ABSTRACT Natural hydrogen (H2) is increasingly considered a promising low-carbon resource, particularly in tectonically active settings where serpentinization of mantle rocks can generate significant H₂. This study investigates the potential for natural hydrogen generation in the active Taiwan orogen, where high heat flow, active faulting, and hydrothermal circulation create favorable conditions. We combine high-resolution seismic tomography, thermodynamic modelling, and geochemical analyses of soils and hot springs to investigate serpentinization beneath southern Taiwan. Three-dimensional S-wave imaging reveals a dome-shaped high-velocity body beneath the eastern Central Range at depths of 6–25 km. Comparison with petrological models indicates a partially serpentinized mantle with an estimated serpentinization degree of 35–65%. At the surface, geochemical analyses reveal hydrogen concentrations in soils and hot springs, reaching up to 798 ppm and 1820 ppm, respectively. H2 anomalies are strongly controlled by structural contacts and extensional faults that likely channel deep fluid migration. Unlike rift-inversion orogens such as the Pyrenees, mantle exhumation in Taiwan appears linked to arc-collision dynamics and possible crustal delamination. These results suggest that active extension combined with shallow mantle emplacement and elevated geothermal gradients can sustain ongoing serpentinization and continuous hydrogen fluxes. Taiwan therefore represents a new type of prospective natural hydrogen system in active orogenic settings, expanding exploration targets beyond classical ophiolite context and stable cratonic environments.

The arc-continent collision and orogenic processes beneath Taiwan result from complex interactions between the Eurasian and Philippine Sea plates, yet the detailed slab morphology and associated crustal tectonic and magmatic processes remain controversial due to the resolution limits of classical travel-time tomography. Here, we present high-resolution tomographic models of density, VP, VS, and VP/VS for Taiwan, obtained by inverting complete teleseismic P and SH waveforms. Beyond the well-documented thickened crust beneath the Central Range, our 3D models reveal fine-scale structural heterogeneities across the crust and upper mantle beneath Taiwan. The VP/VS ratios further illuminate the rheological architecture of the orogen. A distinct low-velocity, high-VP/VS anomaly in the southern Central Range coincides with an aseismic zone, suggesting thermally and fluid-weakened ductile crust. In the shallow crust of northern Taiwan, pronounced low-VS and high-VP/VS anomalies delineate the geometries of magma reservoirs beneath the Tatun Volcanic Group and Turtle Island, providing the first tomographic image of a crustal magma reservoir beneath offshore Turtle Island. In the shallow crust of eastern Taiwan, we image an east-dipping high-velocity structure beneath the eastern Central Range, which is interpreted as the underthrusted forearc basement of the Luzon Arc. At upper-mantle depths north of 24°N, the models reveal a distinct slab gap at depths of 130–200 km, underlain by a detached high-velocity Eurasian slab, establishing a structural window for toroidal mantle flow around the slab edge. These new tomographic models provide improved constraints on the three-dimensional crustal and upper-mantle architecture and slab dynamics of the Taiwan orogen, highlighting the potential of teleseismic full-waveform inversion for imaging complex convergent margins in tectonically challenging, limited-aperture settings.

An unrest period is recorded at Mount Pelée volcano since 2019, characterized by increasing shallow seismicity and a surface deformation rate of a few millimeters per year. Whether this deformation is caused by underlying magmatic activity remains unclear. Here, we use 3D mechanical models and elasto-plastic rheology to explore the volcano's mechanical stability under the gravity field. Once a reference state of stress is established, assuming elastic equilibrium, we explore two extreme cases of effective strength of the volcanic edifice: i) A strong case (φ=20°, c, T=10 MPa), which produces surface displacements consequent to internal compaction of the edifice and compatible with geodetic observations, associated with plastic failure limited to 100 m below the surface. ii) A weak case (φ=10°, c=8 MPa and T=5 MPa) capable of generating plastic failure over the first 2 km below the surface, similar to seismic observations, but overestimating surface displacements by one order of magnitude. Finally, we include an inflating shallow compliant source (ΔP=10 MPa). The radial surface displacements induced by this hypothesized source have relatively low magnitudes, being masked by the gravitational response of the volcanic edifice. All models reveal the development of a shear zone on the western flank of the edifice, spatially similar to landslide scars and local flank destabilizations. Our results demonstrate that variations in effective crustal strength combined with gravitational loading, with and without an inflating source, can reproduce the observed deformation and seismicity patterns within their limited precision range.

