One of the aims of the InSight mission was to seismically constrain the size and character of Mars’ core, however, seismology on Mars is challenging, involving the interpretation of low-amplitude signals with a relatively high noise level from small magnitude marsquakes on a single seismometer. Two recent papers propose a smaller core than previous studies that is overlain by a molten layer at the base of Mars’ mantle. Here we suggest that the seismic observations presented in support of this conclusion can be alternatively explained by scattering in the mantle, and that a basal layer is therefore not required by the data. Furthermore, we discuss several problems for the evolution and dynamics of Mars arising from the presence of the basal layer. We suggest that other possibilities should be considered, and that significant further investigation is required to determine the structure of Mars’ lowermost mantle and core.

Iceland has long been a focus of geophysical studies due to its potential link to a deep mantle plume. However, the source region, morphology, and compositional characteristics-particularly the role of water-remain debated. We analyze a new SS precursor data set containing over 3,000 high-quality waveforms and apply a multi-dimensional reconstruction method to improve the robustness of travel-time (depth) measurements used to image the mantle transition zone (MTZ). Our imaging results reveal a thin MTZ (231 5 km) centered beneath Iceland, extending southward along the Mid-Atlantic Ridge to the Reykjanes Ridge. This thinning is mainly caused by a similar to 11 km uplift of the 660 km discontinuity (642 3 km), whereas the 410 km discontinuity (412 5 km) remains near the global mean. Temperature estimates from discontinuity topography indicate a 194 56 K anomaly at 660 km, while the lack of a thermal anomaly at 410 km contradicts a conventionally warm mantle plume environment, implying that additional factors flatten the discontinuity. We suggest that a moderate water content (similar to 0.2-0.5 wt%) can compensate for the expected thermal depression at 410 km, causing reduced S410S amplitudes (similar to 0.02). In contrast, the large S660S amplitudes (similar to 0.06) are consistent with a model of relatively dry thermal plume beneath Iceland originating from the lower mantle. This deep plume may interact with a hydrated MTZ, entraining volatiles during ascent. Our results underscore the critical roles of temperature and hydration in controlling plume expression and interpreting MTZ structure beneath oceanic hotspots.

Bridgmanite is the most abundant intrinsically-anisotropic constituent of the lower mantle. Its deformation, thus, potentially translates to large-scale anisotropy that would accumulate in high-stress regions, particularly the interaction between subducted materials and the surrounding mantle. While most of the lower mantle is generally considered well-mixed, recent observations suggest structures at mid-mantle depths (800 – 1500 km). Their origin, however, often remains enigmatic. Recent state-of-the-art deformation experiments in bridgmanite at lower-mantle temperatures and pressures reveal a depth-dependent behavior of anisotropy. Coupled with realistic geodynamic models of mantle convection, the large-scale imprint of the depth-dependent fabric reveals a purely deformation-driven seismic discontinuity between 1000 – 1400 km depth that matches observations. The discontinuity appears sharpest in actively deforming regions, and becomes negligible closer to neutral ones. In this work, we investigate its seismic detectability around a subduction zone using three-dimensional global waveform modeling via AxiSEM3D. Given its likely presence across a broad range of frequencies, we assess the suitability of SS precursors for detecting this feature. Enhanced using array seismological methods, results show the presence of SS precursors in the transverse component generated by the anisotropic discontinuity. In addition, a flattened slab geometry produces a shallower SS precursor due to the interface formed between an overlying isotropic slab and an underlying strongly anisotropic (VSH>VSV) layer. We examine the azimuthal dependence of precursor amplitudes and their frequency dependence, with particular emphasis on the microseismic frequency band (~0.05 – 0.25 Hz). In light of these recent findings, we discuss its implications on the nature and origin of mid-mantle discontinuities.

