The crust, despite being the Earth's outermost layer, remains extremely challenging to study given its heterogeneity and complexities. High-resolution integrated tomographic studies at various scales are essential to indirectly obtain robust information on the compositional and physical properties of the crust. The Greater Alpine Crust (GAC), shaped by the Hercynian, Alpine, and Apennine orogenies, provides a natural laboratory for studying geodynamic processes at plate boundaries. These orogenies have driven the continuous evolution of the European crust from the Paleozoic era to the present day.

This study aims to obtain robust seismic constraints withing the GAC to assess lateral physical and compositional variations. Our approach primarily relies on phase velocity measurements of Rayleigh and Love waves derived from Ambient Noise (AN) tomography, and compressional-to-shear wave velocity ratio (Vp/Vs) and crustal thickness measurements obtained from Receiver Functions (RF). We then use a thermodynamic model, together with independent constraints such as petrology and heat flow data to make interpretations in terms of compositional variation of the crust.

46,041 Rayleigh and 40,028 Love dispersion curves were calculated, and maps of phase velocities were obtained from 3 to 35 seconds. Shear-wave velocity (Vs) maps were derived from surface-wave phase velocity measurements, via a Neighborhood Algorithm. The Molasse, Pannonian, Po, Adriatic, and Tyrrhenian basins are characterized by low Vs (< 2.8 km/s) down to 3 km depth. The Po and Adriatic basins are recognized as low velocity zones down to 10 km depth, with velocities below 3.5 km/s. From 15 km depth the highest velocities of the GAC are in the Tyrrhenian basin (> 4.4 km/s), where the Moho is shallow, while the continental crust presents velocities around 3.5 km/s. From 30 km depth the roots of the Alps, Apennines, Dinarides and Carpathians are clearly visible as relative low velocity zones.

Earthquake data recorded from 2015 to 2023 with a minimum magnitude of 5.5 and a maximum of 8.5, with epicentral distances from 25 to 95 degrees of the center of the study area, were used to calculate P-wave RFs at more than 400 seismic stations using iterative deconvolution. H-κ analysis was performed, using a Moho calculated from AN as a prior. Vp/Vs ratio and crustal thickness were obtained beneath each station. Within our study area, the Moho is deepest under the Alps, Apennines and Dinarides (> 50 km), and shallowest under the Hercynian basement and the sedimentary basins. The lowest Vp/Vs are found in the Moldanubian and Saxo-Thuringian belts (average ~1.70), while the sedimentary basins, and the Alpine and Apennine belts present the largest and most variable Vp/Vs (average ~1.78).

Finally, a comprehensive interpretation of crustal composition and temperature was conducted, integrating constraints from petrological data, heat flux measurements, and thermodynamic modeling. This approach resulted in a new, robust physical and compositional characterization of the GAC.

How to cite: Berger Roisenberg, H., Boschi, L., and Cammarano, F.: Physical and Compositional Characterization of the Greater Alpine Crust Using Seismic Observables, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-713, https://doi.org/10.5194/egusphere-egu25-713, 2025.

We present a comprehensive model of lithospheric structure extending to 300 km beneath France, derived from a joint inversion of teleseismic, gravity, and gradiometry datasets. Our analysis incorporates 27,935 relative travel time residuals sourced from the 193 French permanent seismic stations (EPOS-France), alongside 30,351 terrestrial gravity measurements and the complete gravity gradient tensor from GOCE satellite mission. The integration of these three complementary datasets enhances our understanding of lithospheric structures. Our joint inversion method allows for inverting the velocity-density relationship with independent model parametrization.

Through the velocity model, our findings reveal significant lateral variations in P-wave velocity, including a prominent orogen-parallel high-velocity anomaly that extends from the surface to a depth of 135 km, centered beneath the Pyrenees and the Southern Alps. Additionally, we identify a high-velocity body extending from the surface down to 80 km beneath the Massif Central. Notably, our density model highlights several key features, including a narrow high-density body between 10 and 40 km depth, known as the Ivrea body in the Alps. Our results are to compare with previous regional temporary inversions, especially for the northern Pyrenees where velocity and density models are not always coherent. These results contribute to a more nuanced understanding of the lithospheric dynamics in this geologically complex region.

How to cite: Dashti, F., Tiberi, C., Gautier, S., and vergne, J.: Comprehensive Lithospheric Structure of France: Joint Inversion of Seismic, Gravity, and Gradiometry Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1251, https://doi.org/10.5194/egusphere-egu25-1251, 2025.

In 2020, a comprehensive 3D reflection seismic survey was conducted over the Asse salt structure in Lower Saxony, Germany, to support the retrieval of radioactive waste from the salt mine. While the data has already been processed using conventional seismic imaging techniques, we present the results from applying the Fresnel-Volume-Migration (FVM) approach that we extended for considering anisotropy and anelastic attenuation. These enhancements aim to provide a more detailed and accurate characterization of the Asse region’s complex geology, which is crucial for the safe planning and execution of the waste retrieval process.

A wavefront construction (WFC) technique was employed to calculate the required Green’s functions for 3D anisotropic (TTI) velocity models. The WFC method was further extended to also calculate compensation traveltime fields (t*) for spatially varying Q-models. These t*-fields were then incorporated into the migration process to account for amplitude decay, phase shifts and dispersion due to anelastic attenuation, ultimately leading to a more accurate representation of subsurface reflectivity.

The method was applied to both synthetic 2D data as well as 3D subsets of the Asse seismic data set. Migration with anelastic compensation effectively corrected amplitude losses and phase distortions in the synthetic data. Furthermore, applying the anisotropic FVM to the 3D Asse data set significantly improved image quality. Additionally, the migration was performed in Common Offset Gather (COG) domain to facilitate muting of the corresponding Common Image Gathers (CIGs) and thereby significantly enhancing the quality of the resulting images.

Our study highlights the critical importance of integrating both anisotropy and anelastic attenuation into 3D seismic imaging to obtain reliable, high-resolution subsurface images. Accurate positioning and characterization of reflectors are essential for performing further quantitative seismic processing, e.g. AVO (Amplitude Versus Offset) analysis, which, in turn, facilitates more precise geological interpretations. The seismic imaging advancements developed here also offer promising applications for other applications, e.g. for mineral exploration, geothermal reservoir characterization, as well as within the radioactive waste disposal site selection process.

How to cite: Kühne, N., Hlousek, F., Buske, S., Bräunig, L., Becker, V., and Scholze, M.: Anisotropic Anelastic Fresnel-Volume-Migration of the Asse 3D Seismic Data Set, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2738, https://doi.org/10.5194/egusphere-egu25-2738, 2025.