Forearc basins develop at convergent plate boundaries through interactions between subducting oceanic slabs and overriding plates. Their basement composition and sedimentary fill vary significantly across different convergent margins, offering critical insights into the evolution of subduction zone. The Jaz Murian basin in Iran, located above the western Makran subduction zone, is critical to unravel the subduction history of the central Tethys. In this study, we investigate the deep structure of the Jaz Murian basin by integrating teleseismic data from a recent broadband deployment with high-resolution aeromagnetic data, employing innovative analytical methods. Our findings reveal a combination of high seismic velocities, elevated Poisson’s ratios, and high magnetization intensity, collectively indicating the presence of ophiolites beneath the sedimentary cover. By correlating basin provenance analysis with geochronological data from the Makran ophiolite, we propose a model of oceanic lithosphere characterized by discontinuous or locally absent crust transitioning into the basement of Jaz Murian basin. This ophiolitic basement reflects highly focused magmatism and a reduced magma budget typical of modern ultraslow-spreading ridges. Our findings suggest that the subduction and subsequent incorporation of (ultra)slow-spreading mid-ocean ridges into the continental margin played a pivotal role in the formation of the ophiolites and the forearc basin basement systems of the central Neo-Tethys along the southern Eurasian margin.

We present new three-dimensional P- and S-wave velocity models of the northern Iberian Peninsula and Pyrenees using arrival times of local earthquakes and seismic ambient noise.The arrival time dataset has been built in two steps. First, we have merged the existing seismic bulletins of permanent seismic networks in the region (IGN, ICGC, OMP). Second, we have compiled continuous waveforms of all temporary experiments in the region from 2010 to present and have been automatically picked using the deep-learning picker PhaseNet, substantially increasing data coverage in regions with sparse permanent instrumentation. Using this augmented arrival time dataset, we have inverted it simultaneously for P and S wave velocity structure and earthquake relocation. Since the region is too large for the flat-earth approximation used in the tomography code we have obtained multiple overlapping smaller models and calculated the final model by averaging the individual models.To further enhance structural resolution, particularly in areas with limited earthquake ray coverage, we incorporated results from ambient noise tomography based on inter-station surface-wave dispersion measurements. These data provide complementary constraints on the shallow crust and improve lateral continuity of the velocity model, especially in aseismic regions and across major sedimentary basins.The obtained P and S wave velocity models provide detailed images of the subsurface structure of the region, including parts that were poorly imaged before. The addition of arrival times picked at temporary stations has been of fundamental importance to illuminate the central part of the region, because of its low seismic activity and lack of permanent stations. Particularly well imaged are the sedimentary basins, including the southern Aquitaine basin, and the northern Ebro and Duero basins and their connection along the Rioja Trough.

Abstract Mantle rocks undergoing serpentinization can generate significant amounts of natural hydrogen, yet the rates and controlling processes remain poorly understood. Here, we constrain the possible hydrogen generation rates in two distinct mantle rock types, the fertile lherzolites of the Western Pyrenees and the depleted harzburgites of Northern California, to relatively low rates of ~0.1 to ~0.5 tonnes H₂ yr⁻¹ km⁻³ of reactive rock. When integrated over the full reactive volumes, this corresponds to total production rates of ~300 to ~600 tonnes H₂ yr⁻¹. By combining three-dimensional geophysical inversion with numerical modelling of fluid-rock processes, we show that hydrogen generation rates are mainly limited by H₂ saturation in the fluid and reaction kinetics. Under these constraints, hydrogen generation in mantle-derived serpentinization systems proceeds slowly, making rapid large-scale replenishment unlikely and suggesting that large, economically relevant accumulations, would require timescales of thousands to tens of thousands of years to develop.

