Lithospheric Model Research Articles

Seismic lithospheric model across Ukrainian Shield from the Carpathians to the Dnieper-Donets Basin and its tectonic interpretation

The SW-NE directed wide-angle reflection-refraction (WARR) SHIELD’21 profile crosses entire width of Ukraine. It targeted the structure of the crust and uppermost mantle in the southwestern part of the East European Craton (EEC), between Carpathians and Voronezh Massif, across Ukrainian Shield and Dnieper-Donets Basin. The ~660 km-long profile is an extension of the RomUkrSeis profile in Romania and Ukraine. SHIELD’21 experiment, using TEXAN and DATA-CUBE short-period seismic stations, provided high-quality seismic sections from 10 shot points. This allowed construction of a ray-tracing P-wave velocity model, supplemented by Vp/Vs ratio, for the upper part of the lithosphere in the Sarmatia segment of the EEC and constraining geodynamic processes that led to the formation of the Ukrainian Shield and its margins since Archean times.The velocity distribution in the crystalline crust indicates a rather uniform structure, with velocity changing from 6.0 km/s near the surface to 6.8 km/s at the Moho. The entire section shows the lack of a high-velocity lower crust (Vp > 6.8 km/s), presumably resulting from delamination of the primitive mafic lower crust during early evolution of a juvenile continental crust. The seismic boundaries in the upper crust reflect a Paleoproterozoic extensional detachment system below the SW flank of Dnieper-Donets rift basin, initiated in the Devonian. At larger depths, a wide dome of the lower crust with velocities of 6.5–6.8 km/s in the SW-central segment of the profile, probably represents an enormous Palaeoproterozoic(?) granitoid batholith. In the southwesternmost part of the profile, the crystalline crust shows exclusively low velocities. The prominent Moho is undulating and varies in depth between 32 and 50 km. It is underlain by high-velocity bodies (Vp: 8.36–8.40 km/s), against the background of 8.16–8.25 km/s in the mantle. The velocity model corresponds with the anomalies of potential fields and zones of high electric conductivity.

Строение центральной части палеопротерозойского Лосевского террейна по данным плотностного моделирования

The results of detailed three-dimensional density modeling of the upper crust of the central part of the Losevsky terrane located in the northern part of the Voronezh crystal massif are considered. The distribution of absolute crustal density values of to a depth of 16 km was obtained as a result of the inversion of local anomalies of the gravitational field in the Bouguer reduction. The solution of the inverse problem of gravimetry was implemented within the framework of the initial model, which was formed using digital geological and geophysical data from the study area: the regional density model of the East European Platform lithosphere and its corresponding regional gravity field; the field of local gravity anomalies, representing the difference between the observed and regional fields of the lithosphere model; generalized information on the density of sedimentary cover rocks and the crystal basement; the thickness scheme of the «gravity-active» layer, obtained on the basis of statistical analysis of the anomalous field; geological and topographic information. Quantitative interpretation of the gravimetry data allowed us to obtain fundamentally new information about the geological structure of the upper crust of the study area. The reliability of the obtained results is confirmed by the consistency of a priori data and the correspondence of the total model (local plus regional) and observed gravitational fields.

Dominance of transient rheology in far-field postseismic deformation following the 2012 Mw 7.8 Haida Gwaii and 2013 Mw 7.5 Craig earthquakes