The high amplitude J magnetic anomaly is usually described as corresponding to the M3-M0 anomalies of the M-series and marks the boundary with the Cretaceous Normal Superchron in the Central Atlantic. A seafloor spreading nature of J is undisputed, but remains debated a potential extension into the southern North Atlantic west of Iberia, with its implications for the kinematic reconstructions of the rift-to-drift transition of Iberia-Newfoundland margins. To image the structure of the J anomaly, coincident wide angle seismic, multichannel seismic and magnetic data were collected across the Mesozoic oceanic crust at ~31°N during the ATLANTIS cruise in 2022. A seismic tomography model reveals a complex velocity structure with significant lateral crustal thickness variation that is at odds with the classical view of a uniformly thick J anomaly crust. Instead of the invoked excess magma production, the structure supports complex variation of the seafloor spreading processes.

We present new magnetic modeling for the ATLANTIS profile that constrains the geometry of the magnetic layers with the seismic velocity model, the basement and Moho topography, and the crustal thickness. We first show the canonical approach of modeling the oceanic crust as a constant thickness layer with alternating polarity blocks, which is not able to match neither the amplitude nor the wavelength of anomalies, either considering constant depth or integrating top of basement topography for the magnetic layer. This may be related to the slow spreading processes that tend to cancel short wavelength anomalies and decrease the anomaly amplitude, which strongly suggests that the crustal structure should be integrated in magnetic modeling, especially of non-fast spreading crust.

We used the seismic velocity structure to constrain the thickness of the magnetic layer, either from 6.0 or 6.5km/s isovelocity contour, or varying proportionally to total crustal thickness. Our results show that the M-series domain (up to M3n) can be modeled by defining the magnetic layer thickness as ~20% of total crustal thickness and with a simplified alternating polarity sequence. For the J-anomaly domain, however, a simple relation doesn’t apply, and adjustments in the layer thickness and magnetization are required. A progressive increase in magnetization is needed from the time of M3n onwards, reaching maximum values between M1r and M0r anomalies and decreasing towards the CNS. The magnetic layer thickness follows the same tendency. However, the crustal thickness varies in a much distinct way: maximum magnetization values are modeled at the thinner crust, and intermediate magnetization is kept for the region where the crust is thicker, already located at the CNS and offset of the highest anomaly amplitude.

These results challenge the use of classical methods to model oceanic magnetic anomalies generated at slow spreading centers, and more particularly, the classical view of the J-anomaly structure. A temporal lag is suggested between the source mantle processes that originated the alteration of magma composition to higher magnetization (mantle fertility / chemical composition) and the increase in crustal thickness (likely more related to mantle temperature).

Work supported by the Portuguese FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020). 

How to cite: Neres, M., Ranero, C., Prada, M., Grevemeyer, I., and Gómez de la Peña, L.: Magnetic modeling of the J anomaly and M-series at 31°N, NW Central Atlantic, constrained by crustal structure, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3499, https://doi.org/10.5194/egusphere-egu25-3499, 2025.

A wide range of academic and practical applications require that we interrogate the Earth’s subsurface for answers to scientific questions. A common approach is to image the subsurface properties using data recorded at or above the Earth’s surface, and to interpret those images to address questions of interest. Seismic tomograph is one such method which has been used widely to generate those images. In order to obtain robust and well-justified answers, it is important to assess uncertainties in property estimates.

To solve seismic tomographic problems efficiently, mixture density networks (MDNs) have been used to estimate Bayesian posterior probability density functions (pdfs) which describe the uncertainty of tomographic images. However, the method can only be applied in cases where the number of data is fixed, and consequently cannot be used in a large number of practical applications that have variable sizes of data. To resolve this issue, we introduce graph neural networks (GNNs) to solve seismic tomographic problems. Graphs are data structures that provide flexible representation of complex, variable systems. GNNs are neural networks that manipulate graphs. GNNs can be combined with MDNs (called graph MDNs) to provide estimates of posterior pdfs for graph data. In this study we use graph MDNs to solve seismic tomographic problems by representing seismic travel time data using a graph. We demonstrate the method using both synthetic and real data, and compare the results with those obtained using Monte Carlo sampling methods. The results show that graph MDNs can provide comparable solutions to those obtained using Monte Carlo methods for problems with variable number of data. After training, graph MDNs estimate posterior pdfs in seconds on a typical desktop computer. Hence the method can be used to provide rapid solutions for similar problems with variable sizes of data. We therefore conclude that graph MDNs can be an important tool to solve many practical tomographic problems.

How to cite: Zhang, X., Wang, Y., and Zhang, H.: Rapid Bayesian Seismic Tomography using Graph Mixture Density Networks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4114, https://doi.org/10.5194/egusphere-egu25-4114, 2025.

The eastern part of the Nankai Trough is a region of intense tectonic activity where the Philippine Sea Plate subducts beneath the Eurasian Plate. Large interplate earthquakes, such as the Tonankai and Tokai earthquakes, have repeatedly occurred in this area. A notable feature of the region is the paleo-Zenisu Ridge – a ridge subparallel to the trough axis – believed to be subducting beneath the accretionary prism. Previous studies have suggested that the extent of the paleo-Zenisu Ridge is closely linked to the distribution of rupture zones for megathrust and slow earthquakes. However, the precise extent and topography of the paleo-Zenisu Ridge remain poorly constrained.

Within the long-term collaborative project we aim to reconstruct the detailed seismic velocity structure of the eastern Nankai Trough using several dense 2-D wide-angle OBS datasets to better constrain the condition of the paleo-Zenisu Ridge and evaluate its critical role in the region’s tectonic and seismic dynamics. In this study, we apply first-arrival traveltime tomography and time-domain full-waveform inversion to recover high-resolution velocity models along two parallel OBS profiles (spaced ~20 km apart) acquired by JAMSTEC in the eastern Nankai Trough. These profiles integrate OBS data from two legacy seismic dasets (KR07-10_B and KR12-12_Z04; ~4.8 km OBS spacing) with recently acquired coincident profiles (KM23-13), where OBS instruments were deployed between the receiver positions of the legacy surveys. This integration results in datasets consisting of 80 and 97 OBS per profile, with an improved spacing of ~1.6 km.

The dense OBS spacing of the combined datasets enables stable FWI application up to 8 Hz, allowing for detailed reconstruction of the underlying structure of the accretionary wedge and subducting oceanic crust. Our results, combined with other high-resolution velocity models from the Tokai area of the Nankai Trough, reveal variations in the topographic relief of the paleo-Zenisu Ridge along the subduction front axis. These findings suggest a non-uniform impact of the subducting ridge on the overlying wedge, as well as variations in stress distribution, fluid migration, and seismic coupling along the subduction interface. By leveraging additional OBS profiles and advanced inversion techniques, this study enhances our understanding of the paleo-Zenisu Ridge and its role in shaping the tectonic framework of the eastern Nankai Trough.

How to cite: Górszczyk, A., Arai, R., Fujie, G., Shiraishi, K., Nakamura, Y., Nakanishi, A., and Qin, Y.: Structural Characteristics of Subducting Oceanic Ridges in the Eastern Nankai Trough Region Derived from FWI of OBS Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4219, https://doi.org/10.5194/egusphere-egu25-4219, 2025.

