approximately 2% or one event every 20 s. In other data sets, with monitoring geometries and conditions previously unseen by the network, >50% of events identified by manual inspection are detected by the CNN.

SUMMARY We present unambiguous evidence that the Mahogany salt body, located in the Northern part of the Gulf of Mexico, is seismically anisotropic. Evidence of anisotropy comes from shear wave splitting data obtained from a vertical seismic profile VSP. The data set consists of 48 vertically aligned receivers in a borehole drilled through the salt body. Splitting analysis is performed on shear wave phases that are converted from compressional waves at the top and bottom of the salt body. The phase converted at the top of the salt layer shows a clear signature of seismic anisotropy, while the phase at the base of the salt layer shows negligible splitting. We investigate the possibility of rock salt halite LPO as a cause of the observed anisotropy. A finite element geomechanical salt deformation model of the Mahogany salt body is developed, where deformation history is used as an input to the texture plasticity simulation program VPSC. Assuming a halite salt body, a full elasticity model is then calculated and used to create a synthetic VSP splitting data set. The comparison between the synthetic and real VSP data set shows that LPO of rock salt can explain the observed anisotropy remarkably well. This is the strongest evidence to date of seismic anisotropy in a deforming salt structure. Furthermore, for the first time, we are able to demonstrate clear evidence that deforming halite is the most likely cause of this anisotropy, combining data set analysis and synthetic full wave form modelling based on calculated rock salt elasticities. Neglecting anisotropy in seismic processing in salt settings could lead to potential imaging errors, for example the deformation models show an averaged delta parameter of δ = –0.06, which would lead in a zero offset reflection setting to a depth mismatch of 6.2 per cent. Our work also show how observations of salt anisotropy can be used to probe characteristics of salt deformation.

Over recent years, a greater importance has been attached to low-magnitude events, with increasing use of the subsurface for industrial activities such as hydraulic fracturing and enhanced geothermal schemes. Magnitude distributions and earthquake source properties are critical inputs when managing the associated seismic risk of these activities, yet inconsistencies and discrepancies are commonly observed with microseismic activity (M

We present a case study demonstrating the use of an “L”-shaped downhole fibre-optic array to monitor microseismicity. We use a relatively simple method to detect events from continuous waveform data, and develop a workflow for manual event location. Locations are defined with a cylindrical coordinate system, with the horizontal axis of the DAS cable being the axis of symmetry. Events are located using three manual “picks”, constraining (1) the zero-offset “broadside” channel to the event (2) the P-S wave arrival time difference at the broadside channel, and (3) the angle, ? of the event from the array. Because the one-component DAS array is unable to record P-wave energy on the broadside channel, the P-wave pick is made indirectly by ensuring that the modeled P- and S-wave moveout curves match the observed data. The ? angle requires that signal is observed on the vertical part of the array, in our case this is possible because an engineered fiber, rather than standard telecommunications fiber, provided a significant reduction in the noise level. Because only three picks need to be made, our manual approach is significantly more efficient than equivalent manual processing of downhole geophone data, where picks for P- and S-waves must be made for each receiver. We find that the located events define a tight cluster around the injection interval, indicating that this approach provides relatively precise and accurate event locations. A surface microseismic array was also used at this site, which detected significantly fewer events, the locations of which had significantly greater scatter than the DAS array locations. We conclude by examining some other aspects of the DAS microseismic data, including the presence of multiple events within very short time windows, and the presence of converted phases that appear to represent scattering of energy from the hydraulic fractures themselves.

Firn densification profiles are an important parameter for ice-sheet mass balance and palaeoclimate studies. One conventional method of investigating firn profiles is using seismic refraction surveys, but these are difficult to upscale to large-area measurements. Distributed acoustic sensing (DAS) presents an opportunity for large-scale seismic measurements of firn with dense spatial sampling and easy deployment, especially when seismic noise is used. We study the feasibility of seismic noise interferometry on DAS data for characterizing the firn layer at the Rutford Ice Stream, West Antarctica. Dominant seismic energy appears to come from anthropogenic noise and shear-margin crevasses. The DAS cross-correlation interferometry yields noisy Rayleigh wave signals. To overcome this, we present two strategies for cross-correlations: (1) hybrid instruments – correlating a geophone with DAS, and (2) stacking of selected cross-correlation panels picked in the tau-p domain. These approaches are validated with results derived from an active survey. Using the retrieved Rayleigh wave dispersion curve, we inverted for a high-resolution 1D S-wave velocity profile down to a depth of 100 m. The profile shows a ‘kink’ (velocity gradient inflection) at ∼12 m depth, resulting from a change of compaction mechanism. A triangular DAS array is used to investigate directional variation in velocity, which shows no evident variations thus suggesting a lack of azimuthal anisotropy in the firn. Our results demonstrate the potential of using DAS and seismic noise interferometry to image the near-surface and present a new approach to derive S-velocity profiles from surface wave inversion in firn studies.