The Mw 7.8 2012 Haida Gwaii earthquake and the Mw 7.5 2013 Craig earthquake occurred along the Queen Charlotte transpressive margin, two months and 340 km apart. On and near Vancouver Island, hundreds of kilometres southeast of the two events, many continuous Global Navigation Satellite Systems sites underwent no discernable coseismic motion but up to 1 cm southeast postseismic motion (away from epicenters) which lasted for only 2 – 3 years. To resolve the postseismic signal of the two earthquakes, we used a minimum of four years of measurements before the Haida Gwaii earthquake to remove the longer-term motion of these sites that reflect mainly the locking of the Cascadia megathrust. To study the geodynamic process that governed the observed far-field postseismic motion, in particular its short duration, we employ a model of an elastic lithosphere with thickness 20 km and rigidity (μE) 32 GPa overlying Burgers-viscoelastic mantle layers in a self-gravitating spherical Earth which captures the regionally integrated effect of Earth rheology despite the complex and controversial tectonics of the study area. We find that the short-lived far-field postseismic motion was controlled entirely by the transient rheology. The observations can be explained by a steady-state (Maxwell) viscosity ηM of the upper mantle no lower than 1×1019 Pa s with a transient (Kelvin) viscosity ηK of about 6×1017 Pa s (Kelvin relaxation time τK ∼1.2 years) extending to large depths such as > 400 km. The far-field postseismic deformation is the result of the elastic plate serving as a stress guide but resisted by the asthenospheric mantle undergoing transient creep. The lithosphere-asthenosphere coupling is quantified by a “plate Kelvin response time” τKE=ηK/μE. Rheological parameters can be scaled to other values, but as long as τK and τKE stay unchanged, the far-field deformation results remain the same. Our observations presented a unique opportunity to quantify the effect of the transient rheology separately from that of the steady-state rheology.

Lithospheric Structure and Melting Processes in Southeast Australia: New Constraints From Joint Probabilistic Inversions of 3D Magnetotelluric and Seismic Data

AbstractThe thermochemical structure of the lithosphere controls melting mechanisms in the mantle, as well as the location of volcanism and ore deposits. Obtaining reliable images of the lithosphere structure, and its complex interactions with the asthenosphere, requires the joint inversion of multiple data sets and their associated uncertainties. In particular, the combination of seismic velocity and electrical conductivity, along with proxies for bulk composition and elusive minor phases, represents a crucial step toward fully understanding large‐scale lithospheric structure and melting processes. We apply a novel probabilistic approach for joint inversions of 3D magnetotelluric and seismic data to image the lithosphere beneath southeast Australia. The results show a highly heterogeneous lithosphere with deep conductivity anomalies that correlate with the location of Cenozoic volcanism. In regions where the conductivities have been at odds with sub‐lithospheric temperatures and seismic velocities, we observe that the joint inversion provides conductivity values consistent with other observations. The results reveal a strong relationship between metasomatized regions in the mantle and (a) boundaries of geological provinces, elucidating the subduction‐accretion process in the region; (b) distribution of leucitite and basaltic magmatism; (c) independent geochemical data, and (d) a series of lithospheric steps which constitute areas prone to generating small‐scale instabilities in the asthenosphere. This scenario suggests that shear‐driven upwelling and edge‐driven convection are the primary mechanisms for melting in eastern Australia, contrary to the conventional notion of mantle plume activity. Our study presents an integrated lithospheric model for southeastern Australia and provides valuable insight into the mechanisms driving surface geological processes.

Africa's Crustal Architecture Inferred From Probabilistic and Perturbational Inversion of Ambient Noise: ADAMA

AbstractAfrica's continental crust hosts a variety of geologic terrains and is crucial for understanding the evolution of its longest‐lived cratons. However, few of its seismological models are yet to incorporate the largest continent‐wide noise dispersion data sets. Here, we report on new insights into Africa's crustal architecture obtained using a new data set and model assessment product, ADAMA, which comprises a large ensemble of short‐period surface wave dispersion measurements: 5–40 s. We construct a continent‐wide model of Africa's Crust Evaluated with ADAMA's Rayleigh Phase maps (ACE‐ADAMA‐RP). Dispersion maps, and uncertainties, are obtained with a probabilistic approach. This model update, and a crustal taxonomy derived from unsupervised machine learning, reveals that the architecture of Africa's crust can be classified into two main types: primitive (C1: faster velocities with little gradients) and modified (C2–C4: slower velocities in the shallow crust with more pronounced gradients). The Archean shields are “primitive,” showing little variation or secular evolution. The basins, orogens, and continental margins are “modified” and retain imprints of surface deformation. The crustal taxonomy is obtained without a‐priori geological information and differs from previous classification schemes. While most of our reported features are robust, probabilistic modeling suggests caution in the quantitative interpretations where illumination is compromised by low‐quality measurements, sparse coverage or both. Future extension of our approach to other complementary seismological and geophysical data sets—for example, multimode earthquake dispersion, receiver functions, gravity, and mineral physics, will enable continent‐wide lithospheric modeling that extends resolution to the upper mantle.