Full waveform inversion (FWI) has become the standard for high resolution subsurface imaging, in both academia and the industry. FWI is formulated as a data fitting procedure, where the fit between the observed and the synthetic seismograms is improved iteratively. The synthetic seismograms are computed through the numerical solution of a wave equation, they are compared to the observed ones, and the subsurface parameters are updated to improve this fit. This optimization problem is conventionally solved using gradient based optimization strategies, which update a given initial subsurface model iteratively. These optimization approaches often fail to converge to a meaningful solution, when the initial model does not explain the kinematics of the seismic data. That is, the time shift between the observed and synthetic seismograms in the initial model is too large. This is particularly true in active seismic experiments at the crustal scale, where the data lack low frequency content. 

Our strategy relies on modifying the FWI algorithm, in order to help mitigate the ill-posedness of the problem. To do so, we introduce additional parameters to the problem, which help make FWI well behaved. Our strategy makes the receiver position a free parameter, which is included in the optimization. This allows our algorithm to better explain the data kinematically, when the model estimate is poor. Our approach gives rise to a challenging optimization sub-problem, which we solve using stochastic  optimization strategies: namely, Markov-Chain Monte Carlo (MCMC), Simulated annealing methods, and Particle Swarm Optimization (PSO). The latter proved to be a good candidate for our problem. We have also investigated a deterministic optimization strategy, using a dynamic programming approach. This deterministic method is less expensive than the stochastic alternatives. We test our methods using various realistic synthetic cases, obtaining promising results. This has prompted us to extend the method to 3D FWI, and perform synthetic tests, in preparation for a real 3D data application. The preliminary 3D results in synthetic settings are promising. 

How to cite: Benziane, M., Brossier, R., and Métivier, L.: Designing a robust seismic full waveform inversion scheme: an extension approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5885, https://doi.org/10.5194/egusphere-egu25-5885, 2025.

In 1989, as part of a collaborative effort involving 12 research institutions and known as the BABEL Project (Baltic and Bothnian Echoes from the Lithosphere), 2,268 km of crustal-scale seismic lines were acquired in the Gulf of Bothnia and the Baltic Sea. The lines were acquired using near-vertical reflection, and wide-angle refraction methods, providing insights into the evolution of plate tectonic processes during the Paleoproterozoic era.

The offshore, near-vertical seismic data were collected using a 3 km-long cable comprising 60 groups of 64 hydrophones, positioned at a depth of 15 m. An airgun array, consisting of six identical subarrays, was used as the seismic source and towed at a depth of 7.5 m. The group spacing, shot interval, and record length varied between the lines. Specifically, in this study, the record lengths for lines 3 and 4 were 25 s, with a group spacing of 50 m and a shot interval of 75 m, while for line 2, the record length was 23 s with a group spacing of 25 m and a shot interval of 62.5 m, respectively. A sampling rate of 4 ms was used for all three lines.

Lines 2, 3, and 4 in the Bothnian Bay are located between the volcanic-hosted massive sulphide belt of Skellefte in Sweden and Vihanti-Pyhäsalmi in Finland. Given the historic value of the data and within the scope of a mineral systems workflow, we have recovered these data digitally to take advantage of modern processing and imaging solutions. Original processing showed divergent reflectivity reaching the lower crust of a Precambrian crystalline basement in the Baltic Shield. In addition, a prominent dipping reflector extending into the upper mantle was imaged, offsetting the Moho by 5-10 km. These findings led to the suggestion, for the first time, of active plate tectonic processes during the Paleoproterozoic time.

The reprocessing work reveals reflections as shallow as 1 s and shows a series of individual reflections and diffraction signals. The Moho boundary is significantly improved in terms of both its signature and trackability and, as in previous investigations, we show a set of sub-Moho reflections dipping down to 23-25 s. Not only have we brought the data to life, but we have also turned them into compelling narratives, providing an enhanced understanding of lithospheric structures in this mineral-endowed region of the world.

Acknowledgments: This work is supported by the Smart Exploration Research Center. The center has received funding from the Swedish Foundation for Strategic Research (SSF) under grant agreement no. CMM22-0003. This is publication SE25-003.

How to cite: Koufopoulou, A., Malehmir, A., and Markovic, M.: Uncovering fossil subduction in a mineral-endowed Paleoproterozoic terrain: Reprocessing legacy BABEL seismic lines from Bothnian Bay, Sweden, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7164, https://doi.org/10.5194/egusphere-egu25-7164, 2025.

Reflection seismics is indispensable for understanding the structural framework of the crust and providing important constraints on mineral system studies. The non-uniqueness inherent in interpreting data from2D crooked seismic profiles acquired over complex geological structures can be reduced by performing 3D reflector orientation analysis (Calvert, 2017), but this requires good azimuthal coverage, which can be enhanced with the deployment of cross-spreads. A new 70-km long reflection seismic profile was acquired across the Palaeoproterozoic Peräpohja belt in northern Finland to shed new light on its structural framework and contribute to development of the new national mineral exploration program. Single-receiver and single-source acquisition was implemented, resulting in excellent data quality. Survey layout was optimized to extract 3D reflector orientations, and included eight additional cross-spreads extending up to 5 km from the survey line spaced every 7-8 km.

3D reflector orientation analysis was performed for both the inline data (i.e. along the main profile) as well as with the cross-spreads included. The main challenge to processing these data is obtaining an optimal refraction statics solution: in the first pass, a combination of 2D inline statics with 2D statics for each cross-spreads was applied. In the second pass, a 3D tomostatics solution was obtained for the complete dataset. The initial results of the reflection orientation analysis suggests that while the additional effort in acquiring the cross-spreads may not be justified for obtaining the structural image (cross-spreads bring more noisy data), orientation attributes (dip and strike) are better resolved, especially at shallower levels, and where gaps in azimuthal coverage are present (i.e. the profile was too straight). With current acquisition capabilities, cross-spreads can be acquired in a cost-effective manner, yet they should be carefully planted to provide reasonable signal-to-noise ratio data, essential for 3D statics and for the orientation analysis itself.

The new seismic data were acquired as a part of the REPower-CEST “Clean Energy System Transition” project, which received funding by the European Union (number 151, P5C1I2, NextGenerationEU).

How to cite: Malinowski, M. and Calvert, A.: A field test of 3D reflection orientation analysis along a 2D crooked line in northern Finland supplemented with additional cross-spreads, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7991, https://doi.org/10.5194/egusphere-egu25-7991, 2025.

Traditional ambient noise tomography contains two steps: (1) inverting the 2D phase and/or group velocity maps at different periods based on the dispersion curves of each station pair and (2) point-wise inversion to obtain 1D shear wave velocity model at each grid node and then gather together to obtain a 3D velocity model. In the first step, most studies use the travel-time tomography method based on ray theory or 2D finite-frequency sensitivity kernel that assumes the surface wave travels along the great circle path. This could introduce travel-time biases when surface wave propagates away from the great circle in complex media and further affect the imaging results.

To consider the ray bending effect and the finite-frequency effect simultaneously and to balance the computational efficiency and accuracy, we consider modeling the propagation of surface wave by solving the 2-D membrane wave equation. Sensitivity kernels with respect to phase velocity are constructed using the adjoint method, which could capture significant deviation of the ray path from the great circle path when the velocity perturbation is larger than 20%. Checkerboard tests have been applied to demonstrate the effectiveness of the new tomography method, compared with the finite-frequency tomography method based on analytical solutions. We test our method with ambient noise data in Southern California.

How to cite: Li, Z. and Yang, Y.: Membrane Wave Equation-Based Ambient Noise Adjoint Tomography: Verification and Application, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9302, https://doi.org/10.5194/egusphere-egu25-9302, 2025.

Imaging of the Moho discontinuity in the trench-outer-rise region of the Japan Trench is a challenging task due to the structural changes that occur in the oceanic crust. This area is shaped by bending-related faults and petit-spot volcanism, which introduce fractures, hydration, and high porosity to the crust. These processes influence seismic velocities and disrupt sedimentary layers. Volcanic activity adds further complexity by creating uneven structures like cracks, dikes, and sills, which weaken seismic signals and make it harder to detect the Moho. These structural changes call for advanced seismic techniques and detailed data to accurately map the crust-mantle boundary.

In this study we examine the structure of the oceanic plate near the Japan Trench, focusing on identifying the Moho discontinuity and related crustal features. Our study relies on a 100-kilometer-long 2D seismic dataset collected by JAMSTEC in 2017. The data were gathered using 40 Ocean-Bottom Seismometers (OBS) placed 2 kilometres apart, capturing wide-angle seismic signals. Such acquisition setting provides a robust framework for analysing the subsurface with the imaging techniques employed in this study.

We employ two imaging techniques that complement each other in addressing the geological complexities of the region. First, we use Reverse Time Migration (RTM) - wavefield-based imaging approach - to produce highly detailed image of discontinuities in the crust and uppermost mantle. RTM was instrumental in identifying the high-resolution Moho and characterizing the variations in the crust-mantle interface. The method allows for handling areas with complex geological structures, such as those affected by bending-related faults and volcanic intrusions, making it an invaluable tool for this study. In addition, we address the challenges of conventional seismic imaging in regions with highly fractured crusts caused by subduction-related bending. To overcome these challenges, we employ the second technique, known as kinematic migration of slope data. The slope represent the horizontal component of the slowness vector at reciprocal receiver position (air-gun shot position) and is calculated as the difference of picked arrival times of the Moho reflection divided by the receiver distance. This approach significantly reduces uncertainties in identifying the Moho discontinuity.

The combination of RTM and kinematic migration proved highly effective in imaging the Moho discontinuity and revealed valuable details about the crust-mantle boundary. By leveraging these complementary techniques, the study successfully overcame the challenges posed by the region's geological complexity. These results demonstrate the importance of high-resolution imaging in advancing our understanding of Earth's interior. The ability to map the Moho with precision not only improves interpretations of subsurface structures but also contributes to broader tectonic and geophysical research. This study underscores the critical role of innovative methodologies in exploring complex geological environments, paving the way for future discoveries.

How to cite: Amirzadeh, Y. and Górszczyk, A.: Reverse Time Migration and Kinematic Migration Approaches for Imaging the Moho in the Outer-Rise Region of the Japan Trench, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9516, https://doi.org/10.5194/egusphere-egu25-9516, 2025.

Tibetan Plateau is resulted from the collision between India and Eurasia ca 55 Ma. The fate of the subducted Indian crust has long been debated. The Hi-CLIMB seismic experiment (Nábělek et al., 2009) presented images indicating that the southern Tibetan Plateau was under-thrusted by the Indian plate up to ~31°N, and the Indian crust was partially decoupled from the mantle below. However, the structure and the dynamics of the underlying mantle remain enigmatic. Various geodynamic models have been proposed to explain the behavior of the mantle lithosphere across this collision zone. These include hypotheses involving lithospheric delamination, rollback, tearing, etc. Further information about the lithospheric mantle beneath Tibetan Plateau is required to better understand the history and current state of this continental collision. In this study, we conduct full-waveform inversion of P-wave and its coda using 14 teleseismic events recorded by the Hi-CLIMB stations, which mainly consist of an 800-km long sub-linear array of 189 broadband seismometers spaced at 5-15 km. Our analyses yield high-resolution 3-D models for the P- and S-wave speeds along with density in the south-central Tibetan Plateau, which covers key geological features including the Genge basin, the Himalayas, and the Lhasa and Qiangtang terranes. Our model resolves P- and S-wave velocity structures from the surface down to ~400 km depth and the density structure in the uppermost 100 km. Our new 3-D multi-parameter model is integrated with results from geochemical and geothermal simulations to evaluate the existing tectonic models, which sheds new light on the state of the Indian lithosphere beneath Tibetan Plateau.

How to cite: Zhu, Q., Fuji, N., Prigent, C., Singh, S., and Zhao, L.: Full-waveform Box Tomography for the Lithospheric Structure in South-central Tibetan Plateau, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9580, https://doi.org/10.5194/egusphere-egu25-9580, 2025.

Variations of seismic wave speed, in particular shear-wave velocity (v_s), are largely temperature controlled in the Earth’s mantle. Seismic tomography models thus provide us with insights into upper mantle temperature variations, the rheological configuration of the deep lithosphere and, thus, zones susceptible to strain localization within tectonic plates. With this contribution, we introduce V2RhoT_gibbs, an open-source Python tool for converting v_s from upper mantle seismic tomography models to temperature and density in a self-consistent thermodynamic manner.

Our conversion approach utilizes an open-source Gibbs-free energy minimization algorithm (Perple_X), which computes the thermodynamically stable phase and mineral assemblages for a given mantle chemical composition (in terms of major oxides of the (Na2O-)CaO-FeO-MgO-Al2O3-SiO2 system) and a wide range of pressures and temperatures. Users of our tool can choose from different thermodynamic databases to constrain the Gibbs-free energy minimization and thus produce lookup tables for upper mantle pressure-temperature conditions and the associated variations in modal composition and simultaneously calculated bulk rock physical properties (e.g., seismic velocities and thermal conductivities). V2RhoT_gibbs is developed to analyze these lookup tables and hence consider the complex, non-linear relations between v_s, temperature and mechanical properties. The tool corrects the pre-calculated anharmonic seismic velocities for anelastic attenuation effects and partial melts, and finally allocates thermodynamically consistent values for temperature and density to the v_s‑depth-pairs in upper mantle tomographic models.

We will illustrate and discuss the differences between different chemical compositions, representing various degrees of upper mantle depletion, with respect to their effects on the v_s-converted temperature and density fields. In addition, we will show V2RhoT_gibbs conversion results to discuss derived depth variations of the thermal lithosphere-asthenosphere boundary for different tectonic settings (convergent and divergent plate boundaries). Finally, we will discuss challenges and possible solution strategies regarding the interpretation of v_s variations in the shallowest upper mantle.

How to cite: Bott, J., Kumar, A., Zarifikoliaee, S., May, T., Jiménez-Munt, I., Gomez Dacal, M. L., and Scheck-Wenderoth, M.: Upper mantle shear-wave velocity conversions to temperature and density: open-source V2RhoT_gibbs sheds light into challenges and possibilities, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10504, https://doi.org/10.5194/egusphere-egu25-10504, 2025.

The 3D western alpine lithosphere presents a complex structure and remains subject of active geophysical investigation. In this context, we propose and apply new techniques to combine seismic and gravity data. In particular, we set up a Bayesian inversion of Bouguer gravity anomaly data for the 3D distribution of density at crustal and lithospheric scales. We use the Bouguer gravity anomaly map after  Zahorec et al., (2021) across the western Alps. 

In our setup, we introduce a priori information based on existing seismic tomography models (e.g. Nouibat et al., 2022), to guide the exploration of model geometries for target density distributions. We also use flexible constraints based on known ρ-vS conversion laws (e.g. Brocher, 2005), to define a pool of candidate density models consistent with rock-physics constraints and laboratory observations.

The 3D forward gravity modeling is achieved by discretizing the target volume area into unitary voxels of constant density, accounting for surface topography as well. By pre-computing the gravity effect of each voxel, we significantly decrease the computational cost of forward modeling, thus allowing an exploration of the parameter space with a Monte Carlo sampling approach. In particular, we implement a Markov chain Monte Carlo (McMC) algorithm in a Bayesian framework.
To address the lower resolution power of gravity data, we reduce the dimensionality of the model space by describing volumetric structures with a level-set approach, based on the available seismic tomographic models. This allows to 1) incorporate a priori knowledge of the crustal structure from seismic investigations into the inversion setup and 2) define complex laterally-heterogeneous density structures with a lower number of parameters. While we allow deviations from exact ρ-vS conversion laws, the bayesian framework allows to highlight existing trade-offs among density and geometry, and to tackle the non-uniqueness that often affects gravity data inversions. Finally, this setup allows to benchmark a seismic tomographic model against gravity data while providing a new density model.

We produce a new 3D density model of the western alpine lithosphere, including the Ivrea Geophysical Body at the boundary between the European and Adriatic tectonic plates. Our setup allows us to compare the resolved density values with seismic tomography models locally and with surface geology as well, providing new constraints on subsurface rock structure and composition.

How to cite: Scarponi, M., Bodin, T., and Tauzin, B.: Bayesian tomography-driven inversion of Bouguer gravity: application to the western Alps, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11926, https://doi.org/10.5194/egusphere-egu25-11926, 2025.

The Transantarctic Mountains, a major escarpment separating East and West Antarctica, are of enigmatic origin. The component of their topography which arises due to transient mantle processes (dynamic topography) can be constrained by quantifying and removing the isostatic contribution to topography to define residual elevation. In this way, insight into shaping of the Transantarctic Mountains by mantle processes can be gained. This method is dependent on accurately constraining the thickness and density of the crust and overlying ice in order to sufficiently account for isostatic loading. The TAMNNET seismic network offers an opportunity to study the crustal architecture of the northern Transantarctic Mountains using passive seismic techniques. Autocorrelations of spectrally whitened P-wave coda signals and high frequency (2 - 4 Hz) P-S receiver functions utilise ice sheet reverberations to forward model properties and thicknesses of the ice and subglacial layer. This new method allows for the presence and extent of subglacial sediment to be assessed, characterisation of the subglacial geology and hydrology based on seismic velocities and Vp/Vs ratio, and insight into the temperature of the ice sheet, all of which have important implications for ice sheet dynamics.  Crustal architecture is modelled using lower frequency (0.5 - 2 Hz) receiver functions and empirical relationships between seismic velocity, density and pressure. Using these results to correct for isostatic topography and ice loading yields residual elevations of 1 - 2 km, consistent with the presence of Neogene volcanism in the region, mantle upwellings imaged in tomographic models, and thinned lithosphere identified through rare-earth element modelling of basalts from the Erebus and Hallett Volcanic Provinces. Collectively, these observations imply that dynamic mantle convective processes are integral to the origin and evolution of the northern Transantarctic Mountains, shedding light on the interplay between tectonic processes in the West Antarctic Rift System and the margin of the East Antarctic Craton. 

How to cite: Dunn, A., White, N., and Larter, R.: Passive seismic insights into the subglacial environment, crustal architecture, and dynamic topography of the northern Transantarctic Mountains, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12315, https://doi.org/10.5194/egusphere-egu25-12315, 2025.

    The Mariana subduction zone, with its relatively simple oceanic subduction structure and water-rich environment, serves as a unique natural laboratory for studying mantle hydration processes and the lithosphere-asthenosphere boundary (LAB). In this study, we analyze passive-source Ocean Bottom Seismometer data from stations deployed across both the forearc and incoming plate regions to investigate the S-wave velocity structure beneath the central Mariana region. By extracting multi-frequency teleseismic receiver functions and surface wave dispersion data, and applying a transdimensional Bayesian joint inversion method that explicitly accounts for water-layer effects, we achieve a high-resolution characterization of the lithospheric subsurface structure. Our findings confirm significant mantle hydration, consistent with previous studies, and reveal a distinct low-velocity zone at the LAB. Unlike conventional passive-seismic studies, which typically describe the LAB as a single sharp velocity reduction, our results indicate a rapid velocity decrease followed by an equally sharp increase, delineating a ~10 km thick melt-rich zone. Our findings highlight the importance of treating the LAB as a complex system rather than a simple boundary, as the melt-rich zone acts as a lubricant, significantly reducing viscosity and facilitating decoupling of the lithosphere from the asthenosphere, thus enabling plate motion.

How to cite: Zhang, J. and Wang, X.: Ponded Melt-Rich Zone at the Base of Lithospheric Plate in Central Mariana Revealed Using Ocean Bottom Seismometer Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14179, https://doi.org/10.5194/egusphere-egu25-14179, 2025.

Surface wave tomography based on dispersion is an important approach for resolving the velocity structure of the crust and upper mantle. Traditional surface wave tomography methods based on dispersion data typically require first construction of 2D phase/group velocity maps, followed by a point-wise inversion of dispersion data to derive 1D profiles of shear wave velocity as a function of depth at each grid point, and finally forming the 3D velocity model. However, the 2D tomography method based on ray theory has a strong dependence on the selection of the initial velocity model and regularization parameters. Furthermore, the eikonal tomography method requires dense observations. Therefore, we propose a surface wave tomography method based on a physics-informed neural network, which can construct the phase/group velocity maps of multiple frequencies simultaneously, eliminating the need for repeated separate inversion for each frequency. The network comprises two branches, one branch takes in the coordinates of the virtual source and station as well as period as input to fit the observed surface wave travel times, and the other branch takes in the station coordinates and period to predict the phase/group velocity. The two branches are constrained by the eikonal equation. After the training is completed, the velocity of each grid point in each period can be quarried using the neural network and form the group/phase velpcity maps for each period. We tested the new method using data from the Feidong dense array and the Weifang dense array, and compared the tomography results with those of the traditional method. The test results demonstrate that the new method is a meshless tomography method with data adaptive resolution. In addition, this method does not require an initial velocity model or explicit regularizations. It is highly automatic, simple, and easy to use, with potential to combined with existing dispersion curve automatic extraction methods for automatic tomography without human intervention.

How to cite: Yang, S. and Zhang, H.: Physics-Informed Neural Networks for multi-frequency surface wave tomography, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14481, https://doi.org/10.5194/egusphere-egu25-14481, 2025.

Sedimentary basins in the southern Brazilian continental margin have gained special attention in recent years due to hydrocarbon research. Detailed geodynamic models help to understand the basins' evolution, and the choice of the right constraints helps to improve them. However, the oceanic lithosphere structure in this area is poorly investigated by seismological methods, because of the lack of stations. We study the variations in the lithospheric velocity structure along the southern Brazilian passive margin using two types of databases elaborated from the records of 23 stations of the Brazilian Seismographic Network, between 2011 and 2023. The first database is composed of 4781 group velocity dispersion curves generated by 547 teleseismic events of M>5.0 that occurred in the mid-Atlantic ridge and Sandwich Islands, these curves show periods between 15 and 400s, however for higher periods than 100s there are greater uncertainties which makes the dispersion velocities too unstable to be interpreted. The second database contains 226 dispersion curves derived from 30 local events, with magnitudes between 3.0 and 4.0M, that occurred in the offshore region of the Brazilian margin, these curves show periods between 4 and 12 seconds, which sample shallow depths providing important detailed information of the stretched platform region, that can not be sampled by teleseismic data because the absence of short periods.

The regionalization was performed in both databases to identify regional patterns and obtain velocities at different points of interest, between 20 and 40 km are observed velocities with more continental signatures to the south of 20°S, which may be associated with the extension of the continental shelf, while to the north of 20°S are observed higher velocities, indicating a more oceanic lithosphere. Deeper than 40 km, the areas closer to the margin present higher velocities that decrease up to ~100km depth, as far as we have an acceptable resolution. We also observe less strong velocity anomalies with depth, reflecting a more homogeneous lithosphere. Close to 20ºS latitude is observed a negative anomaly for depths greater than 40 km, suggesting a correlation with Trindade's plume. The regionalized curves extracted in points closest to the platform are consistent with PREM continental velocities up to 30 km, while depths between 30 and 70 km present higher velocities than PREM reference model. Previous studies also found similar velocity patterns in the continental margin; however, these do not present results at shallower depths, such as close to the bottom of the crust and the top of the mantle lithosphere.

The regionalization of the local database gives us essential information up to 10km depth, where important basins and the pre-salt region are located. Despite the low quantity of curves, it is possible to observe a clear positive velocity anomaly at 5km depth, that matches with the pre-salt limits as well as with the Brazilian offshore seismicity. The final 1D inverted velocity models will be interpreted and linked with existing geological and geophysical information to improve the knowledge of the basins in the Brazilian margin.

How to cite: Rivadeneyra-Vera, C., Bianchi, M., and Sacek, V.: Lithospheric structure velocity in the Southern Brazilian margin from surface wave analysis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14738, https://doi.org/10.5194/egusphere-egu25-14738, 2025.

It is significant to constrain the anisotropic crustal and lithospheric structures of the North China Craton (NCC) to understand the mechanisms of craton destruction. The NCC consists of the cratonic Ordos block, the Shanxi Rift, and the severely destructed eastern NCC. The Datong volcano zone (DVZ), which is located in the northern Shanxi Rift, suggests active magmatism and volcanism during the Cenozoic. In contrast, the magma-poor southern Shanxi Rift poses a significant challenge in demonstrating the rifting mechanism and processes. Using the database from ChinArray, we obtain anisotropic Rayleigh-wave phase velocity maps by Eikonal tomography and further invert for the 3-D S-wave structure and its azimuthal anisotropy. Strong azimuthal anisotropy with fast polarization directions parallel to the edge of the low-velocity zone is revealed at a depth of 50~60 km to the west of the DVZ, which is the uppermost mantle near Moho. Our results suggest that the magmatic underplating transfers horizontally in the northern NCC and causes ongoing craton destruction by thermal and chemical erosion. We propose that the subduction of the Paleo-Asian Ocean during the Mesozoic, which is a pre-existing structure, may have contributed to lithospheric activation and localized lithospheric thinning in the northern NCC and results in the north-south differential lithospheric deformation. Similar to the Shanxi Rift, other rift systems, such as the Baikal Rift and the Eastern African Rift, may also be the consequence of the lateral motion of an adjacent cratonic block on its margins. The presence of magmatism within the rift zone is mainly dependent on pre-existing structures, such as subduction.

How to cite: Ji, C., Huang, Z., and Bokelmann, G.: Ongoing craton destruction driven by pre-existing structures in the central North China Craton, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15334, https://doi.org/10.5194/egusphere-egu25-15334, 2025.

The eastern margin of the Adriatic plate stands out for its tectonic complexity and geohazard potential in Europe, which are dominated by the northeast-directed subduction and collision of the Adriatic plate with the Eurasian plate beneath the Balkan. Beneath the southern Dinarides (northern Albania), the Adriatic plate is believed to be shallower than 150 km whereas the plate penetrates down to 200 km depth beneath the northern Hellenides (central and southern Albania). Further south, the Kefalonia transform fault system (KTFS, northwestern Greece) is believed to represent the transition from continental subduction of Adriatic plate to the oceanic subduction of the Ionian plate. Recent studies proposed different conception models including horizontal and vertical slab tearing beneath these transitions. Despite the importance of this region, seismic imaging is still insufficient to resolve these fundamental geodynamic processes.

To answer these important geodynamic questions, we employ the advanced wavefield-injection teleseismic full waveform inversion (TELEFWI) to image the seismic velocity structure beneath Albania. The TELEFWI explores waveform recordings from 9 high-quality teleseismic earthquakes recorded by 50 broadband stations from the ANTICS array (Albanian Tectonics of Continental Subduction, FDSN code X3, 2022-2024). TELEFWI reveals detailed structures from the crust down to 220 km in depth, with spatial resolution of 20 km for the P wave velocity from crust down to the upper mantle and 15 km for the S wave in the crust and uppermost mantle.

The new velocity model displays a strong eastward-dipping high-velocity anomaly in the upper mantle down to at least 150 km for the whole study domain, which we suggest to be the expression of the subducting Adriatic plate. The Adriatic plate displays westward-retreating pattern in the upper mantle to the coast beneath southern Albania. Meanwhile, multiple high velocity anomalies in depth probably indicates multi-phase slab break-off events at depths of 100 to 150 km beneath southern Albania. In contrast, the Adriatic plate remains relatively flat for over 100 km from the coast to the inland and then dips into the upper mantle beneath northern Albania, but the anomaly is relatively weaker compared to the south. Strong low-velocity anomalies in the upper crust are observed beneath the basins, probably representing the thick sediment layer.

In summary, this study provides a high-resolution velocity model for the first time based on teleseismic full waveform inversion and sheds light on the Adriatic plate configuration beneath its eastern margin.

How to cite: Gao, Y., Rietbrock, A., Frietsch, M., Agurto-Detzel, H., Kufner, S., Dushi, E., Rama, B., Koxhaj, D., Tilmann, F., Schurr, B., Yuan, X., Heit, B., and He, B.: Unraveling the Adriatic Plate Configuration beneath Albania with Teleseismic Full Waveform Inversion, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16082, https://doi.org/10.5194/egusphere-egu25-16082, 2025.

Glaciers, which store nearly 70% of the Earth's freshwater, are undergoing significant changes due to accelerating melting caused by climate change. A better understanding of their behavior and mechanisms is therefore crucial for the years to come. To study these processes, a dense seismic array was deployed on the Argentière Glacier in the Mont Blanc massif (French Alps). The sensor array consists in 98 3-component seismic stations which continuously recorded surface displacements over 35 days in early spring 2018. This period coincided with rising temperatures and rapid glacier evolution, so the sensors captured thousands of events, mostly icequakes.

The aim of this study is to reconstruct the glacier's structure and study icequake mechanisms using elastic Full Waveform Inversion (FWI) on the 3-component data. As the data come from a passive seismic experiment, we have no information about the sources. Before reconstructing the structure it is therefore necessary to work on source parameters estimation. These parameters include spatial localization and mechanism. 

First, we detect and localize the icequakes inside the glacier using a beamforming method called Matched Field Processing (MFP). The detected icequakes are observed to be located mostly at the positions of crevasses on the glacier's surface. Then, we decompose the icequake mechanism into a moment tensor and a time signature wavelet. To estimate these two parameters, we develop a joint inversion method based on waveform analysis using an iterative alternating minimization algorithm. The type of mechanism and the source orientation are then interpreted through the eigenvalue and eigenvector decomposition of the estimated moment tensor. The Fundamental Lune representation is employed to statistically study the distribution of more than 14,000 icequake mechanisms within the glacier, revealing a significant proportion of opening and closing mechanisms associated with crevasses. In certain areas, Double-Couple (DC) mechanisms can also be observed, potentially corresponding to crevasse fault slip events.

Using the estimated source parameters, FWI can be applied to reconstruct the glacier structure. A 3D synthetic crevasse model was created to mimic reality, incorporating the three observed crevasse clusters on the glacier, to evaluate the effectiveness of FWI in reconstructing the model in a given frequency-band. The parameters used in the model include S-wave and P-wave velocities, as well as density. The inversion results reveal several key findings: first, multi-parameter inversion with both S-wave velocity and density yields better results. Second, crevasses can be accurately reconstructed within the considered frequency band, provided the source parameters are well-estimated. Finally, the accuracy of source mechanism estimation significantly impacts the quality of crevasse reconstruction. Importantly, iterating between mechanism estimation and structure reconstruction yields improved results, providing promising insights.

How to cite: Grange, A., Brossier, R., Métivier, L., and Roux, P.: Glacier structure and icequakes characterization at Argentière using elastic full waveform inversion, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16712, https://doi.org/10.5194/egusphere-egu25-16712, 2025.

The Xinfengjiang Water Reservoir (XWR) in the northwest of Heyuan city, Guangdong province, China, has hosted a large number of earthquakes since its impoundment. It is one of the reservoirs that have experienced earthquakes of magnitudes greater than 6, making it one of the most active seismic zones in Guangdong. After the significant increase in seismic activity since the reservoir water storage, many researchers have conducted a series of geophysical studies in the area. However, because of the station coverage restrictions, the detailed seismic structures within XWR have not been resolved, and the understanding of its seismic mechanism and future earthquake disaster risk are still unclear. In this study, we construct a high-resolution shear-wave velocity model in the whole crust at depths from the surface to ∼30 km based on both permanent and temporary stations deployed surrounding the XWR using ambient noise tomography. The permanent stations belong to the Guangdong Earthquake Early Warning Network, including stations equipped with broadband velocity instruments and stations equipped with acceleration strong motion instruments. The temporary stations belong to a short-period seismic array deployed surrounding the XWR in 2023, with a continuous recording duration of 30 days. The imaging results above 5 km show that, with the Heyuan-Shaowu Fault as the boundary, the XWR shows a high-speed anomaly and the Heyuan Basin shows a low-speed anomaly. We also found significant low wave velocity anomalies below the XWR at depths of 5-15 km. Although we have only obtained preliminary velocity models at the crustal scale in the area and made some discoveries, it can still promote a deeper understanding of the crustal structural characteristics and seismic mechanisms of the XWR and its adjacent areas.

References

Ye, X. W., Deng, Z. H., Huang, Y. M., Liu, J.-P., Wang, X.-N., Liu, J., & Tan, Z.-G. (2017). The characteristics of 3D P-wave velocity structure of Middle-upper crust and reservoir water infiltration-diffusion in Xinfengjiang Reservoir of Guangdong. Chinese Journal of Geophysics, 60(9), 3432–3444.

He, L., Sun, X., Yang, H., Qin, J., Shen, Y., & Ye, X. (2018). Upper crustal structure and earthquake mechanism in the Xinfengjiang Water Reservoir, Guangdong, China. Journal of Geophysical Research: Solid Earth, 123, 3799–3813.

Dong, S., Li, L., Zhao, L., Shen, X., Wang, W., Huang, H., et al. (2022). Seismic evidence for fluid-driven pore pressure increase and its links with induced seismicity in the Xinfengjiang Reservoir, South China. Journal of Geophysical Research: Solid Earth, 127, e2021JB023548.

How to cite: Lyu, Z., Ye, X., and Wen, G.: Seismic ambient noise imaging of Xinfengjiang reservoir and adjacent areas, Guangdong, China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16767, https://doi.org/10.5194/egusphere-egu25-16767, 2025.