Supervised deep learning models have become a popular choice for seismic phase arrival detection. However, they do not always perform well on out-of-distribution data and require large training sets to aid generalization and prevent overfitting. This can present issues when using these models in new monitoring settings. In this work, we develop a deep learning model for automating phase arrival detection at Nabro volcano using a limited amount of training data (2,498 event waveforms recorded over 35 days) through a process known as transfer learning. We use the feature extraction layers of an existing, extensively trained seismic phase picking model to form the base of a new all-convolutional model, which we call U-GPD. We demonstrate that transfer learning reduces overfitting and model error relative to training the same model from scratch, particularly for small training sets (e.g., 500 waveforms). The new U-GPD model achieves greater classification accuracy and smaller arrival time residuals than off-the-shelf applications of two existing, extensively-trained baseline models for a test set of 800 event and noise waveforms from Nabro volcano. When applied to 14 months of continuous Nabro data, the new U-GPD model detects 31,387 events with at least four P-wave arrivals and one S-wave arrival, which is more than the original base model (26,808 events) and our existing manual catalog (2,926 events), with smaller location errors. The new model is also more efficient when applied as a sliding window, processing 14 months of data from seven stations in less than 4 h on a single graphics processing unit.

Corbetti is currently one of the fastest uplifting volcanoes globally, with strong evidence from geodetic and gravity data for a subsurface inflating magma body. A dense network of 18 stations has been deployed around Corbetti and Hawassa calderas between February 2016 and October 2017, to place seismic constraints on the magmatic, hydrothermal and fault slip processes occurring around this deforming volcano. We locate 122 events of magnitudes between 0.4 and 4.2 were located using a new local velocity model. The seismicity is focused in two areas: directly beneath Corbetti caldera and beneath the east shore of Lake Hawassa. The shallower 0-5km depth below sea level (b.s.l.) earthquakes beneath Corbetti are mainly focused in NW-elongated clusters at Urji and Chabbi volcanic centres. This distribution is interpreted to be mainly controlled by a northward propagation of hydrothermal fluids from a cross-rift pre-existing fault. Source mechanisms are predominantly strike-slip and different to the normal faulting away from the volcano, suggesting a local rotation of the stress-field. These observations, along with a low Vp/Vs ratio, are consistent with the inflation of a gas-rich sill, likely of silicic composition, beneath Urji. In contrast, the seismicity beneath the east shore of Lake Hawassa extends to greater depth (16 km b.s.l.). These earthquakes are focused on 8-10 km long segmented faults, which are active in seismic swarms. One of these swarms, in August 2016, is focused between 5 and 16 km depth b.s.l. along a steep normal fault beneath the city of Hawassa, highlighting the tectonic hazard for the local population.

Injecting fluids into the subsurface is necessary for a number of industries to facilitate the energy transition (e.g., geothermal, geologic CO2 sequestration or hydrogen storage). One of the biggest challenges is that fluid injection induces seismicity, which can lead to damaging events. It is currently not possible to predict the exact nature of seismicity that will occur due to fluid injection prior to operations.Using laboratory friction experiments and in-situ microseismic analyses, we investigate the role frictional behaviour may have on the rate and magnitude of induced seismicity. This study focuses on the Horn River Basin shale gas play (British Columbia, Canada), where hydraulic fracturing activity has resulted in felt induced seismicity. Microseismic data from this field highlights fault planes that cut across the stratigraphy, including overburden and reservoir shales of varying mineralogy and underburden dolomites.Our experimental friction results on samples recovered from core at reservoir depths show that both the frictional strength and stability vary considerably across the different lithologies; transitioning from very velocity-strengthening with friction coefficients of 0.3 – 0.4 in the overburden shales to more velocity-weakening and friction coefficients of 0.55 – 0.7 in the reservoir shales and an analogue of the underburden dolomite.Spatial clustering analysis of the microseismicity allowed us to discriminate the operationally induced fracturing from fault reactivation events. We then examined the variations in the seismic b-value of the event magnitude-frequency distribution. These events were further differentiated by depth, separating them into their lithological horizons. The results show, for both fracturing and faulting events, higher seismic b-values of 1.4 – 1.5 occur in the overburden shales, which then decrease into the upper reservoir shales to 0.8 – 1.1, and then increase into the lower reservoir shales and underburden dolomite to 1.1 – 1.4. These trends correlate well with the laboratory measurements of frictional a-b values that define the degree of velocity-strengthening to velocity-weakening in the different gouges across the same lithological units.These results suggest that knowledge of the frictional behaviour of the subsurface prior to operations, derived from mineralogical compositions and laboratory testing on cored material, may help improve our understanding of the potential rate and magnitude of induced seismicity that may occur due to subsurface fluid injection.

Induced seismicity due to fluid injection, including hydraulic fracturing, is an increasingly common phenomenon worldwide; yet, the mechanisms by which hydraulic fracturing causes fault activation remain unclear. Here we show that preexisting fracture networks are instrumental in transferring fluid pressures to larger faults on which dynamic rupture occurs. Studies of hydraulic fracturing-induced seismicity in North America have often used observations from regional seismograph networks at distances of 10s of km, and as such lack the resolution to answer some of the key questions about triggering mechanisms. To carry out a more detailed analysis of the mechanisms of fault activation, we use data from a dense sensor array located at a hydraulic-fracturing site in Alberta, Canada. The spatiotemporal distribution of event hypocenters, coupled with measurements of seismic anisotropy, reveal the presence of preexisting fracture corridors that allowed communication of fluid-pressure perturbations to larger faults, over distances of 1 km or more. The presence of preexisting permeable fracture networks can significantly increase the volume of rock affected by the pore-pressure increase, thereby increasing the probability of induced seismicity. This study demonstrates the importance of understanding the connectivity of preexisting natural fractures for assessing potential seismic hazards associated with hydraulic fracturing of shale formations and offers a detailed case exposition of induced seismicity due to hydraulic fracturing.

Summary Here, we present the first results of DAS deployed in Antarctica to interrogate icequake microseismicity. Distributed Acoustic Sensing (DAS) can provide much higher density spatial sampling of an earthquake’s seismic wavefield than conventional seismometer networks, and is therefore promising for microseismicity studies generally. We first present a methodology for microseismic event detection and location using DAS. We then demonstrate the potential of DAS for discriminating source mechanisms and anisotropic path effects. To our knowledge, this is the first full waveform source mechanism inversion of a microseismic source using DAS. We also investigate the potential of DAS for ambient noise studies. In each case, we compare the performance of DAS to conventional geophones. Our results have implications for future microseismicity studies, highlighting both the promise and limitations of DAS for such applications.

Microseismicity, induced by the sliding of a glacier over its bed, can be used to characterize frictional properties of the ice-bed interface, which are a key parameter controlling ice stream flow....

Transform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid‐ocean ridges in ocean basins worldwide. The current consensus is that oceanic transform faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of transform fault formation. Here we integrate evidence from surface faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto‐transform fault is initiating during the final stages of continental breakup. This is the first direct observation of proto‐transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late‐stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading., Geochemistry, Geophysics, Geosystems, 19 (12), ISSN:1525-2027

How an earthquake rupture propagates strongly influences the potentially destructive ground shaking. Complex ruptures often involve slip along multiple faults, which masks information on the frictional behaviour of fault zones. Geometrically smooth ocean transform fault plate boundaries offer a favourable environment to study fault dynamics, because strain is accommodated along a single, wide fault zone that offsets the homogeneous geology. Here we present an analysis of the 2016 Mw 7.1 earthquake on the Romanche fracture zone in the equatorial Atlantic, using data from both nearby seafloor seismometers and global seismic networks. We show that this rupture had two phases: (1) upward and eastward propagation towards a weaker region where the transform fault intersects the mid-ocean ridge, and then (2) an unusual back-propagation westwards at a supershear speed towards the centre of the fault. We suggest that deep rupture into weak fault segments facilitated greater seismic slip on shallow locked zones. This highlights that even earthquakes along a single distinct fault zone can be highly dynamic. Observations of back-propagating ruptures are sparse, and the possibility of reverse propagation is largely absent in rupture simulations and unaccounted for in hazard assessments. In one earthquake, an oceanic transform fault ruptured in one direction and then backwards at a speed exceeding that of shear-wave propagation, according to an analysis of data recorded by nearby seafloor and global seismometers.

Lower crustal seismicity is commonly observed in continental rift zones despite the crust at such depths being ductile enough to prohibit brittle failure. The source of such deep seismicity across the East African Rift remains an outstanding question. Here we present analysis of an isolated cluster of lower crustal earthquakes located on the eastern border faults of the Main Ethiopian Rift, near the Corbetti caldera. Lower crustal earthquakes have not previously been observed in this area. Phase arrival times were determined using an automated picking approach based on continuous wavelet transform and statistical changepoint detection methods. We overcome misinterpretations from large hypocentre depth errors by considering mixture distributions for all events and their associated uncertainties. These mixture distributions represent probability density functions of any event occurring at a given depth. The mixture distribution mode for a variety of different velocity models and error parameters remained stable at a depth of 28 – 32 km, with the vast majority of maximum likelihood estimates for individual hypocenters located at depths of 25 – 35 km. Most events occur over a two‐month period, with 90% of cumulative seismic moment occurring during March and April 2012. The ephemeral and localized nature of this seismicity, combined with low event magnitudes and regional hydrothermal/magmatic activity, all suggest that these lower crustal events are likely related to fluid or magmatic processes. Plausible mechanisms include the movement of magma and/or exsolution of volatiles at depth causing transient high strain rates and pore fluid pressures that induce seismicity.