Terrestrial heat flow and subsidence of the Eastern Mediterranean Sea

We analysed the bathymetry and both marine and continental heat-flow data of the Eastern Mediterranean Sea basins. Bathymetry was corrected for the subsidence caused by sediment deposition to obtain the water-loaded seafloor depth. The thermal measurements were critically reviewed and corrected for sedimentation by 1–13 mW m−2 and for climatic changes by 4–5 mW m−2 to infer the purely conductive terrestrial heat flow. The seafloor depths and thermal data of the Ionian, Herodotus and Levantine basins were compared to reference models of continental lithosphere stretching and ocean plate cooling. In the Levantine Basin, the seafloor depth of 5.0 km argues for a continental crust that thinned ∼100 Ma ago by a factor of 2.7. The seafloor depth of the Herodotus and Ionian basins (7.5 and 6.3 km, respectively) does not match the continental stretching model predictions, suggesting they are floored by oceanic lithosphere. The reference models for the plate cooling and subsidence only partly account for the seafloor depth of the Eastern Mediterranean Sea basins. The seafloor of the Ionian agrees or is slightly deeper than the reference models, while in the Herodotus, it may exceed the expected subsidence of about 1 km. Compared to the reference models of plate cooling, the heat flow of the Herodotus and Ionian basins is lower by about 10 mW m−2. Together with the seafloor depth and gravity anomaly pattern, this argues for a cold and thick lithosphere beneath the Ionian and Herodotus basins. The lower heat flow may suggest that the basins escape from classical schemes of oceanic lithosphere cooling or that the reference models cannot be used for such old seafloors.

Constraints from the Ntaba Mhlope Syncline and the Mkomozane Homocline, Moodies Group (3.22 Ga), Eswatini, on the deformation of the southern margin of the Barberton greenstone belt

The Barberton Greenstone Belt (BGB) has been classically thought to be a first-order synclinal structure, therefore constraining vertically-dominated models of early Earth lithosphere (de-)formation. Geological mapping, strain analysis, U-Pb zircon age dating, and Raman spectroscopy of carbonaceous matter (RSCM) of the Ntaba Mhlope Syncline (NMS) and the Mkomozane Homocline (MH) close to the southeastern linear margin of the BGB in Eswatini assess this interpretation by constraining the type and timing of the dominant late-phase deformation in this region. Geologic mapping shows that the NMS is a steeply doubly plunging, highly strained syncline composed of basal Moodies conglomerates overlain by sandstones and subordinate shales and siltstones. Deformation of conglomerate clasts is markedly constrictional and of uniform orientation throughout the syncline, suggesting that regional deformation occurred in zones of ductile strain. RSCM indicates maximum peak paleotemperatures of ca. 500 °C, somewhat elevated compared to temperatures measured throughout the BGB. Shortening may have occurred contemporaneously with lower crustal extension.

Structure of the Lithosphere and Conditions of Formation of Oceanic Rises in the Sub-Antarctic Sector of the South Atlantic Using Density and Physical Modeling