Bridgmanite, the main mineralogical phase in the Earth’s lower mantle, is intrinsically anisotropic and may therefore generate seismic anisotropy in actively-deforming regions, particularly subduction zones. Bulk of the lower mantle is widely regarded as well-mixed but recent seismic observations suggest structures at mid-mantle depths whose origin remains unclear. Here, we propose a new mechanism that could generate seismic discontinuities beneath subduction zones with depth distributions consistent with some of the observed mid-mantle reflectors. By coupling thermo-mechanical subduction models with pressure- dependent fabric calculations, we show that gradual slip transitions in bridgmanite with depth can manifest anisotropic discontinuities near 1000 km, which can deepen depending on the extent of accumulated deformation. This discontinuity is most pronounced in regions of strong slab-driven flow as revealed by its reflection coefficients, the strength of which varies with distance and azimuth. While our models predict ∼1.5% contrast in fast versus slow shear wave velocities in terms of radial anisotropy, our full waveform simulations show that the discontinuity is detectable with array seismological methods. These results suggest that mid-mantle reflectors near subduction zones may be strain-induced, providing an additional mechanism that links mantle flow, mineral physics, and seismic observations.

The D" region, located just above the core-mantle boundary (CMB), is a geologically interesting region that has been imaged using both tomographic and reflection techniques. However, reflection studies often rely on array analysis techniques, and the lack of suitable seismic arrays in the oceans has left large areas of D" unmapped. One notable area, that is currently sparsely sampled, is beneath the Indian Ocean, where ancient subducted lithosphere has been imaged near the CMB in global tomography studies. We take advantage of the long-running history of five GEOSCOPE stations located in the western Indian Ocean and Antarctica, to investigate the possibility of using source arrays to detect P-wave reflections from the discontinuity above the D" layer. Despite restricting the selected earthquakes around Indonesia to a 120 km depth range and implementing several source normalization techniques, source-array stacks (i.e., source vespagrams) were difficult to interpret. We infer that this complication arises from differing earthquake depths, violating the plane wave assumption made when constructing these stacks. Therefore, we extend our method to a source-array scatter imaging method, which we call source migration, that does not rely on travel-times calculated for a plane wave. Using this technique in conjunction with source normalization, we found clear evidence for a D" P-wave reflector at four of the six GEOSCOPE stations considered in the study. The depth of the reflector for our imaged region varies between 190 km above the CMB beneath the Great Australian Bight and 220 to 270 km beneath the Indian Ocean west of Australia. Our determined depth in the northern portion of our study area is consistent with previous studies of D" depths using S-waves. We suggest that our D" reflections are the result of the previously imaged subducted lithosphere in the region and find that this lithosphere likely thins to the southeast. Additionally, our work more broadly indicates that the long-running history of single global seismic stations combined with source array techniques may be utilized to compliment and extend previous work imaging D" using conventional receiver-array techniques.

Mantle convection is driven by buoyancy forces in Earth’s interior. The resulting radial stresses generate vertical deflections of the surface, leaving traces in the geological record. Utilizing new data assimilation techniques, geodynamic inverse models of mantle flow can provide theoretical estimates of these surface processes, which can be tested against geological observations. These inverse models are emerging as powerful tools, providing the potential for tighter constraints on the relevant physical parameters governing mantle flow.The geodynamic inversions mentioned above require an estimate of the present-day thermal state of the mantle, which can be derived from seismic observations. Using thermodynamically self-consistent models of mantle mineralogy, it is possible to convert the seismic structure imaged by global tomographic models to temperature. However, both seismic and mineralogical models are significantly affected by inherent limitations and different sources of uncertainty. In addition, owing to the complexity of the mineralogical models, the relation between temperature and seismic velocities is highly non-linear and not strictly bijective: In the presence of phase transitions, different temperatures can result in the same seismic velocity, making the conversion from seismic heterogeneity to thermal structure non-unique.We investigate the theoretical ability to estimate the present-day thermal state of the mantle based on tomographic models in the case of isochemical convection. The temperature distribution from a 3-D mantle circulation model with earth-like convective vigour serves as the “true” temperature field. Using a closed-loop experiment, we aim to recover this initial model after: 1) mineralogical mapping from the “true” temperatures to seismic velocities, 2) application of a tomographic filter to mimic the effect of limited tomographic resolution, and 3) mapping of the “imaged” seismic velocities back to temperatures. We test and quantify the interplay of smoothed seismic structure due to tomographic filtering with different approximations for the conversion from seismic to thermal structure. Additionally, owing to imperfect knowledge of the parameters governing mineral anelasticity, we test the effects of changes to the anelastic correction applied in the mineralogical mapping. The observed mismatch between the recovered and initial temperature field is dominated by the effect of tomographic filtering, with a depth-dependent average error of up to 200 K. Additionally, we observe systematic large errors in the vicinity of phase transitions. Our results highlight that, given the current limitations of tomographic models and the incomplete knowledge of mantle mineralogy, magnitudes and spatial scales of a temperature field obtained from global seismic models will deviate significantly from the true state, even under the assumption of purely thermally driven mantle flow. Strategies to estimate the present-day thermodynamic state of the mantle must be carefully selected to minimize additional uncertainties.

