Seismic Tomography Models Research Articles

Crustal melting and continent uplift by mafic underplating at convergent boundaries.

The thick crust of the southern Tibetan and central Andean plateaus includes high-conductivity, low-velocity zones ascribed to partial melt. The melt origin and effect on plateau uplift remain speculative, in particular if plateau uplift happens before continental collision. The East Anatolian Plateau (EAP) has experienced similar, more recent uplift but its structure is largely unknown. Here we present an 80 km deep geophysical model across EAP, constrained by seismic receiver functions integrated with interpretation of gravity data and seismic tomographic, magnetotelluric, geothermal, and geochemical models. The results indicate a 20 km thick lower crustal layer and a 10 km thick mid-crustal layer, which both contain pockets of partial melt. We explain plateau uplift by isostatic equilibration following magmatism associated with roll-back and break-off of the Neo-Tethys slab. Our results suggest that crustal thickening by felsic melt and mafic underplate are important for plateau uplift in the EAP, Andes and Tibet.

The Influence of Mantle Structure on Dynamic Topography in Southern Africa

AbstractDue to relatively high terrain and negligible active tectonics, the southern Africa region boasts over 30 independent estimates of dynamic topography. These published estimates display a wide variance due to both the variety of methods used in computation and a lack of constraints on the regional mantle structure. Here we show that a focus on regional mantle structure is important to generate models of lithospheric and mantle dynamics. Global average mantle properties are not representative of a particular region, and it is necessary to generate viscosity profiles specific to a region of interest. We develop a Bayesian inversion using dynamic geoid kernels, existing seismic tomography models, and Slepian functions to invert for a localized radial viscosity profile that best explains the geoid in southern Africa. With an understanding of viscosity uncertainty, we constrain dynamic topography in southern Africa to lie between 1,000 and 2,000 m. Additionally, we model vertical displacements from 112 Global Navigation Satellite System stations across our region to examine the long‐term, long wavelength pattern of present‐day vertical motion, suggesting that a mean of 1.5 mm/yr (1: 0.8–2.0 mm/yr) of vertical motion may be related to ongoing dynamic topography. Our study demonstrates the utility of dynamic geoid kernels in local nonlinear inversions of non‐unique geophysical data. Furthermore, we present evidence that the mantle beneath southern Africa is generating significant dynamic support for and vertical displacement of the lithosphere in this region.

Density structures of the upper mantle in the East African Rift System: implications for the evolution of intracontinental rifting

The East African Rift System (EARS) provides an ideal natural laboratory for studying the mechanisms of tectonic plate breakup and continental drift, as well as a unique perspective for exploring the maturation process of continental rifting and its drivers. This study combines high-resolution satellite gravity data and seismic tomography model with an integrated geophysical approach to reveal the density structures in the upper mantle of the EARS. The results show that the northeastern to central Congo and Zimbabwe Craton exhibit significant high-density anomalies extending up to 250 km, which is indicative of a thicker and more intact lithosphere. In contrast, the Uganda, Tanzania, eastern and southern Congo, and Kaapvaal Craton show shallow high-density anomalies underlain by low-density anomalies that are clearly derived from the deeper mantle, indicating a thining of the lithosphere with some degree of possible melting at the base. The various rift segments of the EARS exhibit different rift morphologies. The Main Ethiopian Rift and the Kenya Rift of the Eastern Rift Branch show strong low-density anomalies, indicating intense melting, which is much stronger than that observed in the Western Rift Branch. However, the two rifts may have originated from the same mantle uplift in which the low-density anomalies of the Eastern and Western Rift Branches connected in the deep upper mantle. The lower portion of the Malawi Rift exhibits weaker low-denstiy anomalies, which can be observed to the south of the Malawi Rift, extending further south as a continuation of the EARS. Combining the results of previous kinetics simulations and our density perturbation results, it can be inferred that the Eastern Rift Branch is mainly affected by active rifting, while the Western Rift Branch is affected by both active and passive rifting.

Pliocene shorelines and the epeirogenic motion of continental margins: a target dataset for dynamic topography models

Abstract. Global mean sea level during the mid-Pliocene epoch (∼3 Ma), when CO2 and temperatures were above present levels, was notably higher than today due to reduced global ice sheet coverage. Nevertheless, the extent to which ice sheets responded to Pliocene warmth remains in question owing to high levels of uncertainty in proxy-based sea level reconstructions as well as solid Earth dynamic models that have been used to evaluate a limited number of data constraints. Here, we present a global dataset of 10 wave-cut scarps that formed by successive Pliocene sea level oscillations and which are observed today at elevations ranging from ∼6 to 109 m above sea level. The present-day elevations of these features have been identified using a combination of high-resolution digital elevation models and field mapping. Using the MATLAB interface TerraceM, we extrapolate the cliff and platform surfaces to determine the elevation of the scarp toe, which in most settings is buried under meters of talus. We correct the scarp-toe elevations for glacial isostatic adjustment and find that this process alone cannot explain observed differences in Pliocene paleo-shoreline elevations around the globe. We next determine the signal associated with mantle dynamic topography by back-advecting the present-day three-dimensional buoyancy structure of the mantle and calculating the difference in radial surface stresses over the last 3 Myr using the convection code ASPECT. We include a wide range of present-day mantle structures (buoyancy and viscosity) constrained by seismic tomography models, geodynamic observations, and rock mechanics laboratory experiments. Finally, we identify preferred dynamic topography change predictions based on their agreement with scarp elevations and use our most confident result to estimate a Pliocene global mean sea level based on one scarp from De Hoop, South Africa. This inference (11.6 ± 5.2 m) is a downward revision and may imply that ice sheets were relatively resistant to warm Pliocene climate conditions. We also conclude, however, that more targeted model development is needed to more reliably infer mid-Pliocene global mean sea level based on all scarps mapped in this study.

Plate‐Scale Imaging of Eastern US Reveals Ancient and Ongoing Continental Deformation

AbstractEastern North America was constructed over several Wilson cycles, culminating in the breakup of Pangea. Previous seismological imaging lacked the resolution to depict precisely how ancient tectonic boundaries manifest throughout the lithosphere, how continental breakup modified the plate, or how ongoing mantle dynamics shapes the continental margin. We present a high‐resolution, plate‐scale seismic tomography model of the eastern US by combining an unprecedented suite of complementary data sets in a Bayesian framework. These data provide detailed resolution from crust to asthenosphere, identifying the base of the lithosphere and mid‐lithospheric discontinuities. The plate thins in steps that align with ancient orogens. The lithospheric step at the Appalachian front is associated with cells of mantle upwellings, likely edge‐driven convection, that erode the base of the plate and shape modern Appalachian topography. Low‐velocity structures in the lithospheric‐mantle align with the Grenville front and may be remnants of Rodinia assembly.

Three-dimensional density model of the mantle beneath the Ukrainian shield

Purpose. Mantle density models are key tools for understanding the fundamental geological and physical processes occurring within the Earth and are essential to our scientific and applied understanding of the planet. Methodology. The tasks were solved by a complex research method, including analysis and generalization of literary and patent sources, analytical, experimental studies, using computer and mathematical modelling methods. Findings. One-dimensional models simplify the mantle density distribution by assuming that it is uniform only in the vertical direction. This limitation does not allow for horizontal variations in mantle density, which may be important on a regional scale. 3D models are more complex and require more data and computational resources, so their use may be limited. In this study, we present a quasi-three-dimensional model of mantle density beneath the Ukrainian Shield. This 3D model is obtained using a basic set of one-dimensional seismic tomographic velocity models calculated for 21 mantle domains in the depth range from 50 to 2,600 km. The process of converting the P-wave velocity model into a density model includes the following stages: 1) determining seismic boundaries in the mantle based on P-wave velocity curves for each mantle domain; 2) creating a synthetic mantle model beneath the Ukrainian Shield for the P,S-wave velocity curves; 3) solving the Adams-Williamson equation for each domain, considering polynomial corrections to extract heterogeneities during its solution; 4) analysing existing models by comparing the calculated gravitational potential at the central point of the Ukrainian Shield as the standard reference for selecting one of 5 reference models. Here, we focus on the final stages of constructing the mantle density model by: 1) balancing the mass of the upper and lower mantle for each domain when determining density using the Adams-Williamson equation and introducing polynomial corrections; 2) calculating densities for each of the 21 mantle domains and their 3D integration. Originality. The obtained mantle-density model of the Ukrainian Shield aligns well with the division of the mantle into three main layers: lithosphere, upper mantle, and lower mantle. Each of the mantle’s structural layers has its representation pattern in density heterogeneities. Anomalies of decreased density in the lithosphere of the Ukrainian Shield correlate with thermal anomalies, whereas anomalies of increased density correspond to tectonic zones dividing its megablocks. Practical value. Regions of increased density gradient are associated with mantle thrust faults, which in some cases can be boundaries between different petrological formations and serve as channels for magma ascent into the Earth’s crust at certain stages of geological development of the Ukrainian shield and, in turn, be sources of minerals.

Gondwanan flood basalts linked seismically to plume-induced lithosphere instability

Delamination of the continental lithospheric mantle is well recorded beneath several continents. However, the fate of the removed continental lithosphere has been rarely noted, unlike subducted slabs reasonably well imaged in the upper and mid mantle. Beneath former Gondwana, recent seismic tomographic models indicate the presence of at least 5 horizontal fast-wavespeed anomalies at ~600 km depths that do not appear to be related to slab subduction, including fast structures in locations consistent with delamination associated with the Paraná Flood Basalt event at ~134 Ma and the Deccan Traps event at ~66 Ma. These fast-wavespeed anomalies often lie above broad slow seismic wavespeed trunks at 500 to 700 km depths beneath former Gondwana, with slow wavespeed anomalies branching around them. Numerical experiments indicate that delaminated lithosphere tends to stagnate in the transition zone and mid-mantle above a mantle plume where it shapes subsequent plume upwelling. For hot plumes, the melt volume generated during plume-influenced delamination can easily reach ~2 to 4 × 106 km3, consistent with the basalt eruption volume at the Deccan Traps. This seismic and numerical evidence suggests that observed high-wavespeed mid-mantle anomalies beneath the locations of former flood basalts are delaminated fragments of former continental lithosphere, and that lithospheric delamination events in the presence of subcontinental plumes induced several of the continental flood basalts associated with the multiple breakup stages of Gondwanaland. Continued upwelling in these plumes can also have entrained subcontinental lithosphere in the mid-mantle to bring its distinctive geochemical signal to the modern mid-ocean spreading centers that surround southern and western Africa.

Multilevel transcrustal magmatic system beneath the Geysers-Clear Lake area

Magmatism in the Quaternary Clear Lake volcanic field (CLVF), with its youngest eruption having only occurred c. 10 ka ago, is commonly invoked as the heat source for the world's largest commercial geothermal reservoir, The Geysers, in northern California. A shallow silicic magma reservoir in the upper-middle crust has been discovered for some time, but the location and mechanism of a potential deep mafic magma reservoir have remained elusive. Here, we present a seismic tomographic model that images the entire crustal column, clearly revealing a multilevel transcrustal magmatic system beneath the Geysers-Clear Lake area. Upwelling melts from the mantle traverse across the crust-mantle boundary and accumulate in the lower crust underneath the southeastern part of Clear Lake, resulting in a hot Moho in between. Mafic melts primarily ascend westward due to the extensional regime in the west and physical barrier effect from the overlying rigid ophiolite fragment, ultimately forming a shallow silicic magma reservoir underlying and heating The Geysers geothermal field. In addition, this study also links compositionally diverse volcanism in a continental setting to differentiation in a multilevel transcrustal magmatic system.

Influence of mantle plume on continental rift evolution: A case study of the East African rift system

Mantle plume is an essential component of the mantle convection system, and its influence on the geodynamics of continental rifts is of great significance for understanding the crust–mantle interaction. The East African Rift System, as the largest continental rift in the Cenozoic and in the initial stage, provides an excellent option for studying the interaction between the mantle plume and the continental crust. Based on the data such as GPS, seismic tomography, and global crustal model, a viscoelastic-plastic 2D thermodynamic numerical model is established to reconstruct the evolution of the Afar depression, Ethiopian Rift, and Kenyan Rift. By comparing the differences between the models of the Afar depression, Ethiopian Rift, and Kenyan Rift, the relationship between the mantle plume and pre-existing structures and their influence on the evolution of continental rifts are discussed. The results show that the mantle plume can increase the depth of the rift faults, concentrate the distribution of the faults, and strengthen the control of main faults on the rifts, allowing the possibility of narrow rifts. Pre-existing structures control the fault styles and symmetry of the rifts and also the morphology of the mantle plume.

Location and polarity of Variscan sutures based on petrological and seismological data from the Bohemian Massif and the implications for the European Variscides

The high- to ultrahigh-pressure ((U)HP) metamorphic rocks are present within the European Variscan belt between the Bohemian and Iberian massifs (the Galicia-Moldanubian zone) and they are partly incorporated into the Alpine orogenic system. Due to their involvement in various allochthonous units, the affiliation of the (U)HP rocks to the suture zones that were the sites of their initial exhumation, is not always clear. The Bohemian Massif preserves the best evidence of Variscan sutures with clear relationships to the exposed (U)HP rocks. They are the Moldanubian and the Saxo-Thuringian sutures bounding the Teplá-Barrandian block from the SSE and NNE, respectively. The distribution of (U)HP rocks coincides with the boundaries of mantle lithosphere domains, delimited from large-scale seismic anisotropy, and reveals the NW-ward inclination of the Moldanubian mantle lithosphere domain beneath the Teplá-Barrandian block and thus a subduction polarity to the NW. The eastern margin of the Teplá-Barrandian block contains a magmatic arc, which is in direct contact with the Moldanubian orogenic wedge, and both are penetrated by lamprophyre dykes (∼340 Ma), which dates the cessation of the collision-related shortening and crustal consolidation. The overall crustal geometry of the Saxo-Thuringian suture implies the SE-ward polarity of subduction during its formation. However, based on seismic tomography and anisotropy model, the suture at mantle depths appears as a sub-vertical boundary between the Saxo-Thuringian and the Teplá-Barrandian lithosphere domains. The Saxo-Thuringian zone bears evidence of blueschist facies metamorphism in the (para)autochthonous units, which are strongly retrogressed. Compared to the Moldanubian zone, (U)HP rocks are less common in the Saxo-Thuringian zone and occur as nappes and klippes, some of which are exposed near the Moldanubian suture. The similarities of the Saxo-Thuringian (U)HP rocks to those in the Moldanubian zone and their allochthonous positions favour formation of some of the (U)HP rocks along the Moldanubian suture and their subsequent emplacement into the Saxothuringian zone. The Moldanubian suture appears to control the distribution of most of the (U)HP rocks exposed along the European Variscan Belt. They all show similarities regarding lithology, mainly fragments of mantle rocks included in felsic materials, and their granulite-amphibolite facies thermal overprint.

Basal Mantle Flow Over LLSVPs Explains Differences in Pacific and Indo‐Atlantic Hotspot Motions

AbstractSurface hotspot motions are approximately a factor of two faster in the Pacific than the Indo‐Atlantic, and the Indo‐Atlantic large low shear velocity province (LLSVP) appears to be significantly taller than the Pacific LLSVP. Hypothesizing that surface hotspot motions are correlated with the motion of plume sources on the upper surface of chemically distinct, intrinsically dense LLSVPs, we use 3D spherical mantle convection models to compute the velocity of plume sources and compare with observed surface hotspot motions. No contrast in the mean speed of Pacific and Indo‐Atlantic hotspots is predicted if the LLSVPs are treated as purely thermal anomalies and plume sources move laterally across the core‐mantle boundary. However, when LLSVP topography is included in the model, the predicted hotspot speeds are, on average, faster in the Pacific than the Indo‐Atlantic, even when modest topography is assigned to both LLSVPs (e.g., 100–300 km). The difference in mean hotspot speed increases to a factor of two for larger and laterally variable LLSVP topography estimated from seismic tomographic model S40RTS (up to 1,100–1,500 km for the Indo‐Atlantic region vs. 700–1,400 km for the Pacific region) and our results also broadly reproduce the convergence of Pacific hotspots toward the center of the Pacific LLSVP. These largescale features of global hotspot motions are only reproduced when ambient mantle material flows over large, relatively stable topographical features, suggesting that LLSVPs are chemically distinct and intrinsically dense relative to ambient mantle material.

Resolution and trade-offs in global anelastic full-waveform inversion

Summary Improving the resolution of seismic anelastic models is critical for a better understanding of the Earth’s subsurface structure and dynamics. Seismic attenuation plays a crucial role in estimating water content, partial melting, and temperature variations in the Earth’s crust and mantle. However, compared to seismic wavespeed models, seismic attenuation tomography models tend to be less resolved. This is due to the complexity of amplitude measurements and the challenge of isolating the effect of attenuation in the data from other parameters. Physical dispersion caused by attenuation also affects seismic wavespeeds, and neglecting scattering/defocusing effects in classical anelastic models can lead to biased results. To overcome these challenges, it is essential to account for the full 3D complexity of seismic wave propagation. Although various synthetic tests have been conducted to validate anelastic Full-Waveform Inversion (FWI), there is still a lack of understanding regarding the trade-off between elastic and anelastic parameters, as well as the variable influence of different parameter classes on the data. In this context, we present a synthetic study to explore different strategies for global anelastic inversions. To assess the resolution and sensitivity for different misfit functions, we first perform mono-parameter inversions by inverting only for attenuation. Then, to study trade-offs between parameters and resolution, we test two different inversion strategies (simultaneous and sequential) to jointly constrain the elastic and anelastic parameters. We found that a sequential inversion strategy performs better for imaging attenuation than a simultaneous inversion. We also demonstrate the dominance of seismic wavespeeds over attenuation, underscoring the importance of determining a good approximation of the Hessian matrix and suitable damping factors for each parameter class.

A New Lithospheric Density and Magnetic Susceptibility Model of Iran, Starting From High‐Resolution Seismic Tomography

AbstractWe propose a new model for the crust and upper mantle in Iran by joint inversion of gravity and magnetic fields, constrained with a seismic tomography model. We then calculate shear modulus from the Vs velocities and densities. The crust and mantle tomography model is first converted to a density cube through empirical and petrological velocity‐density relations. The starting susceptibility is assigned to a two‐layer homogeneous model, above a heat flow‐derived Curie depth. Considering the uncertainties in the density‐velocity relations, and the starting layered susceptibility variation, we refine the model by a constrained inversion of the gravity and magnetic fields with a Bayesian approach, producing the final 3D density and susceptibility model. The area is tectonically active with high seismicity and active faulting which are regulated by the crustal density and rigidity variations. Higher rigidity matches lower seismicity and extended deserts and basins, suggesting the control of their development. The Neo‐Tethys suture, extending ∼1,500 km long, as well as the Paleo‐Tethys suture match crustal scale density variations, defining characteristics of the lighter Arabian plate and denser Eurasian crust. The South Caspian Basin is enigmatic, due to focusing on the seismicity along all its borders, but with relatively low average rigidity, which is contrary to what is observed for Iran, where the reduced rigidity correlates with higher seismicity. The 3D density model will be useful for numerical geodynamic models and obtaining geologic inferences from the crustal‐scale units.

The deep structure of the Zagros mountain system according to Taylor approximation seismic tomography data

The seismic tomographic model obtained by the Taylor approximation method provides new constraints on the Earth’s crust and mantle structure beneath the Zagros mountain system at depths of 50—850 km. Based on structural geology and seismic tomography data, we divide the Zagros into Northwestern (Kirkuk, Lurestan, and Dezful regions) and Central (Kazerun and Fars regions) parts. The Central Zagros was formed due to subduction-collision tectonics, and the Northwestern Zagros was formed due to a collision-shear (transpression) tectonic process. Based on the 3D P-velocity model of the Zagros and its surrounding mantle, a high-velocity distal southeastern part of the Arabian Plate was established, subducting under the Central Zagros and the Iranian Plate, following the subduction of the Neotethys. This slab covers an area of 23—32°NL — 49—58°EL, i.e., the Bay of Bengal, the Central Zagros, and the southern part of the Iranian Plate. The maximum horizontal extent of the submerging part of the Arabian slab is 800—1100 km, and the maximum submersion depth is 450—500 km. A number of projections on the horizontal plane of the lines of intersection of the surface of the subducting Arabian Plate with horizontal sections of 50, 100, 200, 300, and 400 km were constructed. We obtained the direction of plate plunging, which in different areas varies from northern to northeastern 30—45°. In the Northwestern Zagros, according to the model, traces of subduction do not appear; however, right-lateral strike-slip deformations along the Main Recent Fault are visible and continue now in the crust. The difference in the types of deformation of the Northwestern and Central Zagros is most likely due to the features of the outlines of the eastern edge of the Arabian Plate and the complex nature of its movements. Analysis of earthquake mechanisms in 1977—2001 shows that the movement of the Arabian Plate could have occurred with frequent alternation in the S-N and N-E directions due to the alternation of spreading processes in the Gulf of Aden and the southeastern part of the Red Sea.

Thermal conductivity of Fe-bearing bridgmanite and post-perovskite: Implications for the heat flux from the core

The thermal conductivity of the lowermost mantle controls the energy transfer from the core to the mantle, and provides insight into the thermal history, evolution, and magnetic field of the Earth. Bridgmanite (Brg) and post-perovskite (PPv) are the most abundant minerals in the lowermost mantle. However, their thermal conductivities, as well as the impact of Fe impurities, are highly controversial. In this study, the thermal conductivities of Fe-free and Fe-bearing Brg and PPv at high pressure and temperature were predicted, through a combination of non-equilibrium molecular dynamics and machine learning potential (MLP) trained with data from first-principles calculations. The thermal conductivities of Fe-free Brg and PPv are in agreement with those calculated by the Green-Kubo method based on the MLP. We found that the presence of 12.5 mol% Fe in the lowermost mantle decreases the thermal conductivities of Brg and PPv by ∼10% and ∼14%, respectively. Furthermore, the phase transition from Brg to PPv increases the thermal conductivity of pyrolite by ∼22%. Incorporating the distribution of minerals, temperature, and iron content obtained through the inversion based on mineral elasticity and seismic tomography models, a global heat flow with substantial lateral variation at the core-mantle boundary was reported. The total heat flux from the core was found to be 7.1 ± 0.5 TW, implying a geologically young inner core of 0.75 ± 0.35 Ga.

Lithospheric Structure of the Circum‐Pannonian Region Imaged by S‐To‐P Receiver Functions

AbstractThe lithosphere‐asthenosphere boundary and mid‐lithospheric discontinuities are primary attributes of the upper mantle. The Pannonian region is an extensional sedimentary basin enclosed by collisional orogens. Here, we estimate the negative phase depth of S‐to‐P receiver functions to image the lithospheric thickness and other discontinuities with high resolution, based on the recent dense seismological broadband networks. The lithosphere‐asthenosphere boundary is relatively shallow (<90 km) in the Pannonian Basin system, and deeper (∼90–140 km) in the surrounding orogens, where average surface heat flow values are higher (120 mW/m2) and lower (50–70 mW/m2), respectively. The 1D and 2D common conversion point migration with 3D velocity model provide comparable but different resolution images beneath the wider region of the Pannonian Basin. We obtained deeper values in the Western (∼120 km) and Southern‐Carpathians orogens (∼135 km). Furthermore, we provide new information on the lithospheric thickness and its seismic properties in the eastern part of the study region (e.g., Apuseni Mountains (∼95 km), Eastern‐Carpathians (∼120 km), Moesian Platform (∼90 km) and Transylvanian Basin (∼85 km). The shallower negative phase depth can be interpreted as the lithosphere‐asthenosphere boundary beneath the Pannonian Basin system in agreement with its high heat flow values. In contrast, the deeper negative phase depth estimates in the colder surroundings can be interpreted as intra‐ or mid‐lithospheric discontinuities, when compared with local seismic tomography models. In this region, the correlation with heat flow implies that the observed negative phase depth is of thermo‐chemical or rheological nature.

GLOBAL GEODYNAMIC MODEL OF THE EARTH AND ITS APPLICATION FOR ANTARCTICA

A geodynamic model of the modern Earth is constructed based on the SMEAN 2 global seismic tomography model. On the basis of mantle temperature anomalies, numerical modeling of spherical mantle convection is performed for the parameters of the real Earth. For Antarctica, an explanation was obtained for the existence of the West Antarctic Rift System, which contains one of the largest and least known volcanic provinces on Earth, which is consistent with the increased surface heat flow and modern volcanism of the West Antarctic Rift System and part of the Antarctic Peninsula. The increased heat flow and volcanoes in this region are causing instability and accelerating the flow of glaciers from the West Antarctic Ice Sheet into the ocean, which could lead to a significant rise in global sea level.