The kinematic reorganization of plate boundaries, accompanied by the cessation of old spreading centers and the formation of new ones, and manifestations of plume magmatic activity in the southeastern part of the Antarctic sector of the South Atlantic led to the formation of a complex structural plan of the region. As a result of these processes, a system of ridges, rises, and plateaus with varying morphological expressions and different geophysical characteristics were formed. Results of density modeling of the crust and lithosphere structure along profiles extending from the Falkland Plateau to the Mozambique Ridge and crossing a series of rises and ridges separated by deepwater basins showed that rises have different crustal structures, indicating different origins. The conditions for the formation of different types of submarine rises were studied based on physical modeling. A new experimental model of lithosphere and submarine rises formation in the region was constructed, in which the fracture of the large Agulhas magmatic province into the Agulhas Plateau and the Northeast Georgia rise played an important role, as did the accretion of oceanic crust on the Agulhas spreading ridge and subsequent jump of the spreading axis, leading to the cessation of spreading on this ridge and the formation of the southern segment of the Mid-Atlantic Ridge and its associated Meteor and Islas Orcadas rises. Jumps of spreading axes, accompanied by the periodic activity of hotspots, played an important role in the formation of submarine rises of different genetic types, which determined the different structures of their crust.

Thermal Conductivity and Thermal Diffusivity of Tremolite at High Temperature and Pressure and Implications for the Thermal Structure of the Venusian Lithosphere

AbstractThermal conductivity (κ) and thermal diffusivity (D) of tremolite were measured at up to 2.5 GPa and 1,373 K using the transient plane‐source method in a multi‐anvil apparatus. Thermal conductivity and thermal diffusivity of tremolite decrease monotonically before dehydration (<1,173 K) and increase significantly after dehydration. Tremolite exhibits positive pressure dependence before dehydration. Heat capacity (C) of tremolite calculated fromκandDshows a positive pressure dependence and is controlled by an almost constant thermal expansion coefficient (α) with temperature. Conductive heat transport and radiative heat transport dominate the heat transport process before dehydration, and the significant increase in thermal conductivity after dehydration is attributed to convective heat transport. A compositional model of the Venusian lithosphere composed of a basaltic crust and peridotite mantle with or without tremolite was established. The thickness of the Venusian lithosphere with or without tremolite for Venus was calculated by combining the heat flow (from 20 to 80 mW/m2) at a certain depth (from 5 to 25 km) of crust, ranging from 24.4 to 184.6 km.

The Moho Architecture and Its Role for Isostasy—Insights From the Lofoten‐Vesterålen Rifted Margin, Norway

AbstractThe crustal structure of the Nordland and Troms region, Norway, has received growing scientific attention because (a) the region is one of the most seismically active areas of mainland Norway, and (b) there are differing interpretations of the crustal structure but none of the proposed models simultaneously satisfy gravity, topography and crustal isostasy. At the core of the puzzle is the Lofoten‐Vesterålen archipelago, which exhibits considerable variations in crustal thickness, seemingly inconsistent with the topographic expression along this geomorphic structure. The prevalent view has been that the crust beneath the southern Lofoten is extremely thin (∼20 km). This has recently been disputed. Here, we address this debate by producing new lithospheric models in the region from joint inversion of receiver functions and P‐wave polarizations at 62 seismic stations. Our results are consistent with the regional trends from other models, including a shallow Moho in the southern Lofoten. Moreover, our results detect a low‐velocity layer in the uppermost mantle, which appears to be highly relevant to isostasy in the region. We conclude that the crustal structure in the region may not be as controversial as the recent debate suggested. What appears more urgent to understand is how the concept of isostasy is defined, and how it relates to the layered structure of the lithosphere. In particular, our findings emphasize the importance of conceptualizing the Moho as a transition zone with considerable thickness and internal structural variations, rather than a simple velocity discontinuity.

Lithospheric Elastic Thickness Beneath the Caloris Basin: Implications for the Thermal Structure of Mercury

AbstractThe elastic thickness of a planet's lithosphere is essential to the investigation of its thermal evolution. Our work focuses on the largest visible impact basin on Mercury, Caloris, which has undergone complex magma‐tectonic deformation after its formation. Using a flexural lithosphere model with the initial mantle plug loading, we estimated the elastic thickness in the Caloris region to be 18.0 4.4 km. The successful application of the mantle loading model and the small load ratio indicates that major load in the Caloris region is the mantle plug rather than the lateral density anomaly. The 18 km effective elastic thickness indicates the lithospheric temperature at the time of loading. The current heat production rate in Mercury mantle is estimated as 2.2–4.2 × 10−12 W/kg. Comparison between the effective elastic thickness predicted from the thermal model and loading model suggests that the megaregolith in the Caloris basin region has a certain thickness (>3 km) to ensure the formation of a lithosphere with 18 km elastic thickness. The temperature model results also indicate that the mantle heat production rate at 3.6 Ga is closer to that calculated based on a model with heat‐producing elements abundance close to CI chondrite‐like primitive silicate portion of Mercury. The surface heat flow in the Caloris region is consistent with other megaregolith‐covered regions.

Lithosphere Structure of the Southern Dinarides and Continuity of the Adriatic Lithosphere Slab Beneath the Northern Dinarides Unravelled by Seismic Modelling

Currently recognised lithospheric models hypothesise the non-existence of a lithospheric slab (a so-called slab gap) in the area of the Northern Dinarides, and the possible existence of a shallow slab in the Southern Dinarides. These geological models are mostly based on previous regional and global 3D velocity models obtained from teleseismic tomography. Recent local tomographic models providing a good resolution have regularly shown the existence of a fast anomaly underneath the entire Dinarides, directly indicating the existence of a lithospheric slab. To avoid interpretation pitfalls and increase reliability, forward seismic modelling based on new tomographic models was performed. Seismic modelling indicates a continuous lithospheric slab along the entire Dinarides in the shallow mantle, but it is not continuous vertically. In the Northern Dinarides, the shallow lithospheric slab extends at least to a depth of 150 km. In the Southern and Central Dinarides, there is a deep fast anomaly that can be interpreted in two ways due to the weak vertical resolution of teleseismic tomography. The first model suggests a steeply dipping continuous Adriatic lithospheric slab whereas the second model shows that the slab consists of two separate blocks, meaning that the deeper block was formed by delamination of the Adriatic lithospheric slab. Due to a similar correlation between the inverse velocity models for the synthetic model and the observed data, preference is not given to any model. The second model could indicate two independent Dinaridic subduction phases, as opposed to viewing subduction as a single long process during the geological past.

Architecture of the subcontinental lithospheric mantle of the Archean segment of the Fennoscandian Shield: Analysis of seismic data

Here, Karelian seismological network data were studied to reveal the architecture of the subcontinental lithospheric mantle (SCLM). The receiver functions method was used, which is based on the identification of converted waves from teleseismic earthquakes and their joint inversion into a velocity model. Original one-dimensional (1D) velocity models for the continental lithosphere of the central Karelian Craton (KC) and the western Belomorian Mobile Belt (BMB; both part of the Archean segment of the Fennoscandian Shield) were presented, analysed and interpreted in geological and petrological terms.The obtained results reveal that the SCLM is >200 km thick both in the KC and the BMB. The mantle in both structures was shown to contain a well-defined, contrasting boundary with the crust. However, the boundary between the SCLM and the asthenosphere was not identified in this study. The SCLM was found to be stratified and divided into upper, middle and lower layers in both structures. The boundary between the upper and middle layers was identified at a depth near the upper line of the garnet stability limit in mantle peridotites; the boundary between the middle and lower layers corresponds to the graphite/diamond equilibrium line.The SCLM of the BMB was shown to exhibit a distinct lowered velocity zone in the middle layer. The 1D velocity model of the central KC was found to be in good agreement with the evidence for the SCLM stratigraphy, based on the study of mantle xenoliths and xenocrysts from kimberlites and lamproites from the area.