Rupture propagation of an earthquake strongly influences potentially destructive ground shaking. Variable rupture behaviour is often caused by complex fault geometries, masking information on fundamental frictional properties. Geometrically smoother ocean transform fault (OTF) plate boundaries offer a favourable environment to study fault zone dynamics because strain is accommodated along a single, wide zone (up to 20 km width) offsetting homogeneous geology comprising altered mafic or ultramafic rocks. However, fault friction during OTF ruptures is unknown: no large (Mw>7.0) ruptures had been captured and imaged in detail. In 2016, we recorded an Mw 7.1 earthquake on the Romanche OTF in the equatorial Atlantic on nearby seafloor seismometers. We show that this rupture had two phases: (1) up and eastwards propagation towards the weaker ridge-transform intersection (RTI), then (2) unusually, back-propagation westwards at super-shear speed toward the fault’s centre. Deep slip into weak fault segments facilitated larger moment release on shallow locked zones, highlighting that even ruptures along a single distinct fault zone can be highly dynamic. The possibility of reversing ruptures is absent in rupture simulations and unaccounted for in hazard assessments.

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.

Volcanoes produce widely varying seismic signals due to the presence of complex and non-linear physical processes. The temporal distribution of seismicity at volcanoes ranges from individual transients to swarms of many small events and protracted volcanic tremor. The spectral range of volcanic signals is unequivocally broadband, with coincident high (>20 Hz) and very low (down to periods of hundreds of seconds) frequency signals frequently observed at many volcanic systems. As such, interpretations of volcano-seismic source and process require suitable characterisation in the time-frequency (T-F) domain. The adoption of automated approaches to routine seismic processing at volcanoes also creates the need to evaluate how we suitably extract discriminatory features of interest from such diverse volcano-seismic signals. Here we assess the performance of the continuous wavelet transform (CWT) for spectral representations of volcano-seismic signals. The localisation property of wavelet transforms gives the CWT a distinct advantage over commonly used moving-window Fourier transforms, enabling it to capture sharp changes in signal and represent signals over a wide range of timescales. Examination of seismic data for typical volcano-seismic phenomena, such as volcano-tectonic earthquakes (VTs), shows that CWT scalograms have better T-F resolution across broader frequency ranges than Fourier transform spectrograms, which suffer from greater spectral smearing in the time domain at higher frequencies. The inherent log-scale representation of CWT scalograms is also better suited for detection and representation of very-long-period (VLP) signals and for distinguishing volcanic signals from ambient microseismic noise. When applied to seismic data from Santiaguito volcano in Guatemala, CWT analysis reveals pre- and syn-eruptive signals across a wide range of frequency bands, ranging from 600 s to 50 Hz, with ultra-long-period signals (ULPs; 30 to 600 s) detected on instruments up to 1.9 km from the active vent, which is beyond the range of previously detected ULPs at this volcanic system. The CWT scalogram conveniently represents these simultaneous syn-eruptive spectral features in a single plot, which can aid exploratory analysis and inform source models. Furthermore, the ‘edge detection’ capabilities of the CWT accurately identify sharp changes in the raw signal over the VLP-ULP frequency range (5 to 600+ s), thought to represent sudden deflation associated with eruption, providing a useful tool for ‘picking’ explosive eruptions. The addition of an average wavelet energy distribution to CWT scalograms, which reveals the average energy across the whole signal at each wavelet scale, is also useful for characterising spectral content and identifying spectral peaks, as its smooth appearance is easier to interpret than FFT spectral amplitude plots. We conclude that wavelet transform methods are underutilised in volcano seismology, where their T-F localisation properties would be particularly well-suited, and suggest potential applications in terms of automated event detection and classification.

Seismicity can be used to better understand interactions between magma bodies, hydrothermal systems and their host rocks—key factors influencing volcanic unrest. Here, we use earthquake data to image, for the first time, the seismic velocity structure beneath Aluto, a deforming volcano in the Main Ethiopian Rift. Traveltime tomography is used to jointly relocate seismicity and image 3D P- and S-wave velocity structures and the ratio between them (V P/V S). At depths of 4–9 km, the seismicity maps the top of a large low velocity zone with high V P/V S, which we interpret as a more ductile and melt-bearing region. A shallow (

Uturuncu volcano is situated in the Bolivian Andes, directly above the world's largest crustal body of silicic partial melt, the Altiplano-Puna Magma Body (APMB). Uturuncu last erupted 250,000 years ago, yet is seismically active and lies at the centre of a 70 km diameter uplifted region. Here, we analyse seismicity from 2009 to 2012. Our earthquake locations, using a newly developed velocity model, delineate the top and bottom of the APMB, reveal individual faults, and reconcile differences in depth distribution between previous studies. Spatial clustering analysis of these earthquakes reveals the orientations of the faults, which match stress orientations from seismic anisotropy. Earthquake b-values derived from moment magnitudes ( 1.44 ± 0.06 ) differ significantly from those using local magnitude measurements ( 0.80 ± 0.03 ). From these observations and theoretical justification, we suggest that, if possible, moment magnitudes should be used for accurate b-value analysis. We interpret b-values > 1 in terms of fluid-enhanced seismicity. Shallow seismicity local to Uturuncu yields b-values > 1.1 with some temporal variation, suggesting fluid migration along pre-existing faults in a shallow hydrothermal system, likely driven by advection from the APMB. Intriguingly, events deeper than the APMB also yield large b-values (1.4), mapping the ascent into the lower crust of fluids that we infer as originating from a subducting slab. Cumulatively, these results provide a picture of an active magmatic system, where fluids are exchanged across the more ductile APMB, feeding a shallow, fault-controlled hydrothermal system. Such pathways of fluid ascent may influence our understanding of arc volcanism, control future volcanic eruptions and promote the accumulation of shallow hydrothermal ore deposits.

The physical signatures of unrest in large silicic magma systems are commonly observed in geophysical surveys, yet the interactions between magmatic processes and crustal stresses are often left unconstrained. Stresses in the mid and upper crust exert a strong control on the propagation and stalling of magma, and magma ascent can in turn change the magnitude and orientation of these stresses, including those associated with hydrothermal systems. This study assesses the state of stress at the restless Uturuncu Volcano in the Bolivian Andes with space, depth and time using observations of seismic anisotropy and the magnitude–frequency distributions of local earthquakes. Shear-wave splitting measurements are made for 677 events in the upper crust (1–25 km below sea level) between June 1, 2009 and March 10, 2012, and b-values are calculated using the Aki maximum likelihood method for a range of catalog subsets in the entire crust (−5 to 65 km below sea level). The b-value of the crustal events is unusually low ( b = 0.66 ± 0.09 ), indicating that the seismogenic region features strong material with high stresses that are released with limited influence from hydrothermal fluids. The 410 good quality shear-wave splitting results have an average delay time of 0.06 ± 0.002 s and an average percent anisotropy ranging from 0.25 ± 0.04% to 6.2 ± 0.94% with a mean of 1.70 ± 0.32%. Fast shear-wave polarization directions are highly variable and appear to reflect a combination of tectonic and magmatic stresses that overprint the regional E–W compressive stress associated with the convergence of the Nazca and South American Plates. The shear-wave splitting results and b-values suggest that the upper crust beneath Uturuncu (∼0–7 km below the summit) is characterized by a weak and localized hydrothermal system in a poorly developed fracture network. We conclude that stresses imposed by crustal flexure due to magmatic unrest above the Altiplano-Puna Magma Body activate crack opening on a pre-existing fault beneath the volcano, generating seismicity and a spatially variable 1–10% anisotropy above the brittle–ductile transition zone. These results suggest that strong stresses in relatively unfractured upper crustal rocks may locally inhibit fluid migration in large silicic magma systems, leading to pluton emplacement and effusive volcanism rather than explosive eruptions.

The Aquistore project in Saskatchewan, Canada provides carbon dioxide (CO2) storage for the world's first combined commercial power plant and carbon capture and storage (CCS) project. CO2 has been injected at a depth of 3.2 km since April 2015 and a permanent near surface geophone array provides passive seismic monitoring. The ability to identify any containment breach is a vital part of risk management and reduction for CO2 storage sites. We therefore investigate the potential to monitor seismic velocity changes following a hypothetical leak of CO2 from the reservoir using passive monitoring methods. We estimate the expected shear-wave velocity change with CO2 saturation, and using data from the geophone array we investigate whether ambient noise interferometry (ANI) and a tomographic inversion for Rayleigh wave group-velocity maps could provide a suitable CO2 leakage detection tool. To assess the repeatability of the method, we conduct, for the first time, a time-lapse ambient noise tomography survey of a CO2 storage site to cover time periods preceding and following injection start-up. Sensitivity analysis results indicate that usable surface wave data derived from the current array configuration are sensitive to depths of ∼400 m and shallower. We do not expect to observe any changes due to CO2 migration at such shallow depths and the estimated seismic velocities pre- and post-injection agree to within 60 m s−1, which is on the order of double the predicted velocity change with CO2 saturation. Therefore, due to uncertainties in travel-time picks (5–15%) and variations in the obtained velocity structure between consecutive days (up to 20%), we would be unable to resolve the expected seismic velocity change with an influx of CO2 at 400 m (∼3–4%). Additionally, the noise source variability does not allow stable velocity estimates to be made in the time-frame of currently-available data. Consequently, in the event of a CO2 leak at the Aquistore site, using the standard ambient noise analysis methods applied herein, Rayleigh wave tomography could be deployed to detect velocity changes due to CO2 saturation only if (a) a wider aperture surface array was in place to allow longer period surface waves to be used, providing sensitivity at greater depths, (b) arrival times of interferometrically-synthesised surface waves could be picked with increased accuracy, and (c) there is stability of the noise source distribution between repeated surveys. However, a map of three-dimensional near surface velocities, as obtained in this study, could nevertheless be useful for near surface static corrections when using active-source seismic reflection surveys to image and monitor the reservoir. More generally, further similar studies are required to assess the applicability of ANI for leak detection at other CO2 storage sites.

