Damped Least-squares Inversion Research Articles

Shallow crustal model of the DehDasht in Zagros, Iran, using Rayleigh wave tomography

Surface wave tomography is considered an efficient method to reveal the physical properties of subsurface geological structures, particularly beneath geologically complex areas where conventional active seismic methods are problematic. The salt-rich DehDasht structural basin, in the SW of Iran, is an example of this area, which is characterized by high seismicity. Despite the complex geology and the great thickness of evaporitic deposits in this region, surface wave tomography can retrieve high-resolution images by applying a dense network of stations. Therefore, >11,000 micro-earthquakes (M ≤ 3.0) recorded by 116 broadband stations in 9 months were processed to detect geological structures and a crustal model beneath the DehDasht region. After selecting strict data selection criteria, the observed dispersion curves of Rayleigh waves were then measured using the multiple filter analysis method, and the 2D tomography was done for periods of 0.3 to 5.0 s. A non-linear iterative damped least-squares inversion procedure was then applied to each local dispersion curve to obtain the 1D VS model of the uppermost 5 km, and the results were inserted into the original cells to construct a quasi-3D VS model. Although our resulted VS model has high correlations with known geological units and tectonic features of the study area, we validated it by inverting the observed gravity data independently and consequently recovering the location of the anomalies at the basin and/or around faults, anticlines, and synclines. Generally, the observed low-and high-velocity anomalies are associated with the sedimentary materials.

Integrating neural networks in least-squares inversion of airborne time-domain electromagnetic data

Airborne time-domain electromagnetic surveys produce extremely large data sets with thousands of line kilometers of data and millions of possible models to explain the data. Inversion of such data sets to obtain the resistivity structures of the subsurface is computationally intensive and involves calculation of a significant number of forward and derivative responses for solving the least-squares inverse problem. The flight altitude of the airborne system needs to be included in the modeling, which adds further complexity. We propose to integrate neural networks in a damped iterative least-squares inversion framework to expedite the inversion process. We train two separate neural networks to predict the forward responses and partial derivatives independently for a broad range of resistivity structures and flight altitudes. Data inversion is not only used for producing the final subsurface models but also used during data processing, or to produce intermediate results during a survey. With these purposes in mind, we provide three inversion schemes with a tunable balance between computational time and modeling accuracy: (1) numerical forward responses used initially in combination with neural network derivatives, and the derivatives switched to a numerical solution in final iterations, (2) numerical forward responses in combination with neural network derivatives used throughout the inversion, and (3) only neural network forward responses and derivatives used in inversion. Experiments on field data find that we improve inversion speed without any loss in modeling accuracy with our first approach, whereas the second scheme gives a significant speedup at the cost of minor and often acceptable deviations in the inversion results from the conventional nonlinear inversion. The last approach is the fastest and captures the overall resistivity structures quite well. Therefore, depending on the modeling accuracy, inversion speedup factors of up to 50 are realized by using the proposed schemes.

Historic Underground Silver Mine Workings Detection Using 2D Electrical Resistivity Imaging (Durango, Mexico)

This paper presents an underground silver mining operation outside Gomez Palacio, Durango, Mexico, terminated around the 1930s, of which previous knowledge of its operations was poor. Durango’s current silver exploration campaigns are likely to overlook historic silver mining sites due to interest in specific prospect regions. A two-dimensional (2D) Electrical Resistivity Imaging (ERI) survey coupled with reconnaissance of the area was performed at this historic silver mining site. The exploration campaign aimed to find the abandoned mineshaft, map its subsurface extent, and explore the occurrence of mineralization zones (silver ore). The ERI survey comprised five profiles measured with the extended dipole-dipole array with a consistent electrode spacing of 5 m. The smooth, robust, and damped least-squares inversion methods were used to invert the 2D data. Our field observations and ERI survey results collectively reveal the following findings: (a) reconnaissance reveals mining infrastructure consistent with historical mining activity; the infrastructure includes a complex of habitational rooms, an ore-processing pit near a concrete slab next to a dirt road, and two limestone-wall structures interpreted as the entrance of abandoned backfilled mineshafts named Mesquite and Lechuguilla; (b) high-resistivity anomalies suggest vestiges of shallow, underground mine workings including backfilled mineshafts that connect a mine gallery complex, and (c) various low-resistivity anomalies, juxtaposed against mine galleries, suggestive of unmined shallow vein-type and manto-type mineralization of hydrothermal origin. The imaging depth is estimated at ~65 m. Underground silver mining moved southwards and was limited to ~40 m depth.

Analysis of the shallow crustal structure of the Bam Fault Zone, Iran, using the surface wave and first-arrival P-wave tomography approaches

The crustal structure around the Bam fault located southeast of Iran was investigated by applying surface and P-wave tomography to locally recorded aftershocks of the destructive Bam earthquake (Mw 6.6) on December 26, 2003. The aftershock dataset was recorded for 1 month after the main shock by 23 temporary seismic stations operated by the International Institute of Earthquake Engineering and Seismology (IIEES). After applying the selection criteria, 331 well-located events, including more than 1700 waveforms, remained. By calculating the surface wave dispersion measurements in the period range of 0.6–5 s, a tomographic procedure was then performed, and the local dispersion curves were extracted for each geographic grid point (1.8 km × 1.8 km). We applied an iterative damped least-squares inversion procedure to obtain the VSV and VSH models at depths of up to 6.5 km. The discrepancy between these VSV and VSH models also resulted in the radial anisotropy model. Moreover, the P-wave first-arrival times were inverted by the computer program SIMULPS to calculate the VP model for a depth range of 6.5–14 km, while the calculated VP model does not have sufficient resolution at depths of up to 6 km. The tomographic results indicate that a ~ 2 km thick upper sedimentary layer with a shear wave velocity of less than 2.4 km/s and relatively negative radial anisotropy covers the study area (except Bam City, with a thin 500 m positive radial anisotropy layer), which overlay on relatively positive radial anisotropy. Our results indicate that the main shock occurred on the Bam fault escarpment; then the rupturing was shifted to the co-seismic surface rupture (approximately 5 km west of the escarpment) at a depth of ~8–10 km. This dividing of energy on the fault plane (an oblique Y shape) leads to damping most of the energy as a deformation in the lower sedimentary layer.

Seismic Imaging of Lithospheric Structure Beneath Central-East Java Region, Indonesia: Relation to Recent Earthquakes

The Central and East Java region, which is part of the Sunda Arc, has an important role in producing destructive earthquakes and volcanic complexes in Indonesia as a result of the convergence between the Indo-Australian plate that subducts under the Eurasian plate. In this study, the 3-D seismic velocity structure (Vp, Vs, and Vp/Vs) of the crust and upper mantle was determined to reveal the presence of the subducted slab, volcanic sources, and seismogenic features in the Central and East Java region. We have manually re-picked P- and S-arrivals of 1,488 events from January 2009 to September 2017 recorded at 27 stations of the BMKG network. An iterative damped least-squares inversion method was applied to simultaneously calculate both hypocenter relocations and velocity structure beneath this high-risk region to a depth of 200 km. We then compare the tomographic results and seismicity to interpret structural features in the seismic zones. The subducted slab is dipping toward the north, imaged by the high-velocity regions with low Vp/Vs at depths of about 50–100 km. Low-velocity anomalies with high Vp/Vs above the slab at a depth of ∼100 km, imply the possible location of partial melting from slab dehydration. Fluids and melts are ascending to feed the volcanoes i. e., Merapi-Merbabu, Wilis, Pandan, Semeru, Bromo, and Ijen that also have similar low-velocity anomalies at 10–30 km depths, suggesting the presence of the sedimentary basin or magma reservoir. We also have redetermined the hypocenter location of the 2021 (Mw 6.1) Malang earthquake at 8.94oS, 112.45oE, with a depth of 59.7 km. The location error in the x, y, and z directions are 3.08, 6.39, and 11.91 km, respectively. This intraslab event with a thrusting mechanism is located in the high-velocity region and close to the intermediate-depth seismic clusters, which indicates the geometry of the oceanic slab. In the region of 1994 (Mw 7.8) Banyuwangi earthquake, we found a low-velocity anomaly at ∼ 50 km depth that might be associated with the presence of subducting seamount that is more hydrated than the surrounding slab. The slip over this subducting seamount caused the tsunamigenic earthquake.

Contemporary crustal stress of the Greek region deduced from earthquake focal mechanisms

A massive dataset of over 1900 focal mechanisms of crustal earthquakes with M ≥ 3.5 in the Greek region was employed to resolve the contemporary stress-field using a damped least-squares inversion. The results are in good agreement with the strain-rate field of the Global Strain Rate Model, which was used as reference, both in terms of their principal axes orientations and expected faulting styles. Dual stress-states were identified using the Multiple Inverse Method in regions delineated by joining neighboring Area Sources of the European Seismic Hazard Model 2013 (ESHM13). North-western Greece is mostly affected by transpressional tectonics characterized by NE-SW contraction. Northern/central Greece and the Corinth Rift are dominated by E-W normal faulting, with secondary oblique-normal to strike-slip faulting at the western margin of the latter. North and central Aegean are mainly governed by transtensional regime, characterized by stable N-S extension.The stress-tensor was found to be compatible with the Fault Sources (FS) of ESHM13, in terms of orientation and expected faulting type. Differences were observed in regions of low strain-rate, such as the Southern Aegean, where left-lateral, E-W strike-slip instead of normal faulting was inferred. Discrepancies in areas with strong local heterogeneities were highlighted by anomalies in the stress-ratio, