Geophysical Journal International

SummaryThe estimation of topographic gravity field models has attracted significant interest in recent years due to its growing relevance in Earth sciences. In this study, we present a robust methodology for the computation and comprehensive validation of global, complete spherical Bouguer and isostatic gravity anomalies that are essential for accurately interpreting subsurface mass distributions therefore geological structures. We synthesize these crucial gravitational functionals by leveraging spherical harmonic coefficients from high-resolution global gravity field models and various topographic/topographic-isostatic gravity field models. Our findings underscore the critical role of comprehensive terrain corrections in deriving physically meaningful, complete Bouguer gravity fields. The calculated global anomalies demonstrate strong coherence with established benchmark datasets, such as the World Gravity Map 2012. Residual differences are primarily attributed to variations in input Digital Terrain Models. Comparisons with regional Bouguer datasets reveal systematic biases that are largely explained by differing terrain correction methodologies. After removing this effect, there is a high level of consistency between the calculated global and published regional datasets, highlighting the utility of our global solutions, particularly in regions with sparse terrestrial data. Furthermore, the globally computed isostatic gravity anomalies exhibit significant agreement with both external global and diverse regional datasets, notably without the large systematic biases observed in Bouguer comparisons. This agreement reflects the effectiveness of the combined topographic and isostatic corrections in capturing Earth’s mass balance. This research provides valuable tools for new studies in the geoscience community by offering globally consistent and complete Bouguer and isostatic gravity field anomalies that have been rigorously validated for the ICGEM service.

SummaryDestabilization of volcanic edifices can generate debris avalanches with catastrophic impacts on their environment. We present the first high-resolution muography of Mount Unzen, Japan, conducted to characterize the structure of lava lobes formed on the volcano’s summit and flank during the 1990-1995 eruption. A multi-wire-proportional-chamber-based muon tracking system was operated for 203 days. The obtained high-resolution muographic image shows the internal density structure of Mount Unzen with a spatial resolution of 12 meters. Mean densities were respectively measured as 2,470 kg m−3 and 2,290 kg m−3 for the base rock and a fracture zone, and both were consistent with the results of prior drilling and sampling experiments. The mean density of lava lobes was measured significantly lower value of 1,570 kg m−3, indicating post-eruptive structural weakening. A comparison between the time-series of muographically measured density-lengths and daily precipitation records suggest that rainfall-induced gravitational destabilization did not occur during the observational period. This work demonstrates that long-term (multi-year) muon monitoring of the lava lobes can provide valuable complementary information for volcanic stability assessments.

SummaryThe asthenosphere is a weak layer in the upper mantle that supports the movement of the overriding tectonic plates and facilitates mantle convection. In this study, we compile a global dataset of SS precursors reflected at the base of the asthenosphere, also known as the 220-km discontinuity. The global dataset includes the oceanic SS precursors from Sun & Zhou (2025a) and new measurements with bounce points in continental regions. Similar to the oceanic dataset, the continental SS precursors are observed on about 45% of the SS waves, with bounce points distributed across all tectonic regions — from orogeny belts to stable cratons. We image the depth of the discontinuity at a global scale using finite-frequency tomography. In oceanic regions, the depth of the 220-km discontinuity agree well with the previous study, with discontinuity depth structure characterized by alternating linear bands of shallow and deep anomalies that roughly follow seafloor age contours. In continental regions, the variations are not spatially oscillatory but are instead much broader, with prominent perturbations associated with convergent plate boundaries. The base of the asthenosphere is shallow along the southern border of the Eurasian plate, from the Mediterranean region to Southeast Asia. Shallow discontinuity anomalies are also observed in the continental interiors – in Eurasia, from the northern Tian Shan through Mongolia to eastern Siberia, and in North America east of the Rocky Mountains. These anomalies form a linear structure roughly parallel to the Pacific subduction zones. The average depth of the discontinuity, as well as the velocity contrast across the interface, is globally consistent across both oceans and continents, with an average depth of approximately 251 km and a velocity increase of about 7%. Given that the continental lithosphere has been cooling for much longer than the oceanic lithosphere, the observed consistency in the average depth of the discontinuity implies that secular cooling does not significantly impact the thermal structure at the base of the asthenosphere.

SummaryA very frequent approach for studying lithospheric processes is to deploy temporary seismological networks in dedicated areas and to map the mantle structures with different approaches. One of them is the well-established relative travel time body wave tomography. Different circumstances often lead to a non-uniform deployment of stations both in space and time, and a wish to combine data which have been acquired asynchronously. This is the situation in Patagonia where two distinct seismic experiments provide complementary seismic data over the region covering the Patagonia slab window. Combining these data in one regional relative body wave tomography is however problematic as the two data sets are a priori with respect to two different reference models. In this contribution, we show that the number of finite-frequency relative travel time residuals varies very strongly from station to station for this data set, violating the assumption implicit in relative travel time tomography of a unique reference model due to an even data distribution for all events. We demonstrate the superiority of the inversion using relative sensitivity kernels compared with a traditional approach with absolute kernels and event terms. A resolution test proves how this is crucial for resolving the important issue of the eastern extent of the slab window. In addition, we discuss potential issues related to interference of the direct phases with core phases when measuring finite-frequency travel time residuals by cross-correlation of waveforms in necessarily relatively large time windows. We also briefly outline our preferred strategy for performing crustal correction, keeping in mind that finite-frequency residuals require frequency-dependent crustal corrections.

SummaryP-wave receiver functions (RFs), which utilize P-wave conversions to probe subsurface structures, face significant challenges in sedimentary environments. Specifically, strong reverberations generated by ultra-low-velocity sedimentary layers distort RF waveforms and obscure crustal signals, posing challenges for robust shallow crustal imaging. We develop a novel Bayesian joint inversion framework that simultaneously utilizes three complementary datasets—reverberant receiver functions, dereverberated receiver functions, and surface wave dispersion—to address this challenge. Our approach employs Unscented Kalman Inversion, a derivative-free method that efficiently handles nonlinear joint inversion problems. Synthetic tests demonstrate that our joint inversion recovers sediment thickness and Moho depth with uncertainties of ±0.50 km and ±1.0 km, respectively. Application to real data from the Songliao Basin verifies the approach, successfully reconstructing sediment thickness and Moho depth beneath sedimentary cover. This methodology demonstrates potential for advancing crustal investigations in complex sedimentary settings, such as continental rift basins and oceanic margins, where sedimentary sequences of variable thickness often obscure deeper structures.

SummaryThree-dimensional (3-D) forward modeling of magnetotelluric (MT) data remains a computationally challenging task, particularly when accurate broadband MT responses are simulated for real-world problems that often involve complex multi-scale bathymetry and/or topography. To overcome this challenge, we developed a new efficient numerical approach for 3-D MT forward modeling that combines high-order Nédélec-type finite elements and high-order meshes, allowing us to obtain superior accuracy and account for complex material boundaries and interfaces. Despite gains in accuracy, higher-order FE solvers are often considered impractical owing to higher memory consumption and a more ill-conditioned system. To overcome these limitations, we use an iterative solver accelerated by the Low-Order-Refined (LOR) preconditioner, which uses spectrally equivalent low-order operators, rendering the complexity independent of the polynomial degree. Another key novelty is a matrix-free implementation, where the action of the high-order operator is computed efficiently without explicit matrix assembly. The low-order system is solved using an Auxiliary Space Maxwell (AMS) solver based on a multigrid solver. We demonstrate the efficiency in a series of numerical experiments. Scalability analysis on a 3-D benchmark model demonstrates that the LOR preconditioner significantly outperforms the current state-of-the-art AMS preconditioner in terms of CPU time and memory usage, especially for higher polynomial degrees. Excellent scalability is confirmed by solving a problem with up to 1.5 × 109 degrees of freedom in less than 2 minutes using 16,384 CPU cores, which is, to our knowledge, the largest 3-D MT problem reported to date. We also illustrate that high-order hexahedral meshes allow for accurate discretization of complex geometries, such as topography, with substantially fewer elements than conventional linear meshes. Finally, the capability of the integrated approach is demonstrated on a real 3-D model crossing the ocean trench in the Aleutian subduction zone. The proposed methods pave the way for more efficient and accurate 3-D MT modeling that is crucial for the inversion of complex MT data sets.

SummaryAtmospheric models are based on various types of geophysical data, including lidar and radar. Infrasounds, acoustic waves that can propagate over large distances, have not yet been used in atmospheric models, although they provide valuable information. Besides their sensitivity to atmospheric phenomena such as gravity waves, infrasound also presents the advantage of being omnipresent. Previous studies explored the use of infrasound packet arrival properties for model estimation. However, properties such as arrival times present less information than full waveforms. We aim here to investigate, for the first time, the sensitivity of a full infrasound waveform to model parameters and to use these sensitivities in an inverse problem to recover atmospheric structure. For this purpose, infrasound propagation is modeled by Euler equations (i.e. Navier-Stokes equations in the absence of attenuation effects), and discretization is carried out here using the finite-differences method. Waveform sensitivity to atmospheric parameters is computed through the adjoint method via a novel and optimized double checkpointing-based procedure and validated by comparison with a small perturbation method. As an illustration, these sensitivity kernels are computed for the idealized case of an explosion in Finland, recorded by a CTBT station. These first results demonstrate the high sensitivity of infrasound waveforms to the atmospheric perturbations generated by gravity waves. Moreover, the sensitivity kernels of infrasound waveforms allow us to recover the variations of model parameters by solving an inverse problem. To demonstrate this capability, full waveform non-linear inversions are performed using the Limited Broyden-Fletcher-Goldfarb-Shanno method (L-BFGS): wind and sound speed profiles are inverted for a test case with idealized conditions and a synthetic dataset. These estimates of infrasound sensitivity kernels are closing a knowledge gap that allows the use of infrasound full waveforms to constrain atmospheric models.

SummaryPortland cement remains the most widely used construction material globally, valued for its well-documented properties and performance. However, its production generates substantial CO₂ emissions, mainly due to the decomposition of limestone (CaCO₃) into calcium oxide during clinker formation. In response to these environmental concerns, researchers have been actively exploring ways to lower cement’s carbon footprint and improve its sustainability. One effective strategy involves reducing the clinker content by incorporating supplementary cementitious materials (SCMs). To ensure SCMs enhance performance without compromising safety, it is essential to investigate the properties of blended cements. Natural zeolites have emerged as promising SCMs. Although they do not possess inherent cementitious properties, finely ground zeolites can react with calcium hydroxide in the presence of water, contributing to strength development. This study examines the potential of natural zeolites as SCMs and utilizes the spectral induced polarization (SIP) method to monitor cement hydration and reaction mechanisms. Portland cement mortars containing 25% zeolite were prepared and compared against two reference mixes. Zeolites were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), while SIP monitoring was conducted continuously over 28 days. Our results reveal that SIP responses are influenced by the specific chemical composition of the mortar. The incorporation of SCMs alters cement chemistry, significantly influencing SIP signals. Over time, we observed an increase in the imaginary conductivity component and a decrease in the real conductivity component. SEM analysis showed the formation of new fibrous mineral habits in zeolite-blended samples, alongside a reduction in pore fluid content. These observations suggest a strong connection between SIP signals and mineralization processes, likely associated with the formation of secondary gels and calcium monosulfoaluminate. The interaction of zeolites with calcium hydroxide promotes the development of calcium aluminate hydrates, which then react with ettringite to form calcium monosulfoaluminate. These results emphasize the importance of studying SIP behavior in cement systems containing SCMs, as assumptions based on ordinary Portland cement may lead to misinterpretations. Our research underscores the potential of SIP as a valuable tool for monitoring cement hydration while offering new insights into the chemical transformations in zeolite-containing mortars. Ultimately, this work contributes to the advancement of more sustainable cement formulations, supporting environmentally responsible construction practices.

SummaryElastic rock physics models are widely used to estimate the saturation of hydrate in isotropic sediments. However, for isotropic media, the influence of heterogeneously distributed hydrate on the P- and S-wave velocities remains unclear, leading to uncertainties in hydrate saturation estimates. To address this issue, in this work we proposed a double-solid-matrix model for predicting the velocities of sediments hosting heterogeneously distributed hydrates. A comparison of simulated velocities of our model and two rock physics schemes designed for homogeneous distributed hydrate (i.e. matrix-supporting and pore-floating models) show that, our model predicts higher S-wave velocity than matrix-supporting and pore-floating models, but yields similar P-wave velocity estimates as matrix-supporting model. We apply our model to two marine hydrate sites in the Cascadia margin: Site 1245 from Ocean Drilling Program Leg 204 and Site U1328 from International Ocean Drilling Program Expedition 311. Two locations yield similar results: velocity estimates from our model are much closer to downhole measurements than matrix-supporting and pore-floating models. Moreover, we estimate in situ hydrate saturation and clay concentration using our model, matrix-supporting model, and pore-floating model independently, and find that (i) hydrate saturations predicted by our model conform better with the saturations from chloride concentration and (ii) clay contents calculated by our model fit the best with results from smear slide analysis. This study demonstrates that our double-solid-matrix model can be an effective tool to understand the effect of heterogeneously distributed hydrates on velocities, as well as obtain accurate hydrate content in marine isotropic sediments.

SummaryThe Hainan volcanic field (HNVF) is one of China’s most active Holocene volcanic areas. Due to a lack of comprehensive geophysical research, questions persist regarding the deep magma system of the HNVF. For example, it is unclear whether the intense seismic activity in its eastern part may be a precursor to renewed volcanic activity. We present new three-dimensional electrical conductivity images, derived from magnetotelluric data, that provide a new understanding of the deep magma system in the HNVF. Our results reveal the presence of multiple sets of low-resistivity structures in both shallow and deep regions. Although once associated with past volcanic activity, a widespread shallow low-resistivity layer on the northwest side of the HNVF is not currently indicative of shallow magma chambers. Instead, a deeper large-volume low-resistivity structure in the western part of the HNVF may represent the current crustal magmatic plumbing system. Our analysis suggests that the intense seismic activity in the east of HNVF lacks corresponding low-resistivity structures, which indicates that there is no direct correlation between seismicity and movement of magma. Recent volcanic eruptions are primarily concentrated near the Changliu-Xiangou fault, which may indicate that the migration of magma has utilized crustal weak zones.

SummaryUnderstanding the effects of pore pressure changes on soil stability is important in geohazards and geotechnical studies. In situ measurements of PP are difficult at large scales. Geophysical methods can offer an indirect approach in understanding the effects of PP in soils. In this laboratory study, we investigate the complex conductivity (CC) signatures of soils undergoing increasing pore pressure inside a rigid cylinder. We experimented on different synthetic soil mixtures (with various clay percentages) as well as a natural soil sample collected from central Oklahoma, United States. We measured the CC response of the soil as we increased the pore pressure in small increments starting from atmospheric pressure up to 200 kPa. Our results show that the CC method is sensitive to changes in pore pressure values with imaginary conductivity magnitude increasing with increasing pore pressure for the samples containing clay minerals. The pure sand soil sample showed a less pronounced yet similar trend to clayey mixtures. The natural soil sample and samples with montmorillonite showed a direct relationship between imaginary conductivity and PP while real conductivity and PP showed an inverse relationship. In the samples without montmorillonite, we observed no changes in the characteristic relaxation time (τpeak) indicating no pore geometry changes in these samples. However, the samples with montmorillonite showed a direct linear relationship between PP and τpeak. Our findings indicate that under our controlled conditions, the CC measurements are sensitive to PP changes in clayey natural and synthetic soils, and although further research in the field with site-specific calibration, a wider spectrum of natural soil types and larger PP increments, is needed to validate our results; this is a starting point to evaluate the possible sensitivities of CC measurements to PP changes in earth materials.

SummaryThe Neuwied Basin within the East Eifel Volcanic Field (EEVF) is characterized by increased microseismicity, long hypothesized to be linked to the subsurface Ochtendung Fault Zone (OFZ). However, the source of this unrest remained elusive due to limited hypocenter resolution. Here, we present an extended local earthquake catalogue, compiled from a year-long Large-N deployment and a machine learning-based detection and location approach, including over 1,000 microearthquakes recorded between September 2022 and August 2023. This high-resolution dataset reveals new seismicity clusters, repeated waveforms, and distinct temporal bursts of activity, suggesting fluid-induced earthquake triggering. Probabilistic moment tensor inversion for 192 high-quality events (Mw 0.6-2.7) resolves predominantly strike-slip faulting along the OFZ, with localised clusters of normal faulting nearby, potentially associated with a previously unknown border fault of the NWB. Notably, we observe systematic rotations in P-axis orientations along the OFZ, which we interpret as localized stress perturbations induced by an overpressured reservoir beneath the Laacher See volcano - the youngest explosive eruption centre in the EEVF. These patterns, coupled with elevated magmatic CO2 emissions in the region and high waveform similarity, suggest that active magmatic and transcrustal fluid processes are influencing the stress regimes and driving the seismicity in the NWB. Our high-resolution seismicity and moment tensor catalogue offers new insights into the interplay between tectonics and fluid-driven processes beneath the youngest volcanoes in the EEVF.

SummaryThis study presents the first comparative measurements of seismic attenuation between Mauna Loa and Kīlauea volcanos on Hawai’i Island. The focus is on key physical variables found within Kīlauea, and extending our knowledge of these from Kīlauea to Mauna Loa. The measurements of attenuation, elastic/anelastic moduli (µ rigidity and K bulk), T temperature, P pressure, basalt activation energy, are uniformly applied to these adjacent volcanos (34 km separation) for comparative analyses. While numerous seismic attenuation studies have been conducted at Kīlauea, Mauna Loa has remained unexamined in this context until now. We extend previous methodologies to measure both shear (Qµ) and bulk (QK) attenuation over propagation paths from both volcanic calderas to the Aloha Cabled Observatory (ACO), located 442-464 km away at 4728 m depth. Utilizing earthquake displacement source spectra from shallow (near sea level) events beneath both calderas, we derive frequency-dependent effective Q values across the 2-35 Hz frequency band. Our analytical approach employs the t* formulation (ratio of travel time to Q) to separate attenuation along path segments, allowing direct comparison between the two volcanic systems. Results reveal that Mauna Loa exhibits substantially higher attenuation (lower Q values) than Kīlauea for both bulk and shear waves. At 10 Hz, Qµ is approximately four times higher for Kīlauea (∼400) than Mauna Loa (∼115), while QK displays even greater contrast with Kīlauea (∼425) exceeding Mauna Loa (∼25) by a factor of 17. Both volcanoes demonstrate QK < Qµ across most frequencies, emphasizing the significance of bulk losses in volcanic environments. This contradicts traditional assumptions held, that bulk attenuation is negligible in Earth. The pronounced difference in attenuation between these adjacent volcanoes, which share the same hot spot origin, cannot be explained solely by temperature-pressure dependent activation energy models. While we calculated expected Q variations using established basalt activation energies (59-68 kJ/mole), the observed differences exceed predictions by an order of magnitude. This suggests additional mechanisms are at work, likely involving partial melting processes. Our findings indicate that the internal structure of Mauna Loa may contain a greater proportion of partial melt or different melt geometry than Kīlauea, significantly affecting seismic wave propagation. At higher frequencies (17-33 Hz), both volcanoes show evidence of comparable scattering effects. This research provides new insights into the internal composition and dynamics of Hawaiian volcanoes, demonstrating that despite their proximity and shared magmatic source, Mauna Loa and Kīlauea possess distinctly different attenuation characteristics that reflect fundamental differences in their internal structure and melt distribution. These findings enhance our understanding of volcanic processes and contribute to improved interpretation of seismic data in volcanic environments.

SummaryCentral America’s tectonic complexity arises from the interaction of multiple plates and diverse plate boundaries, resulting in high seismic activity and intricate subduction processes. Three-dimensional seismic velocity models can provide critical constraints on subduction processes and associated earthquake hazard models. Although regional tomographic studies have offered insights into seismic activity and lithospheric processes in Central America, there have been few studies that image the entire region in a consistent manner, likely due to the geological complexity and numerical challenges. In this study, we develop a new high-resolution three-dimensional (3-D) compressional (P)-wave velocity model to investigate the subduction dynamics of the region. We apply the teletomoDD method, which uses both local and global body-wave arrivals to resolve velocity structures. We use the International Seismological Center’s EHB catalog to extract data from 6,026 regional earthquakes from 1965 to 2019, recorded by seismic stations both inside and outside of our study area. Our model is further constrained by incorporating about 30,000 global events recorded by the seismic stations within our study area. We perform both checkerboard and restoration tests to assess the resolution of the model and find that the main features are resolved robustly regardless of the initial models. The model shows a coherent high-velocity anomalies along the Middle America Trench at 50 km depth, suggesting cold, dense subducting slabs. It also captures notable variations in slab geometry, including a slab window in the southern Cocos region starting at ∼75 km depth. Low-velocity anomalies beneath major volcanic systems such as the Central American Volcanic Arc and the Trans-Mexican Volcanic Belt point to slab dehydration, fluid migration, and partial melting processes, whereas the discontinuous distribution of volcanism in Mexico and Central America appears to be influenced by the subduction of the Cocos Plate. Additionally, we identify high-velocity anomalies near the Siqueiros and Clipperton Transform Faults on the East Pacific Rise, possibly caused by mafic magmatic cumulates. The high-velocity anomaly near Swan Islands Transform Fault may reflect locally increased density inferred from previous gravity studies. Our new velocity model offers a consistent seismic structural foundation for further investigations into seismogenic processes, slab geodynamics, petrology, and rheology in Central America.

SummaryRecently, semi-airborne transient electromagnetic (TEM) systems have gained attention in geophysical investigations due to their ability for fast mapping and minimal ground access requirement. These systems consist of a ground-based transmitter source and an inductive receiver coil, carried by an uncrewed aerial vehicle. This study investigates how transmitter source selection in field-based semi-airborne TEM systems affects model parameter uncertainty, using synthetic subsurface models. The simulated dB/dt responses highlight distinct signal characteristics between the galvanic-based system (herein galvanic source) and the inductive-based system (herein inductive source), with differences observed across varying subsurface conditions. An analysis of four synthetic 3-layer models highlights that the inductive-based system resolves shallow conductors better at short offsets, whereas the galvanic-based system is better at resolving parameters for deeper targets at large offsets. Both systems, however, face challenges in accurately resolving resistive targets embedded between conductors, highlighting the need for strategic selection of the transmitter source. The galvanic-based system consistently achieves a higher signal-to-noise ratio (SNR), particularly at large offsets, making it better suited for deep exploration. In contrast, the inductive-based system exhibits lower SNR, higher noise susceptibility, and sign-changing dB/dt responses at increasing offsets adding complexity to data processing and interpretation. Despite these limitations, inductive-based systems enable earlier time measurements with signal magnitudes at short offsets comparable to galvanic-based, due to shorter current turn-off times. In this analysis we have used two system setups utilizing inductive and galvanic sources that reflect commonly used systems, but obviously, assumptions regarding transmitter characteristics such as type, size, waveform, and current amplitude, will influence the results when examining details more closely.

SummaryAccurate quantification of natural gas hydrate is essential for resource potential and climate impact assessment. Archie’s empirical equations are commonly used to quantify hydrates from electrical resistivity measurements. One dominant Archie equation parameters, i.e. saturation parameter (n), is generally assumed to be constant for different hydrate saturation range for a given reservoir. However, n actually varies with hydrate saturation and morphology, and the exact relationship between n and hydrate saturation or morphology still remains poorly understood, leading to great uncertainties in resistivity-derived saturations. Here we investigate the effect of hydrate saturation and dominant hydrate morphologies on n using well logs from four sites in both fine and coarse-grained sediments: two sites with fluid-displacing hydrate (site W11 from the third Guangzhou Marine Geologic Survey; and Mallik 5L-38 well in the Mackenzie Delta) and two sites with fracture-filling hydrate (site 10 from Indian National Gas Hydrate Program Expedition 01; and site W08 from the second Guangzhou Marine Geologic Survey). We calculated n value using Archie’s law with hydrate saturation determined from velocity. Our results demonstrate a clear negative relationship between hydrate content and n value. Moreover, n estimates from two fracture-filling sites show greater variability compared to fluid-displacing sites. At a fracture-filling hydrate site, site 10, various trends between n and hydrate saturation are possibly caused by the distint gas compositions of hydrate. Our results demonstrate that significant effects of hydrate morphology and saturation on n that are site specific, and can be used to enhance the accuracy of gas hydrate quantification.

SummaryFull-waveform inversion (FWI) is a method that utilizes seismic data to invert the physical parameters of subsurface media by minimizing the difference between simulated and observed waveforms. Due to its ill-posed nature, FWI is susceptible to getting trapped in local minima. Consequently, various research efforts have attempted to combine neural networks with FWI to stabilize the inversion process. This study presents a bidirectional physics-constrained full waveform inversion (BP-FWI) framework that leverages transfer learning by pre-training on simple initial models and utilizing the results. Additionally, it employs FWI gradients to co-optimize both the neural network and the adaptive residual learning module under bidirectional physics constraints. By eliminating the reliance on a large amount of manually constructed synthetic datasets, the proposed training strategy addresses the challenge of data dependency. Furthermore, through the joint optimization strategy guided by bidirectional constraints, the neural network is able to focus on integrating physically-informed prior knowledge into global stratigraphic representations, while the adaptive residual learning module specializes in learning residual mappings from the network’s output, thereby capturing subtle inter-layer velocity variations in local geological structures. Evaluating the method on two benchmark models under various conditions, including absent low-frequency data, noise interference, non-uniform receiver configurations, and differing initial models, along with corresponding ablation experiments, consistently demonstrates the superiority of the proposed approach.

SummaryA comprehensive full-waveform inversion model of the seismic velocity, covering nearly the entire tectonic domain of the western Pacific (FWP24) is developed using an optimized many-core version of SPECFEM3D_GLOBE on the New Generation Sunway supercomputer. Taking the global adjoint tomography model GLAD-M25 as the initial model, the three-component seismograms from 1 228 earthquakes recorded at 3 687 stations are employed in iterative gradient-based inversions for three period bands: 40-100 s, 17-40 s, and 10-60 s. A total of 36 iterations are carried out using the conjugate gradient method to update the velocities of horizontally and vertically polarized P-waves and S-waves (Vph, Vpv, Vsh, and Vsv) in the FWP24 model. This process systematically reduces the phase difference between the synthetic and observed seismograms within the phase measurements. Compared with existing region inversion results, the FWP24 model realizes a wider, more continuous, and higher-resolution inversion range, including all subduction zones in the western Pacific (e.g. Kurile-Japan, Izu-Bonin-Mariana, New-Britain-Solomon, New-Hebrides, and Tonga-Kermadec). Furthermore, compared to the initial model, FWP24 reveals more detailed structures particularly in oceanic regions around the Philippine Sea Plate, the Caroline Sea Plate and the Ontong-Java Plateau by applying more seismic data.