The high topography of the central Andes (15-28 degrees S) is supported by thick crustal roots. The recent uplift of the Altiplano is often explained by crustal shortening and subsequent delamination of the lower crust, though its deep structure and plateau formation processes remain debated. The mechanism of flat subduction is also controversial in geodynamics. The well-documented transition from normal to flat subduction beneath southern Peru provides an ideal setting to investigate these processes. In this study, we present a new multi-parameter tomographic model of density, , , and beneath southern Peru. The model was obtained by inverting teleseismic P and SH waveforms recorded by temporary and permanent stations across 78 degrees W-66 degrees W and 10 degrees S-19 degrees S. Time and amplitude issues in the PeruSE data set were identified and corrected before inversion. The final model shows a normally dipping oceanic lithosphere in the south, with a continuous transition to flat subduction beneath central Peru, without a slab tear. The flat subducting lithosphere appears anomalously thin and hot, likely reflecting thermal erosion associated with the formation of the Nazca Ridge. This suggests that the buoyancy of the flat slab is not due to crustal thickening and delayed eclogitization reactions, as previously hypothesized, but rather to a thermally eroded and thinned oceanic lithosphere. We also detect a thin forearc crust (14 degrees S-19 degrees S) overlying a thick, cold mantle wedge. The Altiplano thickening may result from squeezing the crust between this strong forearc block and the Amazonian craton. Finally, the mantle lithosphere beneath the Altiplano is abnormally thin or absent, suggesting partial removal or delamination, which could explain the regional uplift over the past 10 Ma.

Eastern China has undergone significant continental reworking since the Mesozoic, characterized by pronounced lithospheric thinning and widespread magmatism, largely attributed to mantle flow driven by the Paleo-Pacific plate subduction. However, the detailed mechanisms remain elusive. Here, we provide new insights into these processes through a high-resolution lithospheric shear-wave velocity model for central and eastern China, constructed using Love waves, which are highly sensitive to shear-wave velocity variations. To overcome mode interference challenges, we apply a high-resolution linear Radon transform method to identify and exclude records with clear higher-mode interference. Combining earthquake and ambient noise data from the dense China National Seismic Network, we construct Love wave phase-velocity maps (15-150 s periods) using the newly developed eikonal method and invert them to build a 3-D model. The resulting structural features correlate well with surface geology and key tectonic provinces. Notably, the Yangtze craton and the Cathaysia block show a pronounced seismic velocity contrast along the northern margin of the Jiangnan orogenic belt, marking a suture zone between these two Precambrian blocks formed during their Neoproterozoic collision. Low-velocity anomalies in the uppermost mantle beneath both the North and South China cratons align with the distribution of Precambrian orogenic and rifting systems as well as Mesozoic magmatism. These findings indicate that mantle upwelling preferentially exploits pre-existing zones of weakness, highlighting the critical role of tectonic inheritance in continental reworking in eastern China.

Over the last decade, the expansion and densification of broadband seismic networks has led to an increased interest in tomographic approaches based on gradiometry, which relies on the spatial derivatives of the phase and amplitude of surface wavefronts. Nevertheless, the use of amplitude information remains challenging. In this study, we present a novel approach to Helmholtz tomography based on the use of generalized wave equation smoothing splines. The application of these generalized smoothing splines results in the generation of smooth amplitude and phase fields that satisfy the Helmholtz and transport equations, and hence the wave equation. This enables the direct derivation of phase velocity maps using the Helmholtz equation. We apply this new Helmholtz tomography approach to Rayleigh waves recorded by permanent and temporary networks in Western Europe from 2008 to 2022 to obtain phase velocity maps for periods between 25 and 120 s. These maps reveal the detailed structure of the crust and upper mantle in this region. Compared to eikonal tomography, the phase velocity anomalies obtained by Helmholtz tomography are stronger in amplitude and more sharply defined.