Many geophysical studies require knowledge on the present-day temperature distribution in Earth's mantle. One example is geodynamic inverse models, which utilize data assimilation techniques to reconstruct mantle flow back in time. The thermal state of the mantle can be estimated from seismic velocity perturbations imaged by tomography with the help of thermodynamic models of mantle mineralogy. Unique interpretations of the tomographically imaged seismic heterogeneity can either be obtained by incorporating additional data sets or requires assumptions on the chemical composition of the mantle. However, even in the case of (assumed) known chemical composition, both the seismic and the mineralogical information are significantly affected by inherent limitations and different sources of uncertainty.Here, we investigate the theoretical ability to estimate the thermal state of the mantle from tomographic models in a synthetic closed-loop experiment. The 'true' temperature distribution of the mantle is taken from a 3-D mantle circulation model with Earth-like convective vigour. We aim to recover this reference model after: (1) mineralogical mapping from the 'true' temperatures to seismic velocities, (2) application of a tomographic filter to mimic the effect of limited seismic resolution, and (3) mapping of the 'imaged' seismic velocities back to temperatures. We test and quantify the interplay of tomographically damped and blurred seismic heterogeneity in combination with different approximations for the mineralogical 'inverse' conversion from seismic velocities to temperature. Owing to imperfect knowledge of the parameters governing mineral anelasticity, we additionally investigate the effects of over- or underestimating the corresponding correction to the underlying mineralogical model. Our results highlight that, given the current limitations of seismic tomography and the incomplete knowledge of mantle mineralogy, magnitudes and spatial scales of a temperature field obtained from global seismic models deviate significantly from the true state, even in the idealized case of known bulk chemical composition. The average deviations from the reference model are on the order of 50-100 K in the upper mantle and depending on the resolving capabilities of the respective tomography-can increase with depth throughout the lower mantle to values of up to 200 K close to the core-mantle boundary. Furthermore, large systematic errors exist in the vicinity of phase transitions due to the associated mineralogical complexities. When used to constrain buoyancy forces in time-dependent geodynamic simulations, errors in the temperature field might grow nonlinearly due to the chaotic nature of mantle flow. This could be particularly problematic in combination with advanced implementations of compressibility, in which densities are extracted from thermodynamic mineralogical models with temperature-dependent phase assemblages. Erroneous temperatures in this case might activate 'wrong' phase transitions and potentially flip the sign of the associated Clapeyron slopes, thereby considerably altering the model evolution. Additional testing is required to evaluate the behaviour of different compressibility formulations in geodynamic inverse problems. Overall, the strategy to estimate the present-day thermodynamic state of the mantle must be selected carefully to minimize the influence of the collective set of uncertainties.

We investigate the properties of mantle discontinuities beneath the Arctic ocean and the Aleutian-Alaska subduction zone with underside reflections of PP and SS waves. The depth distributions of the 410-km and 520-km discontinuities suggest a relatively normal mantle transition zone beneath the Arctic ocean and a cold mantle transition zone with the subducted Pacific plate beneath Aleutian-Alaska subduction. The depth of the 660 km discontinuity shows normal behavior beneath the Arctic Ocean. However, the detection of deep reflectors with opposite polarities in depth range of 720~770 km beneath the eastern Aleutians introduces additional complexity for explaining the slab morphology. We test several plausible compositions using mineralogical modeling along a subduction geotherm. The deep reflectors are interpreted as mid-ocean ridge basalt (MORB) crust associated with the Pacific slab that may deform or buckle at the bottom of the mantle transition zone beneath the eastern Aleutians. Meanwhile, an uplifted 660-km discontinuity observed in the adjacent Alaska region suggests a different subduction depth, where the slab may penetrate the 410-km discontinuity but does not reach the 660-km discontinuity, consistent with previous regional studies. Our observations thus depict a complex slab geometry along the Aleutian-Alaska trench, that is, the slab may reach the top of the lower mantle beneath eastern Aleutian but remains at the base of the transition zone underneath central Alaska.

Directional variation in PKP differential times provides compelling evidence that Earth's inner core is anisotropic. These phases, however, are also sensitive to Earth's heterogeneous mantle. Disentangling the causal structures of PKP differential time anomalies is difficult but nonetheless important if we are to fully understand the structure and evolution of Earth's inner core. A large proportion of earthquakes used to study the inner core originate from subduction zones, which are associated with strongly positive upper mantle seismic velocity anomalies, but the effect of these on the measurements has not yet been investigated.In this study, we use AxiSEM3D to simulate the effect of source-side subduction on PKP differential time measurements. We find that some combinations of slab parameters result in artefacts up to several seconds in magnitude, while for others the effect is negligible. We subsequently examine existing data sets of measurements to assess if source-side subduction has a detectable influence on the data, and also to assess if the Scotia slab is the cause of observed anomalous measurements originating from the South Sandwich Islands. We find that the signal of source-side subduction in the data is possibly present but weak, and that the magnitude of anisotropy required by the data is not affected by source-side subduction. Furthermore, the Scotia slab is unlikely to be the cause of the anomalous measurements from the South Sandwich Islands. Nevertheless, we advise caution for future studies as the artefacts caused by source-side subduction may, in some cases, be significant.

Detection of D" anisotropy is usually carried-out with shear wave splitting analysis. To constrain azimuthal anisotropy and infer mineralogy and deformation style, a number of crossing paths is necessary. Here we use an approach that utilises the polarity of P- and S- wave reflections from the D" discontinuity, compared with the main phases P and S, and combines these measurements with ScS splitting results. Using deformation scenarios for a number of lower(most) mantle candidate materials, we calculate the reflection coefficient for P and S-wave reflections and ScS splitting predictions. From our modelling, a clear distinction between different anisotropic media is possible by using both types of observations together. Furthermore, the approach allows to use only a single direction to distinguish between different scenarios. We apply the method to the Central/South Atlantic and South Africa, across the border of the large-low seismic velocity province (LLSVP). Shear wave splitting observations suggest that anisotropy is present in this region of the mantle, in agreement with previous studies that partially sampled this region. Modelling the observations with lattice preferred orientation and shape preferred orientation of materials expected in the D" region, we find two domains of mineralogy and deformation: sub-horizontally aligned post-perovskite outside the LLSVP, beneath the South and Central Atlantic, which is replaced by up-tilted aligned bridgmanite within the LLSVP beneath South Africa.

We used seismic P-wave reflections to search for the discontinuity at the top of the D" region beneath the Indian Ocean. Due to a lack of seismic receiver arrays to target this region, we build source arrays using earthquakes in Indonesia and taking advantage of the long-running history of GEOSCOPE stations located in the western Indian Ocean and Antarctica, as well as three additional stations (Seychelles and Antarctica). Despite restricting the earthquake depth range, source-array stacks were difficult to interpret due to complications arising from differing earthquake depths, violating the plane wave assumption. Therefore, we use a source-array scatter imaging method, that does not rely on travel-times calculated for a plane wave. Using this technique in conjunction with source normalization, we found evidence for a D" P-wave reflector for several stations with reflector depths varying between 230-160 km above the CMB South of Australia and 190 to 270 km above the CMB beneath the Indian Ocean, where the depth of the reflector in the north of our study area is consistent with previously imaged D" depths using S-waves and agrees with receiver array data. We suggest that earlier imaged subducted lithosphere in this region is responsible for our D" reflections.

Seismic anisotropy mainly originates from the crystallographic preferred orientation (CPO) of minerals deformed in the convective mantle flow. While fabric transitions have been previously observed in experiments, their influence on large‐scale anisotropy is not well‐documented. Here, we implement 2D geodynamic models of intra‐oceanic subduction coupled with mantle fabric modeling to investigate the combined effect of pressure ‐and water‐dependent microscopic flow properties of upper mantle and upper transition zone (UTZ) minerals, respectively, on large‐scale anisotropy. We then compare our anisotropy models with anisotropic tomography observations across the Honshu subduction zone. Our results for the upper mantle correlate well with observations, implying that the ‐dependence of olivine fabrics is sufficient to explain the variability of anisotropy. Meanwhile, a dry UTZ tends to be near‐isotropic whereas a relatively wet UTZ could produce up to azimuthal and radial anisotropy. Because water facilitates CPO development, it is therefore likely a requirement to explain the presence of anisotropy in the transition zone close to subducting slabs.

There is growing evidence, both from a modelling perspective and seismic observations, that seismic anisotropy in the lower mantle is localized around penetrating slabs where large straining is anticipated. It is believed that the high stresses experienced near the slab activate dislocation creep mechanisms that drive the crystallographic preferred orientation (CPO) of bridgmanite aggregates. Still, deformation mechanisms in bridgmanite remain enigmatic. In recent years, deformation experiments in bridgmanite subjected to mantle temperatures and pressures suggest that its microstructures evolve with pressure, providing another perspective on the debated structure and deformation in the lower mantle. Using this information, we develop a numerical technique that calculates pressure-dependent large-scale seismic anisotropy in a pyrolitic mantle with variable velocity gradients. As a first test, we use the method to predict seismic anisotropy by calculating anisotropic reflection coefficients of underside reflections off a depth corresponding to 50 GPa where pressure-induced slip transitions in bridgmanite are expected. For this, we consider two simple deformation styles: (1) uni-axial compression, akin to vertically penetrating slabs, and (2) simple shear associated with corner-type flows. Finally, we demonstrate a multiscale approach that calculates large-scale seismic anisotropy from a fully time-dependent thermo-chemical model of free subduction with latent heating and phase transitions. The result is a long-wavelength equivalent azimuthal and radial anisotropy maps that are actually comparable to a seismic tomography model. We demonstrate how such an approach can create discontinuities in anisotropy at ~1000 km and provide insights as to how it relates to the heterogeneous distribution of the 1000-km discontinuity.

Building on the well-established connection between the Hilbert transform and derivative operators, and motivated by recent developments in complex-step differentiation, we introduce the Complex-Step Integral Transform (CSIT): a generalized integral transform that combines analytic continuation, derivative approximation, and multi-scale smoothing within a unified framework. A spectral analysis shows that the CSIT preserves phase while suppressing high-wavenumber noise, offering advantages over conventional Fourier derivatives. We discuss the roles of the real and imaginary step parameters, compare FFT-based and interpolation-based implementations, and demonstrate the method on the advection equation and instantaneous-frequency computation. Results show that the CSIT yields smoother, more robust attributes than Hilbert-based methods and provides built-in stabilization for PDE solvers. The CSIT thus represents a flexible alternative for numerical differentiation, spectral analysis, and seismic signal processing. The method opens several avenues for future work, including non-periodic implementations, adaptive parameter selection, and integration with local interpolation frameworks such as high-order Finite-Element methods.

Passive seismic networks play critical roles in both seismicity monitoring and subsurface imaging. The seismic network in the Netherlands has continuously been expanded, both to monitor induced seismicity related to gas exploitation in the north and to monitor natural (tectonic related) earthquakes in the south of the country. Aside monitoring seismicity, the data delivered by the seismological networks is required for imaging the subsurface to improve our knowledge of the composition and structure. This serves as a crucial layer of information for decision-makers in regional planning, locating resources, and risk assessment regarding current and future projects utilising the Dutch subsurface. Over the years, there has been an increase in the number of geophones and accelerometers for monitoring mostly the high frequency seismic signals (induced seismicity) and imaging the near surface structure. However, the network of broadband seismometers for measuring tectonic events (usually lower frequency signals) and imaging deep subsurface is still limited. We describe an overview of previous temporary and permanent broadband seismometer deployments in the Netherlands, with a focus on the latest NARS-DICTUM array of 25 broadband seismometers, underscoring its design, instruments, installation, and some preliminary subsurface information delivered by the network.

Global seismology mainly uses seismic waves propagating in the sagittal plane along the great circle path (GCP). However, heterogeneities in the mantle laterally deviate the path of seismic signals, which arrive out-of-plane (OOP) at arrays of sensors at teleseismic distances. Detection and back-projection of these signals have, in the past, provided independent evidence for the location of distant subducted slabs in the deep mantle, complementing global tomographic imaging. To infer physical properties of these subducted slabs, 3-D waveform modelling of OOP waves for a finite-thickness slab is needed but still missing. In this study, we conduct a series of synthetic tests using a spectral element solver. We test the detectability of OOP signals and, by progressively adding complexities, we evaluate to which extent these signals can be used to infer physical properties of the modeled slab. We carry out three-component array analysis and investigate focal mechanism dependency. Our results show that the transverse component might be the best candidate for such studies, also for P-to-P OOP signals. Vertical and radial component recordings are usually dominated by P-SV energy arriving from the earthquakes along the GCP, which masks possible OOP signals. Contrary, the transverse component filters out any P-SV energy arriving directly from the source and, owing to its intrinsic directionality, allows for higher resolution measurement of P-to-P OOP signals. This is especially the case prior to the arrival of the S-wavefield. We pick a series of OOP arrivals which are back-projected using a multiphase trial-and-error approach, that is considered successful only when different OOP seismic phases converge to the modelled (true) structure. We retrieve the location of the slab, its bottom and top edges and its thickness in the lower mantle. These inferences are tested against varying topography, orientation and size of the modelled slab. The insights gained with modelling are confirmed with real data examples, supporting higher resolution mapping of 3-D mantle structure based on OOP seismology.

The receiver function technique is widely used to image crustal structure using P-to-S converted phases at the Moho discontinuity. However, the presence of sedimentary layer generates additional P-to-S conversions and reverberations, which can overprint the Moho phases and pose problems in imaging crustal structure with standard receiver function techniques. We introduce a robust two-step method that uses H-kappa stacking to determine average thickness and Vp/Vs of the sedimentary layer, followed by waveform-fitting of the observed receiver function to constrain the average crustal thickness and sub-sediment Vp/Vs. We tested the method using both synthetic data and real-data from stations located on sedimentary layers in the Netherlands and USA. We show that the new method outperforms other common approaches in retrieving accurate Moho depth and sub-sediment Vp/Vs estimates, even in cases where the Moho phases are completely overprinted by large-amplitude phases related to sedimentary layers. We propose a two-step method for modeling crustal structure using receiver functions from stations overlying sedimentary layers. In the first step, the sediment layer thickness and velocity ratio (Vp/Vs) are derived from depth versus Vp/Vs stacking of receiver functions. In the second step, the crust-mantle interface (Moho) depth and sub-sediment Vp/Vs are derived from the waveform-fitting grid search of the observed receiver functions with synthetic ones. We tested this method using synthetic receiver functions and real-data from stations in the Netherlands and USA. The new method works well in retrieving Moho depth and sub-sediment Vp/Vs, even in cases where the Moho signals are completely obscured by signals from the sediment layer. New approach for modeling receiver functions for stations overlying a sedimentary layer The approach is effective even when Moho phases are completely overprinted by positive or negative amplitudes from the sedimentary layer Synthetic tests and real-data applications show the advantages of the developed approach over common methods for addressing sediment impact

The dynamic processes associated with subducting tectonic plates and rising plumes of hot material are typically treated separately in dynamical models and seismological studies. However, various types of observations and related models indicate these processes overlap spatially. Here we use precursors to PP and SS reflecting off mantle transition zone discontinuities to map deflections of these discontinuities near three subduction zones surrounding the Caribbean Plate: 1) Lesser Antilles, 2) Middle America and 3) northern South American subduction zones. In all three regions slow seismic anomalies are present behind the sinking slab within the transition zone in tomographic images. Using array methods, we identify precursors and verify their in -plane propagation for MW >= 5.8 events occurring between the years 2000 and 2020 by generating a large number of source receiver combinations with reflection points in the area, including crossing ray paths. The measured time lag between PP/SS arrivals and their corresponding precursors on robust stacks are used to measure the depth of the mantle transition zone discontinuities. In all three areas we find evidence for upward deflection of the 660 discontinuity behind the sinking slab, consistent with the presence of hot plume material (average temperature anomalies of 180 to 620 K), while there is not a corresponding downward deflection of the 410 km discontinuity. One interpretation of these disparate observations is suggested based on comparison to existing models of mantle convection and subduction: plume material rising across 660 km discontinuity could be entrained by lateral flow in the transition zone induced by the nearby sinking slab, and thus delaying the rise of hot material across the 410 km discontinuity.

Sedimentary basins in collisional settings result from interactions within and between lithospheric plates and sublithospheric mantle. Imaging their structure brings fundamental constraints to both the extraction of hy-drocarbon or geothermal resources and seismic hazard analyses, especially in seismogenic areas affected by fluid percolation. Seismic attenuation is highly sensitive to stress, fluid saturation, and fluid-rock interaction and can often constrain small changes in the Earth's matrix better than seismic velocity. Here, we separate different attenuation mechanisms (scattering and absorption) at multiple frequencies and map them in space to constrain the properties of the Carpathian Orogen and the surrounding basins. The separation is achieved by determining S-wave peak delay times and late-time coda quality factors based on first-order Tikhonov inversion and analytical sensitivity kernels. We analysed 366 small-to-moderate crustal local earthquakes (0.7 < ML < 5.8) recorded by permanent and temporary stations operated by the Romanian Seismic Network between 2008 and 2021. Scattering and absorption appear to be frequency-dependent and highly heterogeneous throughout the region. High scattering and absorption characterise the Vrancea Seismic Zone, located in the Eastern Carpathian bend region, at all frequencies, likely due to high-stress rate and fluid inclusions. The seismically-active bend of the collisional orogen also shows high absorption and high scattering, particularly at low frequencies (similar to 3 Hz). Low scattering and high absorption features are observed across the Danubian section of the South Carpathians, marking the contact with the Pannonian Basin, which sits on top of a thin and highly-extended lithosphere. A transition from high to low-scattering regimes with increasing frequencies could mark small-scale heterogeneous structures in the Transylvanian Basin, an elevated sedimentary unit surrounded by high topography, comprising Cretaceous Neogene sediments deposited on top of oceanic ophiolites.