SummaryThe ratio R of shear-wave to compressional-wave velocity variations (dlnVs/dlnVp) is a useful physical parameter to study the thermochemical properties of the Earth’s interior. Several approaches have been employed to estimate R (or its inverse 1/R), but they either assume the same local resolution in models of dlnVs and dlnVp or assume the same ray paths for S- and P-phases, while excluding valuable data and overlooking uncertainties. We overcome these issues by characterizing both dlnVs and dlnVp through the Backus-Gilbert based SOLA method to obtain R including its uncertainties. This approach enables us to ensure that dlnVs and dlnVp share the same local resolution, making it possible to compute their ratio through division. In addition, SOLA provides uncertainties on dlnVs and dlnVp, which we propagate into our estimates of R using the Hinkley distribution for dlnVs/dlnVp. When resembling a Gaussian, the Hinkley distribution provides Gaussian uncertainties for R, enabling us to interpret tomographic features as for instance in terms of slab morphology or partial melt with greater confidence. To illustrate our new approach, we use a data set of P- and S-phase onset-time residuals from ISC to infer the velocity anomalies and the ratio R (or 1/R) in South-East Asia between 100 and 800 km depth. As the SOLA method is driven by data uncertainties, we reassess the provided ISC uncertainties using a statistical approach before developing models of dlnVs and dlnVp with their uncertainties. Based on our quantitative model estimates, we argue that a large velocity anomaly below the Sumatra slab, with a value of R over 2.5, is resolved given our data and their uncertainties. However, in contrast to previous work, we do not find evidence for a slab hole under Java. Our proposed approach to obtain R with uncertainties using the Hinkley distribution can be applied to a large range of tomographic imaging settings.

SummaryWe investigate the effect of statistically non-stationary turbulence in the Earth’s outer core on the effective turbulent electromotive force generated by the convectively driven flow of liquid iron and the evolution characteristics of the geomagnetic field. The non-stationarity means that interactions of distinct waves are crucial, and the effect of beat induces a slow time variation of the large-scale electromotive force. This provides an attractive and fairly simple physical mechanism for the random appearance of short-lived geomagnetic excursions and reversals separating long periods of relatively stable field, through non-synchronized evolution of the amplifying α-effect and turbulent diffusion. This implies rare and random appearance of simultaneous suppression of the α-effect and enhancement of diffusion which leads to a sudden magnetic energy drop, i.e. an excursion. The turbulent field of what is termed MAR waves (Magnetic-Archemedean-Rossby) is analysed. The dispersion relation and structure of such waves involving the joint effect of the Lorentz, buoyancy, and Coriolis forces together with curvature of the core-mantle boundary are obtained and utilized for estimation of the non-stationary electromotive force in the core. The solutions for the large-scale dipole possess an Earth-like behaviour, magnitude, and timescales, and the physical mechanism of the process, including identification of two dynamically important parameters, is discussed. Similar ideas concerning the dynamics of waves within the so-called Stratified Ocean at the top of the Core (SOC) were considered in the recent work Mizerski (2025). The SOC is an important but thin, strongly stratified layer near the core-mantle boundary, and here, the possibility of global non-equilibrium dynamo mechanisms is analysed. It is possible that the surface and bulk mechanisms coexist in the core, both adding to the complexity of the observed picture of reversal occurrences.