The Early Model of Tomography in Eastern Indonesia Using FMTOMO

Eastern Indonesia has a complicated structure due to the collision of the Eurasian Plate, the Pacific Plate, the Indo-Australian Plate, and the Philippine Plate, creating an active seismic zone. The Banda - Seram segment of eastern Indonesia comprises subduction, rollback, detachment, slab tear, back-arc thrust, and the Banda volcanic arc. Sri Widiyantoro did a previous study in Eastern Indonesia utilizing seismic tomography methods to detect the upper mantle and the 180-degree curved transition zone under the Banda arc. In addition, Robert Hall and Spakman’s research of the upper mantle revealed the existence of a spoon-shaped mechanism in the band arc. Seismic tomography modeling, this research seeks to identify the tectonic features in Eastern Indonesia, such as partial melting, subduction, slab rips, back-arc thrust, Banda volcanic-arc, detachment, and faults, and to establish the most suitable mitigation strategies. This study uses earthquake data from the Meteorology Climatology Geophysics Agency database (BMKG) and the form of P waves. In this research from 1 January 2016 to 31 December 2020, with coordinates 1 - 12 LS and 120 - 135 BT, there were 81 recording stations and 11299 earthquake occurrences consisting of 175643 P waves and 27338 S waves. This work applies the FMTOMO method to a seismic tomographic model. This research indicates that high-velocity anomalies characterize subduction to a depth of 650 kilometers. On Buru Island, the high-velocity anomaly is diagnosed as a slab tear at a depth of more than 400 kilometers. This research identifies a 100-kilometer-deep Back-arc thrust on Wetar Island that produces shallow earthquakes. In addition, a low-velocity anomaly at a depth of 70 kilometers - 90 kilometers is characterized as partial melting due to its parallel location with the Banda volcanic arc. This study examines the connection between subduction and intense seismic and magmatic activity in Eastern Indonesia.

The Boundary Areas Structure of the Malko-Petropavlovsk Fracture Zone from Local Seismic Tomography and Earthquake Foci Mechanisms Data

Abstract —This study continues analysis of the new seismic tomographic structure of the suprasubduction complex of the central zone of Kamchatka, obtained from the dense local networks data of 2018–2020, and is devoted to the analysis of the velocity structure in the Malko-Petropavlovsk fracture zone margins and around them. The seismic tomographic model involves about 98,000 P- and S-wave travel times from 2963 local earthquakes from August 2018 to July 2020. The resolution of this model makes it possible to trace the feeding systems of volcanoes of the South Kamchatka and East Volcanic Belt to the slab surface, as well as to identify subvertical structural faults. To construct the orientations of the compression and extension axes we used the foci mechanisms of 41 earthquakes with М ≥ 4.5 from the catalog of the International Seismological Center for the period 1979–2019. Along the Malko-Petropavlovsk fracture zone, the Avacha transform fault is clearly traced in the geometry and mutual arrangement of velocity anomalies almost throughout the entire depth of the model. Comparison of seismic anomalies with a map of the directions of the compression and extension axes distribution from the earthquake foci mechanisms showed the correlation between the change in the value of the velocity anomalies along the Avacha transform fault with the axes direction change by almost 180°. A near-surface low-velocity anomaly to the depths of 25–35 km was found along the western border of the Malko-Petropavlovsk zone under the southern tip of the Sredinny Ridge. This anomaly probably marks the axes junction zone boundary of the ancient volcanic front along the Sredinny Ridge and the modern active Eastern Volcanic Belt, which formed as a result of the Kronotsky paleoarc accretion. To the west from the Sredinny Ridge southern tip, another low-velocity anomaly was revealed. This anomaly was traced to a depth of ~150 km, has a contrasting southern boundary confirmed by the distribution of the compression and extension axes directions by the earthquake foci mechanisms and apparently marks the southern boundary of the West Kamchatka block.

Compressibility of ferropericlase at high-temperature: Evidence for the iron spin crossover in seismic tomography

The iron spin crossover in ferropericlase, the second most abundant mineral in Earth's lower mantle, causes changes in a range of physical properties, including seismic wave velocities. Understanding the effect of temperature on the spin crossover is essential to detect its signature in seismic observations and constrain its occurrence in the mantle. Here, we report the first experimental results on the spin crossover-induced bulk modulus softening at high temperatures, derived directly from time-resolved x-ray diffraction measurements during continuous compression of (Mg0.8Fe0.2)O in a resistive-heated dynamic diamond-anvil cell. We present new theoretical calculations of the spin crossover at mantle temperatures benchmarked by the experiments. Based on our results, we create synthetic seismic tomography models to investigate the signature of the spin crossover in global seismic tomography. A tomographic filter is applied to allow for meaningful comparisons between the synthetic models and data-based seismic tomography models, like SP12RTS. A negative anomaly in the correlation between Vs variations and Vc variations (S-C correlation) is found to be the most suitable measure to detect the presence of the spin crossover in tomographic models. When including the effects of the spin crossover, the misfit between the synthetic model and SP12RTS is reduced by 63%, providing strong evidence for the presence of the spin crossover, and hence ferropericlase, in the lower mantle. Future improvement of seismic resolution may facilitate a detailed mapping of spin state using the S-C correlation, providing constraints on mantle temperatures by taking advantage of the temperature sensitivity of the spin crossover.