The orogenic evolution of the western Pyrenees and its relationship with ancient rift architecture are well-studied and therefore represent a natural laboratory for investigating the link between structural inheritance and the distribution of present-day seismicity. In this study, we use an automated method to detect and pick P and S waves from local earthquakes to characterize the distribution of seismicity in the region. The P and S picks are used to obtain a catalog of earthquakes and to perform a local earthquake tomography. We analyze 2-D sections and depth slices of the tomographic model and of the seismicity catalog, and confront them to the 3-D geological architecture of the inverted Mauleon rift system. Our results reveal that present-day deformation is mainly extensional, with a main cluster of aligned, steeply north-dipping seismicity localized along the former rift necking domain. In addition, the rift-inherited NNE-SSW Saison and Barlane`s transfer zones either offset or disrupt the seismicity cluster and delimit three seismically active segments. The Cha & icirc;nons Bearnais segment shows localized seismicity, whereas the eastern and western Mauleon segments show a westward extension of earthquake distribution in relation with the widening of both the orogenic wedge and the mantle body preserved at shallow depth. Our results suggest that the rift-inherited necking zone played a predominant role in the localization of the deformation during both orogenic and post-orogenic evolution, and as such represents a critical zone for seismic hazard assessment. Earthquakes diffusely distributed in and around the high-velocity mantle body could be partly related to ongoing serpentinization.

The recent installation of new broadband seismic stations in the French Massif Central (FMC) has resulted in the detection of a few “deep” earthquakes located near the crust‐mantle boundary beneath volcanic regions. Analysis of the spectral content of the respective waveforms has shown that the spectra of these “deep” earthquakes are significantly depleted in high frequencies. Based on these observations of anomalous depth and spectral content, these earthquakes can be classified as Deep Long Period (DLP) events. This is a specific class of volcanic seismicity observed beneath many active volcanoes around the World. While the exact physical origin of this type of earthquakes is still debated, they are often considered as indicators of the presence of magma near the crust‐mantle boundary. Therefore, observation of DLP earthquakes can bring new insights into understanding the state and the activity of the recent FMC volcanoes.

In the framework of the MACIV project, a consortium of French laboratories has deployed a temporary seismic network of 100 broadband stations in the French Massif Central (FMC) for 3-4 years (2023-2027). The project aims at imaging the crust and upper mantle of the FMC to better assess the sources of volcanism, and the impacts of the Variscan inheritance or the Cenozoic rift system on volcanic systems. A large-scale array of 35 broadband stations covers the entire FMC and complements the permanent networks to reach a homogeneous coverage with 35 km spacing. This network, with XP code, is the French contribution to AdriaArray. The XP array is complemented with 3 quasi-linear north-south, east-west and northwest-southeast profiles with inter-station spacing of 5-20 km, making up the XF network of 65 stations. The profiles cross volcanic areas and the main Variscan structures. We describe the experimental setup designed to optimize the performance/cost ratio and minimize the number of field visits, the deployment, the state-of-health monitoring, the data management and the data quality control strategies, outcomes of our 15-years' experience with major temporary seismic experiments in France and neighboring countries, including AlpArray. We also show some preliminary results including hypocenter locations and receiver function analysis. The 2 broadband arrays will be supplemented in 2025 by a month-long deployment of 3 large-N dense arrays of 625 3-C short-period nodes. These dense arrays will complete our multi-scale seismic experiment and illuminate active faults and possible plumbing systems of the youngest volcanoes.

The tectonic of northern Taiwan is in a post-collisional stage and has undergone a subduction polarity flip between the Eurasian Plate (EP) and Philippine Sea Plate (PSP). The shallow crust of northern Taiwan features the Tatun Volcano Group (TVG) and the Turtle Island magma reservoirs, with their proximity to Taipei metropols highlighting the volcanic risks to densely populated regions and critical infrastructure. However, it is challenging to image all these structures from the surface down to several hundred kilometers depth with classical passive tomographic approaches. Here, we present tomographic models of density, P-wave velocity (Vp), S-wave velocity (Vs), and the Vp/Vs ratio beneath northern Taiwan, obtained by inverting complete teleseismic waveforms from 18 P and 9 SH events recorded by 175 broadband stations from the Formosa Array and the permanent stations. In our final model, the plate boundary between EP and PSP is clearly depicted as a west-dipping plane, consistent with the western boundary of slab seismicities. Our model identifies two distinct low-velocity, high VP/VS bodies beneath the TVG and Turtle Island, indicative of underlying magma reservoirs. The reservoir beneath the TVG is beaker-shaped, extending from a depth of 6 to 20 kilometers. The reservoir beneath Turtle Island, located on the island’s eastern side, is larger than TVG's but less well defined due to sparse station coverage. The crust north of the Hsueshan Range is thinner, likely related to the post-collisional delamination of the lower crust. This process leads to increased mantle heat flow, providing the heat source for the TVG. With the new 3-D model, we also relocate the local events by utilizing a nonlinear location method, in order to improve their spatial accuracy and get better constraints on the seismogenic structure.

The southern Peru subduction zone is a complex region, marking the transition between the flat slab associated with the Nazca Ridge subduction in the North and a much steeper subduction in the south. The area has been affected by several large earthquakes over the past 20 years, like the Mw 7.2 earthquake that occurred on June 28th 2024 close to the city of Acari, in an area that already ruptured in 2013 and 2018. Here we use data from 26 seismic stations active from March 2022 to December 2024 as part of the DEEPTrigger project, along with 16 permanent Peruvian stations and 15 permanent Chilean stations, to create a 3-year seismicity catalogue of South Peru. Using PhaseNet for the detection and picking of phases and PyOcto for their association, we obtain a total of 154645 events. These earthquakes are located with NonLinLoc using a new 3D P and S-wave velocity model of the region obtained from full-waveform inversion. They are then relocated using double difference methods with cross-correlation times to obtain precise locations. This allows us to image seismic structures along the subduction zones, thus demonstrating the influence of interseismic coupling and of bathymetric features like the Nazca Ridge on seismicity patterns. We focus in particular on the Acari sequence, which occurred at the edge of the Nazca Ridge. The Mw 7.2 mainshock was preceded by a Mw 6 foreshock on June 16th 2024, with both earthquakes seemingly occurring at the plate interface. We show that both the foreshock and the mainshock activated intraslab seismicity along the whole edge of the ridge down to 100 km depth, thus providing a good example of far-field interactions between deep and shallow regions of the subduction.

Understanding the driving forces and nature of intraplate seismicity remains a major unsolved problem in seismology. In the western Pyrenees, seismicity is concentrated in a narrow region that follows the boundary between the Axial Zone and the North Pyrenean Zone. Despite the presence of a permanent network in the region, the geometry of active faults, and their relationship with crustal structures, remain elusive, owing to significant earthquake location uncertainties. Here, we exploit data recorded by a large-N nodal array deployed in the Chainons Bearnais region during four weeks of 2022 in order to image crustal structures and characterize active faults. We automatically detected and picked P and S waves with PhaseNet, resulting in a catalog of over 500 events, half of which are located beneath the temporary deployment. Tomographic images obtained from the inversion of P and S arrival times provide detailed insight into the geometry of folds and thrusts in the sedimentary cover, as well as the presence of a main fault in the basement which dips northward with an angle of 65 degrees (Chainons Bearnais normal fault). Seismicity relocation within the 3D model obtained by tomography shows that earthquakes are concentrated along this main active fault, extending from the top of the basement to a depth of approximately 16 km. These results demonstrate that passive imaging approaches can offer cost-effective alternatives to traditional controlled source imaging for seismotectonic studies and natural resource exploration in regions with active seismicity.