A shear-wave velocity model for the Scandinavian lithosphere from Rayleigh waves and ambient noise - Implications for the origin of the topography of the Scandes mountain range

We present a new 3D shear-wave velocity model and Moho map of Scandinavia, which is based on the inversion of the merged phase dispersion curves from ambient noise and earthquake-generated Rayleigh waves. A classic two step inversion scheme is used where first maps of phase velocities at different periods are derived, and then a 1D transdimensional Bayesian method is applied to determine the VSV-depth structure. We assess the question of what compensates for the unusual high Scandes mountains and aim to identify the different tectonic domains of the adjacent continental lithosphere (Baltic Shield).While the southern Scandes lacks a pronounced crustal root, we observe a crustal root below the northern Scandes that is decreasing towards the central Scandes. A ∼10 km thick high-density lower crustal layer is present below the northern Scandes and generally thickening to the east below the Baltic Shield. The lithosphere-asthenosphere boundary (LAB) below the Scandes is deepening as well from west to east with a sharp step and a strong VSV decrease with depth of 9% in the north and of 5.5% in the south. The LAB of the thinner lithosphere is at 150 km depth in the north and varies from 90 to 120 km depth in the south. Both LAB steps coincide with the mountain front. The central area shows rather smoothly varying structures (170 km LAB depth, −4% VSV with depth) towards the east and no clear spatial match with the front. We infer therefore distinct uplift mechanisms along the Scandes. The southern Scandes might sustain their topography due to dynamic support from the mantle, while the northern Scandes experience both crustal and mantle lithosphere isostasy. In both cases, we suspect a dynamic support from small-scale edge-driven convection that developed at the sharp lithospheric steps.Beneath the Archean Karelia craton in northern Finland, we find low-velocity areas below 150 km depth while a 250 km deep lithospheric keel is imaged below the Paleoproterozic southern Finland. The Norrbotten craton in northern Sweden can be identified at mantle depths as a unit different from the Karelia craton, Scandes and Paleoproterozic central Sweden.

Quantifying Earth's radiogenic heat budget

Earth's internal heat drives its dynamic engine, causing mantle convection, plate tectonics, and the geodynamo. These renewing and protective processes, which make Earth habitable, are fueled by primordial and radiogenic heat. For the past two decades, particle physicists have measured the flux of geoneutrinos, electron antineutrinos emitted during β− decay. These ghost-like particles provide a direct measure of the amount of heat producing elements (HPE: Th & U) in the Earth and in turn define the planet's absolute concentration of the refractory elements. The geoneutrino flux has contributions from the lithosphere and mantle. Detector sensitivity follows a 1/r2 (source detector separation distance) dependence. Accordingly, an accurate geologic model of the Near-Field Lithosphere (NFL, closest 500 km) surrounding each experiment is required to define the mantle's contribution. Because of its proximity to the detector and enrichment in HPEs, the local lithosphere contributes ∼50% of the signal and has the greatest effect on interpreting the mantle's signal.We re-analyzed the upper crustal compositional model used by Agostini et al. (2020) for the Borexino experiment. We documented the geology of the western Near-Field region as rich in potassic volcanism, including some centers within 50 km of the detector. In contrast, the Agostini study did not include these lithologies and used only a HPE-poor, carbonate-rich, model for upper crustal rocks in the surrounding ∼150 km of the Borexino experiment. Consequently, we report 3× higher U content for the local upper crust, which produces a 200% decrease in Earth's radiogenic heat budget, when compared to their study. Results from the KamLAND and Borexino geoneutrino experiments are at odds with one another and predict mantle compositional heterogeneity that is untenable. Combined analyses of the KamLAND and Borexino experiments using our revised local models strongly favor an Earth with ∼20 TW present-day total radiogenic power. The next generation of geoneutrino detectors (SNO+, counting; and JUNO, under construction) will better constrain the HPE budget of the Earth.

A comparative study of seismic tomography models of the Chinese continental lithosphere

The Chinese mainland is subject to complicated plate interactions that give rise to its complex structure and tectonics. While several seismic velocity models have been developed for the Chinese mainland, apparent discrepancies exist and, so far, little effort has been made to evaluate their reliability and consistency. Such evaluations are important not only for the application and interpretation of model results but also for future model improvement. To address this problem, here we compare five published shear-wave velocity models with a focus on model consistency. The five models were derived from different datasets and methods (i.e., body waves, surface waves from earthquakes, surface waves from noise interferometry, and full waves) and interpolated into uniform horizontal grids (0.5° × 0.5°) with vertical sampling points at 5 km, 10 km, and then 20 km intervals to a depth of 160 km below the surface, from which we constructed an averaged model (AM) as a common reference for comparative study. We compare both the absolute velocity values and perturbation patterns of these models. Our comparisons show that the models have large (> 4%) differences in absolute values, and these differences are independent of data coverage and model resolution. The perturbation patterns of the models also show large differences, although some of the models show a high degree of consistency within certain depth ranges. The observed inconsistencies may reflect limited model resolution but, more importantly, systematic differences in the datasets and methods employed. Thus, despite several seismic models being published for this region, there is significant room for improvement. In particular, the inconsistencies in both data and methodologies need to be resolved in future research. Finally, we constructed a merged model (ChinaM-S1.0) that incorporates the more robust features of the five published models. As the existing models are constrained by different datasets and methods, the merged model serves as a new type of reference model that incorporates the common features from the joint datasets and methods for the shear-wave velocity structure of the Chinese mainland lithosphere.

Benchmark forward gravity schemes: the gravity field of a realistic lithosphere model WINTERC-G

Abstract. Several alternative gravity forward modelling methodologies and associated numerical codes with their own advantages and limitations are available for the solid Earth community. With upcoming state-of-the-art lithosphere density models and accurate global gravity field data sets, it is vital to understand the opportunities and limitations of the various approaches. In this paper, we discuss the four widely used techniques: global spherical harmonics (GSH), tesseroid integration (TESS), triangle integration (TRI), and hexahedral integration (HEX). A constant density shell benchmark shows that all four codes can produce similar precise gravitational potential fields. Two additional shell tests were conducted with more complicated density structures: laterally varying density structures and a crust–mantle interface density. The differences between the four codes were all below 1.5 % of the modelled gravity signal suitable for reproducing satellite-acquired gravity data. TESS and GSH produced the most similar potential fields (<0.3 %). To examine the usability of the forward modelling codes for realistic geological structures, we use the global lithosphere model WINTERC-G that was constrained, among other data, by satellite gravity field data computed using a spectral forward modelling approach. This spectral code was benchmarked against the GSH, and it was confirmed that both approaches produce a similar gravity solution with negligible differences between them. In the comparison of the different WINTERC-G-based gravity solutions, again GSH and TESS performed best. Only short-wavelength noise is present between the spectral and tesseroid forward modelling approaches, likely related to the different way in which the spherical harmonic analysis of the varying boundaries of the mass layer is performed. The spherical harmonic basis functions produce small differences compared to the tesseroid elements, especially at sharp interfaces, which introduces mostly short-wavelength differences. Nevertheless, both approaches (GSH and TESS) result in accurate solutions of the potential field with reasonable computational resources. Differences below 0.5 % are obtained, resulting in residuals of 0.076 mGal standard deviation at 250 km height. The biggest issue for TRI is the characteristic pattern in the residuals that is related to the grid layout. Increasing the resolution and filtering allow for the removal of most of this erroneous pattern, but at the expense of higher computational loads with respect to the other codes. The other spatial forward modelling scheme, HEX, has more difficulty in reproducing similar gravity field solutions compared to GSH and TESS. These particular approaches need to go to higher resolutions, resulting in enormous computation efforts. The hexahedron-based code performs less than optimal in the forward modelling of the gravity signature, especially with a laterally varying density interface. Care must be taken with any forward modelling software as the approximation of the geometry of the WINTERC-G model may deteriorate the gravity field solution.