In continental rifts structural heterogeneities, such as pre-existing faults and foliations, are thought to influence shallow crustal processes, particularly the formation of rift faults, magma reservoirs and surface volcanism. We focus on the Corbetti caldera, in the southern central Main Ethiopian Rift. We measure the surface deformation between 22nd June 2007 and 25th March 2009 using ALOS and ENVISAT SAR interferograms and observe a semi-circular pattern of deformation bounded by a sharp linear feature cross-cutting the caldera, coincident with the caldera long axis. The signal reverses in sign but is not seasonal: from June to December 2007 the region south of this structure moves upwards 3 cm relative to the north, while from December 2007 until November 2008 it subsides by 2 cm. Comparison of data taken from two different satellite look directions show that the displacement is primarily vertical. We discuss potential mechanisms and conclude that this deformation is associated with pressure changes within a shallow (

The characterisation of the subsurface of a landslide is a critical step in developing ground models that inform planned mitigation measures, remediation works or future early-warning of instability. When a landslide failure may be imminent, the time pressures on producing such models may be great. Geoelectrical and seismic geophysical surveys are able to rapidly acquire volumetric data across large areas of the subsurface at the slope-scale. However, analysis of the individual model derived from each survey is typically undertaken in isolation, and a robust, accurate interpretation is highly dependent on the experience and skills of the operator. We demonstrate a machine learning process for constructing a rapid reconnaissance ground model, by integrating several sources of geophysical data in to a single ground model in a rapid and objective manner. Firstly, we use topographic data acquired by a UAV survey to co-locate three geophysical surveys of the Hollin Hill Landslide Observatory in the UK. The data are inverted using a joint 2D mesh, resulting in a set of co-located models of resistivity, P-wave velocity and S-wave velocity. Secondly, we analyse the relationships and trends present between the variables for each point in the mesh (resistivity, P-wave velocity, S-wave velocity, depth) to identify correlations. Thirdly, we use a Gaussian Mixture Model (GMM), a form of unsupervised machine learning, to classify the geophysical data into cluster groups with similar ranges and trends in measurements. The resulting model created from probabilistically assigning each subsurface point to a cluster group characterises the heterogeneity of landslide materials based on their geophysical properties, identifying the major subsurface discontinuities at the site. Finally, we compare the results of the cluster groups to intrusive borehole data, which show good agreement with the spatial variations in lithology. We demonstrate the applicability of integrated geophysical surveys coupled with simple unsupervised machine learning for producing rapid reconnaissance ground models in time-critical situations with minimal prior knowledge about the subsurface.

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, whilst 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 Δt* 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. This article is protected by copyright. All rights reserved

The silicic volcanic centres of the Main Ethiopian Rift (MER) play a central role in facilitating continental rifting. Many of these volcanoes host geothermal resources and are located in heavily populated regions. InSAR studies have shown several are deforming, but regional seismic networks have detected little seismicity. A local network of 12 seismometers was deployed at Aluto Volcano from 2012 to 2014, and detected 2142 earthquakes within a 24-month period. We locate the events using a 1D velocity model that exploits a regional model and information from geothermal boreholes and calculate local magnitudes, b-values and focal mechanisms. Event depths generally range from the near surface to 15 km with most of the seismicity clustering in the upper 2 km. A significant amount of seismicity follows the Artu Jawa Fault Zone, which trends in alignment with the Wonji Fault Belt, NNE–SSW and is consistent with previous studies of strain localisation in the MER. Focal mechanisms are mostly normal in style, with the mean T-axes congruent to the orientation of extension in the rift at this latitude. Some show relatively small left-lateral strike-slip components and are likely associated with the reactivation of NE-ENE structures at the southern tip of the Aluto-Gedemsa segment. Events range from − 0.40 to 2.98 in magnitude and we calculate an overall b-value of 1.40 ± 0.14. This relatively elevated value suggests fluid-induced seismicity that is particularly evident in the shallow hydrothermal reservoir and above it. Subdividing our observations according to depth identifies distinct regions beneath the volcanic edifice: a shallow zone (− 2–0 km) of high seismicity and high b-values that corresponds to the hydrothermal system and is influenced by a high fluid saturation and circulation; a relatively aseismic zone (0–2 km) with low b-values that is impermeable to ascending volatiles; a region of increased fluid-induced seismicity (2–9 km) that is driven by magmatic intrusion from below and a deeper zone (below 9 km) that is interpreted as a partially crystalline, magmatic mush. These observations indicate that both the magmatic and hydrothermal systems of Aluto volcano are seismically active and highlight the need for dedicated seismic monitoring at volcanoes in the MER.