Distributed Acoustic Sensing (DAS) is becoming a standard solution for ocean-bottom seismological acquisition by providing a dense and long-distance measurement of ground deformation along offshore submarine fiber-optic cables. In the context of an offshore deployment in Central Chile, fiber-optic cables provide real-time seismic data dominated by scattered and converted phases. In a previous work, we have developed a methodology to determine both the velocity and thickness of the shallow sedimentary layer under the fiber using surface waves and split P-waves. Our current objective is to enhance the crustal imaging by identifying fault zones characterized by strong wavefront scattering and sharp lateral velocity contrasts, and sedimentary basins geometry at sub-kilometer scales in the same area, using scattered surface waves.

We focus on seismic events recorded along a 150-km-long fiber in Central Chile. After partitioning the wavefield to separate direct waves from surface waves, we compute local backprojections of the scattered wavefield. By analyzing multiple seismic events across different frequencies, we investigate variations in wave propagation at multiple scales. The resulting energy profiles reveal spatially resolved fault zone structures and sharp lateral contrasts that align with topographic and structural features. Additionally, using standard seismic noise processing procedures, we compute time-domain cross-correlation functions, autocorrelations, and spectral densities. These analyses provide further insights into the behavior of surface waves near reflector features. For instance, we identify lateral discontinuities associated with basin edges by measuring their frequency-dependent resonance.

Finally, to assess the seismogenic potential of the imaged structures, we will compare the geographical distribution and extent of the detected structures with the shallow seismicity automatically detected in the area using DAS data.

How to cite: Vernet, C., Rivet, D., Trabattoni, A., and Baillet, M.: Seismic Imaging from scattered waves using  Distributed Acoustic Sensing offshore Central Chile, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16907, https://doi.org/10.5194/egusphere-egu25-16907, 2025.

During the Quaternary, the Rhine Glacier formed several overdeepened valleys, including the Tannwald Basin (ICDP site 5068_1, Germany) about 45 km North of Lake Constance. These structures form sedimentary climate archives and thus help to understand climate dynamics in the Alps.

To obtain very high-resolution images of the sediment, seismic crosshole data was acquired using a high-frequency borehole source that predominantly generates SH-waves. The source was excited very meter at 78 to 143 m depth, and the wavefield was recorded at a depth of 105 to 134 m using an eight-station three-component geophone string in a second borehole 28 m away. Given the receivers are spaced 2 m apart along the receiver string, it was moved by 1 m after shooting at all positions. The orientation of both the source and the receivers was done manually, though a calibrated compass attached to the receiver string facilitated this procedure, in constrast to the source orientation.

The SH-data is characterized by a high level of complexity, despite the lithology from a core obtained from one of the boreholes suggesting a predominantly homogeneous material, i.e., fine glaciolacustrine sediments. Additionally, the high-frequency, large-amplitude, long-coda P-wave masks the SH-wave arrivals.

In preparation for full-waveform inversion (FWI), we mute the trigger peak at time zero, perform data reorientation to account for misaligned sources and receivers, apply a 3D-to-2D spreading correction, delay the wavefield by 0.1 s to ensure convergence of the source-time-function inversion, and normalize the data shot-wise. In a previous study, we derived a traveltime tomography model from an additionally acquired SV-wave dataset, which we use as a starting model.

We apply 2D elastic mono-parameter (v_SH) time-domain FWI using the finite-difference method to invert the transverse component data. To mitigate the non-linearity of the problem, we use the multi-stage approach with frequencies starting at 100 Hz. To reduce the effects of source and receiver coupling, the global correlation norm is chosen as the misfit function. The misfit is minimized iteratively by means of an optimization through the quasi-Newton l-BFGS algorithm, which reduces the memory requirements and provides faster convergence. Furthermore, to reduce short-wavelength artifacts, the gradients are smoothed with a Gaussian filter. Source-time-function inversion is performed by a stabilized Wiener deconvolution in the frequency-domain using the Newton method with Marquardt-Levenberg regularization. Additionally, we apply frequency-adaptive time-windowing to precondition the data.

Despite the limited parameter space in the isotropic SH-case, the FWI does not yield convincing results. In this study, we explore the potential factors contributing to this outcome, including the data quality and properties, as well as our FWI approach.

How to cite: Beraus, S., Köhn, D., Burschil, T., Buness, H., Bohlen, T., and Gabriel, G.: High-resolution SH-wave crosshole seismic full-waveform inversion in the glacially overdeepened Tannwald Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17666, https://doi.org/10.5194/egusphere-egu25-17666, 2025.

The Nankai Trough, where the Philippine Sea Plate converges beneath the Eurasian Plate, is situated off SW Japan and is one of the most seismically active subduction zones in the world, producing earthquakes such as the 1944 Tonankai and 1946 Nankai events. Within the Nankai Trough, the region of Shikoku is a geologically distinct segment located within the Tonankai and Nankai rupture zones. Shikoku is distinct due to the presence of a geological backstop, where the rigid forearc crust of the Eurasian Plate resists deformation, causing the compression and thickening of the accretionary prism. This backstop effect creates a structural boundary that influences sediment accretion, tectonic stress distribution, and seismic rupture behavior. The subducting Shikoku Basin crust is thinner and has a shallower dip angle in comparison to the other parts of the trench. The shallow subduction angle beneath Shikoku leads to highly heterogeneous stress and deformation patterns in the accretionary prism compared to steeper-dipping segments. Previous studies employing wide-angle seismic reflection and refraction surveys and tomographic methods have provided valuable insights into the broad-scale structure of the region. These efforts have delineated the geometry of the subducting Philippine Sea Plate and regional velocity structures within the accretionary prism and forearc. However, conventional methods often fail to resolve heterogeneities and variations in the deep subduction interface that critically influence seismic coupling. Furthermore, the role of Shikoku’s unique backstop configuration and the characteristics of the subduction interface at depths exceeding 40 km remain poorly constrained due to the limited resolution of traditional imaging techniques.

To overcome this, we take advantage of wide-angle data from ocean-bottom seismometers (OBS) spaced at 5 km along seismic profiles SK01, SK02, and SK03, acquired between 2009–2010 by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). We carry out Full Waveform Inversion (FWI) of the OBS data, a cutting-edge seismic imaging method leveraging the complete seismic waveform data to produce high-resolution velocity models of the subsurface. FWI iteratively refines the velocity model by minimizing discrepancies between observed and simulated seismic waveforms, enabling higher resolution at greater depths with unprecedented accuracy.

The obtained high-resolution velocity models provide a clear representation of previously under-resolved features in the off-Shikoku region, particularly the crustal structure and geometry of the Moho. These models overcome the limitations of traditional methods in imaging the subsurface at greater depths, addressing critical gaps in geological interpretation and advancing our understanding of tectonic processes in the region. By revealing fine-scale details of the subducting crust and Moho, this study further aids in developing effective planning for megaquakes and tsunami risk strategies and provides insights that could be applied to other regions with similar tectonic characteristics.

How to cite: Yadav, A., Górszczyk, A., and Almeida, R.: Enhanced Subsurface Imaging of Western Nankai Trough Using Full Waveform Inversion, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21263, https://doi.org/10.5194/egusphere-egu25-21263, 2025.