Centroid Moment Tensor Catalog With 3D Lithospheric Wave Speed Model: The 2016–2017 Central Apennines Sequence

AbstractMoment tensor inversions of broadband velocity data are usually managed by adopting Green's functions for 1D layered seismic wave speed models. This assumption can impact on source parameter estimates in regions with complex 3D heterogeneous structures and discontinuities in rock properties. In this work, we present a new centroid moment tensor (CMT) catalog for the Amatrice‐Visso‐Norcia (AVN) seismic sequence based on a recently generated 3D wave speed model for the Italian lithosphere. Forward synthetic seismograms and Fréchet derivatives for CMT‐3D inversions of 159 earthquakes with Mw ≥ 3.0 are simulated using a spectral‐element method (SEM) code. By comparing the retrieved solutions with those from time domain moment tensor (TDMT) catalog, obtained with a 1D wave speed model calibrated for Central Apennines (Italy), we observe a remarkable degree of consistency in terms of source geometry, kinematics, and magnitude. Significant differences are found in centroid depths, which are more accurately estimated using the 3D model. Finally, we present a newly designed parameter, τ, to better quantify and compare a‐posteriori the reliability of the obtained MT solutions. τ measures the goodness of fit between observed and synthetic seismograms accounting for differences in amplitude, arrival time, percentage of fitted seconds, and the usual L2‐norm estimate. The CMT‐3D solutions represent the first Italian CMT catalog based on a full‐waveform 3D wave speed model. They provide reliable source parameters with potential implications for the structures activated during the sequence. The developed approach can be readily applied to more complex Italian regions where 1D models are underperforming and not representative of the area.

The crustal structure of the Cantabrian Mountains revealed by new magnetotelluric soundings

The upper crustal structure of the central Cantabrian Mountains consists of a block of Variscan basement uplifted over the Duero basin as a consequence of a southward displacement along a major thrust during the convergence between Europe and Iberia in the Eocene-Oligocene (Alpine orogeny). We present the results of the 3-D modeling of thirteen new magnetotelluric sites, five of them being completed by long period data, over a 100 km-long, N-S oriented profile across the central Cantabrian Mountains and the Duero basin. Dimensionality analyses indicated a dominant E-W direction but with influence of 3-D structures at long periods and locally in the Cantabrian Mountains. Accordingly, we performed a 3-D joint inversion of the full impedance tensor and the geomagnetic transfer function following a sequential inversion workflow. The inverse model presents similarities with existing lithospheric models. In the southern part of the area, the conductive sediments of the Duero basin over a high resistive and homogeneous Iberian lithosphere are well delineated. Towards the north, beneath the Cantabrian Mountains, the model reveals a heterogeneous and conductive lithosphere, in which various elongated and dipping conductors in the upper and middle crust are associated with major Alpine thrusts, one being the frontal thrust of the Cantabrian Mountains over the Duero basin. At deeper depths (between 20 and 35 km depth), the Iberian crust appears as subducting to the north beneath a conductive zone interpreted as the hydrated mantle wedge of the north-Iberian continental margin.

Receiver Function Imaging of the Crustal Structure Beneath Northern Taiwan Using Dense Linear Arrays

In order to realize the crustal structure in Taiwan, the receiver function method was used to analyze the teleseismic waveforms recorded by two orthogonal broadband linear arrays deployed in northern Taiwan in the east–west and south–north directions by the TAiwan Integrated GEodynamics Research (TAIGER) project from 2007 to 2009. By incorporation with Common Conversion Point (CCP) stacking, the Moho discontinuities beneath northern Taiwan were imaged. Based on the CCP stack of receiver functions in the east–west direction array, a collision boundary between the Philippine Sea Plate and the Eurasian Plate appears at the east of Taiwan. The Moho depth of the Eurasian Plate in west Taiwan is flat and 30 km; the Moho depth of the Philippine Sea Plate below the Central Mountain Range is about 55 km; in the east, the Moho depth of the Ryukyu arc is about 40 km. The south–north profile shows a brittle–ductile transition zone at depths of 15–20 km beneath central Taiwan from south to north. Moreover, the Moho depth of the Eurasian Plate is about 20–25 km in northern Taiwan. The Moho depth appears to deepen from north to south. The deepest Moho is located at the junction of the two profile lines, the Philippine Sea Plate, and has a depth of 60 km. According to these Moho depths, the crustal structure is thin and flat in the west part of northern Taiwan which is similar to the thin-skin model. However, the lithosphere is deformed and forms the mountain root in the east part which is similar to the lithospheric collision model.