Subduction is integral to mantle convection and plate tectonics, yet the role of the subslab mantle in this process is poorly understood. Some propose that decoupling from the slab permits widespread trench parallel flow in the subslab mantle, although the geodynamical feasibility of this has been questioned. Here, we use the source-side shear wave splitting technique to probe anisotropy beneath subducting slabs, enabling us to test petrofabric models and constrain the geometry of mantle fow. Our global dataset contains 6369 high quality measurements – spanning ∼ 40 , 000 km of subduction zone trenches – over the complete range of available source depths (4 to 687 km) – and a large range of angles in the slab reference frame. We find that anisotropy in the subslab mantle is well characterised by tilted transverse isotropy with a slow-symmetry-axis pointing normal to the plane of the slab. This appears incompatible with purely trench-parallel flow models. On the other hand it is compatible with the idea that the asthenosphere is tilted and entrained during subduction. Trench parallel measurements are most commonly associated with shallow events (source depth 50 km ) – suggesting a separate region of anisotropy in the lithospheric slab. This may correspond to the shape preferred orientation of cracks, fractures, and faults opened by slab bending. Meanwhile the deepest events probe the upper lower mantle where splitting is found to be consistent with deformed bridgmanite.

Like most other industrial activities that affect the subsurface, hydraulic fracturing carries a risk of reactivating pre-existing faults and thereby causing induced seismicity. In some regions, regulators have responded to this risk by imposing Traffic Light Scheme-type regulations, where fracture stimulation programs must be amended or shut down if events larger than a given magnitude are induced. Some sites may be monitored with downhole arrays and/or dense near-surface arrays, capable of detecting very small microseismic events. However, such monitoring arrangements will not be logistically or economically feasible at all sites. Instead, operators are using small, sparse arrays of surface seismometers to meet their monitoring obligations. The challenge we address in this paper is to maximise the detection thresholds of such small, sparse, surface arrays, so that they are capable of robustly identifying small-magnitude events, whose signal-to-noise ratios may be close to 1. To do so we develop a beam-forming-and-stacking approach, computing running short-term/long-term average functions for each component of each recorded trace (P, SH and SV), time-shifting these functions by the expected travel-times for a given location, and performing a stack. We assess the effectiveness of this approach with a case study, using data from a small surface array that recorded a multi-well, multi-stage hydraulic fracture stimulation in Oklahoma over a period of 8 days. As a comparison, we initially used a conventional event-detection algorithm to identify events, finding a total of 17 events. In contrast, the beam-forming-and-stacking approach identified a total of 155 events during this period (including the 17 events detected by the conventional method). The events that were not detected by the conventional algorithm had low signal-to-noise ratios, to the extent that in some cases they would be unlikely to be identified even by manual analysis of the seismograms. We conclude that this approach is capable of improving the detection thresholds of small, sparse arrays, and so can be used to maximise the information generated when deployed to monitor industrial sites.

Seismic anisotropy in and beneath the subducting slabs of the Antilles is investigated using observations of shear-wave splitting. We use a combination of teleseismic and local events recorded at three-component broadband seismic stations on every major island in the area to map anisotropy in the crust, the mantle wedge and the slab/sub-slab mantle. To date this is the most comprehensive study of anisotropy in this region, involving 52 stations from 8 seismic networks. Local event delay times (0.21 ± 0.12 s) do not increase with depth, indicating a crustal origin in anisotropy and an isotropic mantle wedge. Teleseismic delay times are much larger (1.34 ± 0.47 s), with fast shear-wave polarisations that are predominantly parallel to trend of the arc. These observations can be interpreted three ways: (1) the presence of pre-existing anisotropy in the subducting slab; (2) anisotropy due to sub-slab mantle flow around the eastern margin of the nearly stationary Caribbean plate; (3) some combination of both mechanisms. However, there are two notable variations in the trench-parallel pattern of anisotropy — trench-perpendicular alignment is observed in narrow regions east of Puerto Rico and south of Martinique. These observations support previously proposed ideas of eastward sublithospheric mantle flow through gaps in the slab. Furthermore, the pattern of anisotropy south of Martinique, near Saint Lucia is consistent with a previously proposed location for the boundary between the North and South American plates.

Earthquakes induced by subsurface fluid injection pose a significant issue across a range of industries. Debate continues as to the most effective methods to mitigate the resulting seismic hazard. Observations of induced seismicity indicate that the rate of seismicity scales with the injection volume and that events follow the Gutenberg–Richter distribution. These two inferences permit us to populate statistical models of the seismicity and extrapolate them to make forecasts of the expected event magnitudes as injection continues. Here, we describe a shale gas site where this approach was used in real time to make operational decisions during hydraulic fracturing operations.Microseismic observations revealed the intersection between hydraulic fracturing and a pre‐existing fault or fracture network that became seismically active. Although “red light” events, requiring a pause to the injection program, occurred on several occasions, the observed event magnitudes fell within expected levels based on the extrapolated statistical models, and the levels of seismicity remained within acceptable limits as defined by the regulator. To date, induced seismicity has typically been regulated using retroactive traffic light schemes. This study shows that the use of high‐quality microseismic observations to populate statistical models that forecast expected event magnitudes can provide a more effective approach.

We reconstruct crustal structure along the Lesser Antilles island arc using an inversion approach combining constraints from petrology of magmatic crustal xenoliths and seismic receiver functions. Xenoliths show considerable island-to-island variation in xenolith petrology from plagioclase-free ultramafic lithologies to gabbros and gabbronorites with variable proportions of amphibole, indicative of changing magma differentiation depths. Xenoliths represent predominantly cumulate compositions with equilibration depths in the range 5 to 40 km. We use xenolith mineral modes and compositions to calculate seismic velocities ( v P , v S ) and density at the estimated equilibration depths. We create a five-layer model of crustal structure for testing against receiver functions (RF) from island seismic stations along the arc. Lowermost layer (5) comprises peridotite with physical characteristics of mantle xenoliths from Grenada. Uppermost layer (1) consists of 5 km of volcaniclastics and sediments, whose physical properties are determined via a grid inversion routine. The three middle layers (2) to (4) comprise igneous arc crust with compositions corresponding to the xenoliths sampled at each island. By inversion we obtain a petrological best-fit for the RF on each island to establish the nature and thicknesses of layers (2) to (4). Along the arc we see variations in the depth and strength of both Moho and mid-crustal discontinuity (MCD) on length-scales of tens of km. Moho depths vary from 25 to 37 km; MCD from 11 and 32 km. The Moho is the dominant discontinuity beneath some islands (St. Kitts, Guadeloupe, Martinique, Grenada), whereas the MCD dominates beneath others (Saba, St. Eustatius). Along-arc variability in MCD depth and strength is consistent with variation in estimated magmatic H2O contents and differentiations depths that, in turn, influence xenolith lithologies. A striking feature is steep, along-arc gradients in v P similar to those observed at other oceanic arcs. These gradients reflect abrupt changes in rates and processes of magma generation in the underlying crust and mantle. We find no evidence for large, interconnected bodies of partial melt beneath the Lesser Antilles. Instead, the crustal velocity structure is consistent with magma differentiation in vertically-extensive, crystal mush-dominated reservoirs. Along-arc variation in crustal structure may reflect heterogeneous upwelling within the mantle wedge, itself driven by variation in slab-derived H2O fluxes.

Summary Fibre-optic Distributed Acoustic Sensing (DAS) cables are now used to monitor microseismicity during hydraulic fracture stimulations of unconventional gas reservoirs. Unlike geophone arrays, DAS systems are sensitive to uniaxial strain along the fibre direction and thus provide a single-component recording, which makes identifying the directionality of incoming waves difficult to infer. Using synthetic examples, we show some fundamental characteristics of microseismic recordings on DAS systems for purposes of hydraulic fracture monitoring in a horizontal well in anisotropic (VTI) shales. We demonstrate that SH arrivals dominate the recorded signals since their polarization is aligned along the horizontal cable at near offset. The amplitude of the SH phase along the cable exhibits a characteristic pattern with bimodal peaks, the width of which relates to the distance of the event from the cable. Furthermore, we find that shear-wave splitting recorded on DAS systems can be used to infer the inclination of the incoming waves, overcoming a current limitation of event locations which have constrained events to lie in a horizontal plane. Low amplitude qSV arrivals suggest an event depth similar to that of the DAS cable. Conversely, steep arrivals produce higher amplitude qSV waves, with shear-wave splitting increasing with offset along the cable.

Induced seismicity due to fluid injection, including hydraulic fracturing, is an increasingly common phenomenon worldwide. Yet, the mechanisms by which hydraulic fracturing causes fault activation ...

Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreading ridges. However, oceanic plates are more likely to become hydrated as spreading rate decreases.

The Bora ‐ Tullu‐Moye volcanic field is a geothermal energy prospect in the central Main Ethiopian Rift, but little is known about the seismicity of the region. Here we document seismic activity between Feb 2016 and Oct 2017, locating more than a 1000 seismic events of local magnitude 0 to 2.7. This provides new insights into fluid movement and deformation beneath what we only now realize is a complicated volcanic system. A discrete cluster of events lies beneath Tullu‐Moye (TM), but, surprisingly, most of the seismicity lies in two clusters that are beneath neither the Bora or TM edifices. In these regions, we use earthquake cluster orientations, fault‐plane‐solutions and fast seismic shear‐wave orientations to show that seismicity is triggered by hydrothermal circulation of fluids along pre‐existing fractures. The fractures trend in multiple directions and are, in general, not parallel to rifting related structures. Instead, the fractures are parallel to structures created during previous caldera forming eruptions at both Bora and TM. Highly fractured regions such as this could be attractive targets for geothermal power generation. We estimate a minimum depth for a magmatic body beneath TM to be 6.5 km using the mapped brittle‐ductile transition. Frequency analysis of the earthquake waveforms reveal the bulk of the events to be volcano‐tectonic but some low‐frequency (LF) seismicity is present at a depth of 5 km beneath the TM edifice triggered by high pore fluid pressures.