By demonstrating that extensional inheritance plays a decisive role in the formation of orogens, recent studies have questioned the ability of a unique, complete Wilson cycle model to explain the diversity of collisional orogens. For 5 years, the OROGEN Research Project had therefore the ambition to challenge this classical Wilson cycle model. By focusing on the diffuse Africa-Europe plate boundary in the Biscay-Pyrenean-Western Mediterranean system, the project questioned the preconceived "Orogen singularity" assumption and investigated the role of divergent and convergent maturities in orogenic and post-orogenic processes. This work led us to rethink the development of collisional orogens in a genetic (or process-driven) way and to propose an updated version of the " classical Wilson cycle", the Wilson Cycle 2.0, and the ORO-Genic ID concept presented in this paper. The particularity of the Wilson Cycle 2.0 is to take into account the divergence and convergence maturity reached during extensional and orogenic processes in proposing different tectonic tracks associated with different ORO-Genic ID numbers. The ORO-Genic ID is composed of a letter (or track), corresponding to the maturity of divergence reached and a number corresponding to the maturity of convergence reached during the formation of the orogen. This new concept relies on the observed pre- and syn- convergent tectono- stratigraphic and magmatic record and deformation history and can be identified in using diagnostic criteria presented in this paper. It represents therefore a powerful tool that can be used to characterize the evolution and the architectural type of an orogenic system. Moreover, as a mappable concept, it can be easily used worldwide and can help us to explain differences in the style of deformation at crustal scale between orogens. En d & eacute;montrant que l'h & eacute;ritage extensional joue un r & ocirc;le d & eacute;cisif dans la formation des orog & egrave;nes, des & eacute;tudes r & eacute;centes ont remis en question le cycle de Wilson et sa capacit & eacute;, en tant que mod & egrave;le unique, & agrave; expliquer la diversit & eacute; des orog & egrave;nes collisionnels. Pendant 5 ans, le projet de recherche OROGEN s'est donc donn & eacute; pour ambition de questionner ce mod & egrave;le classique du cycle de Wilson. En se concentrant sur la fronti & egrave;re diffuse entre les plaques Afrique-Europe dans le syst & egrave;me Golfe de Gascogne-Pyr & eacute;n & eacute;es-M & eacute;diterran & eacute;e occidentale, le projet a remis en cause l'hypoth & egrave;se pr & eacute;con & ccedil;ue de la << singularit & eacute; orog & eacute;nique >> et a explor & eacute; le r & ocirc;le de la maturit & eacute; divergente et de la maturit & eacute; convergente dans les processus orog & eacute;niques et post-orog & eacute;niques. Ce travail nous a amen & eacute;s & agrave; repenser le d & eacute;veloppement des orog & egrave;nes collisionnels d'un point de vue g & eacute;n & eacute;tique (ou ax & eacute; sur les processus) et nous a amen & eacute; & agrave; proposer une version actualis & eacute;e du << cycle de Wilson classique >>, appel & eacute;e Cycle de Wilson 2.0 et le concept d'ID ORO-g & eacute;nique pr & eacute;sent & eacute; dans cet article. La particularit & eacute; du Cycle de Wilson 2.0 est de prendre en compte la maturit & eacute; de la divergence et la maturit & eacute; de la convergence atteinte au cours des processus d'extension et d'orog & eacute;n & egrave;se, en proposant diff & eacute;rents parcours tectoniques associ & eacute;s & agrave; diff & eacute;rents num & eacute;ros d'identit & eacute; ORO-g & eacute;nique. Le num & eacute;ro d'identit & eacute; ORO-g & eacute;nique est compos & eacute; d'une lettre (ou d'un parcours), correspondant & agrave; la maturit & eacute; de la divergence atteinte, et d'un num & eacute;ro correspondant & agrave; la maturit & eacute; de la convergence atteinte lors de la formation de l'orog & egrave;ne. Ce nouveau concept repose sur l'enregistrement tectono-stratigraphique et magmatique avant et pendant la phase de convergence, ainsi que sur l'histoire de la d & eacute;formation observ & eacute;e, et peut & ecirc;tre identifi & eacute; en utilisant les crit & egrave;res diagnostiques pr & eacute;sent & eacute;s dans cet article. Il constitue donc un outil puissant pouvant & ecirc;tre utilis & eacute; pour caract & eacute;riser l'& eacute;volution et le type architectural d'un syst & egrave;me orog & eacute;nique. De plus, en tant que concept cartographiable, il peut & ecirc;tre facilement utilis & eacute; dans le monde entier et nous aider & agrave; expliquer les diff & eacute;rences de style de d & eacute;formation & agrave; l'& eacute;chelle crustale entre les orog & egrave;nes.

The dehydration of subducting slabs expels a massive amount of water into the forearc and backarc mantle which is responsible for the serpentinization of the mantle wedge, as well as the production of melt and arc magmatism. These processes are expected to have characteristic signatures in density and seismic velocities models, which remain largely elusive to date due to the limited spatial resolution of classical passive tomographic approaches. Here we present a tomographic model of density, V P , V S , and V P / V S beneath central Oregon, obtained by inverting complete teleseismic P and SH waveforms recorded by the CASC93 temporary experiment. The final model shows an east‐dipping low‐velocity layer less than 10 km thick that can be associated with the fluid‐saturated Juan de Fuca oceanic crust. The distribution of tremors at the surface closely coincides with the horizontal extent of this low‐velocity layer. Below 40 km depth, seismic velocities and density increase progressively to the values of a typical mantle. This transitional domain corresponds to the eclogitization of the oceanic crust. Silica‐saturated fluids released by pore collapse migrate upward, producing serpentinization reactions in the forearc mantle that lower the density and seismic velocities. The very low V P / V S ratio documented in the Cascadia forearc crust is evidence that these silica‐saturated fluids reach the crust, where they produce extensive quartz mineralization. At greater depth, a low seismic velocity and high V P / V S ratio anomaly provides evidence for partial melting at around 75 km depth beneath the volcanic arc.

SUMMARY The inversion of complete seismic waveforms offers new perspectives to better constrain the elastic properties of Earth’s interior. However, models of density and seismic velocities obtained from full waveform inversions are generally characterized by very different and uneven spatial resolutions. Because the 3-D structure of the Earth represents small deviations from average reference Earth models, the absolute values of density, VP and VS in the Earth are strongly correlated. Here, we exploit this strong correlation between model parameters as a priori information introduced into a new full waveform inversion algorithm, by considering a non-diagonal 3-D model covariance matrix in which the spatial correlations of elastic properties are described with an exponential covariance function. The inverse of such a model covariance matrix is easy to compute, and we thus have all the ingredients to construct a consistent Bayesian full waveform inversion scheme. We show that taking into account the correlations between density and seismic velocities can lead to dramatic improvements on the reconstructed models of density, seismic velocities and VP/VS ratio. This new imaging approach opens new perspectives for refining tomographic images of density and seismic velocities in the lithosphere and upper mantle on a regional scale by full waveform inversion of teleseismic body waves.

ABSTRACT During the large-N MAUPASACQ passive seismic experiment in the foreland of the western Pyrenees (southwest France), an unusual swarm-type seismic sequence was serendipitously recorded in a normally quiet area. Thanks to the density of the deployment and the proximity of all events, it was possible to relocate the hypocenters with a very good relative accuracy through template matching, cross-correlation phase picks, and double-difference algorithm. The four-month seismic activity consists of more than 600 events with local magnitudes ranging between −1.4 and 2.1, clustered in an extremely small volume, and rooted at 4 km depth. The sequence can be divided in two phases of similar durations and event occurrence rates, but of different magnitude–frequency distributions. The presence of an asperity is suggested by the relative abundance of stronger events during the second phase. Fine mapping suggests a small but clear geographic offset of a few tens of meters between the events of the two phases and a very slow migration suggesting a process involving fluids. Changes in the correlation matrices and waveforms of late arrivals at a specific station are also observed, arguing either (and again) for migration of the hypocenters or for changes in the propagation medium between the two phases. The geographical coincidence with the repeated observation of hydrogen leaks on the surface, almost above the swarm, suggests a connection through channels that could carry fluids.

In the South‐Central Andes, the crustal structures driving the tectonic evolution of the Andean Cordillera remain unresolved. So far, most seismological studies focused on the subduction interface, leaving crustal seismicity and its relationship with crustal deformation and Andean volcanism mostly unconstrained. However, because of their large number compared to higher magnitude events, the characterization of small‐magnitude crustal earthquakes is key to identify active structures and better constrain the tectonic models. In this work, we exploit 53 months of continuously recorded, three‐component waveforms from the permanent seismic network in central Chile using a deep‐learning approach to improve the detection of small‐magnitude earthquakes. To increase station coverage, we also use the seismic phases obtained from a previous temporary seismic deployment. We use the obtained seismicity catalog to refine tomographic models of that region, revealing a more detailed architecture of the Chilean forearc. Travel times calculated in the new 3‐D velocity model allowed us to locate ∼14,000 earthquakes. Refined double‐difference relocations of ∼4,900 events located beneath the West Andean Thrust suggest a large‐scale, west‐dipping structure which, together with the west‐verging tectonic front, likely contributed to the uplift and crustal deformation during the past ∼20 Myr.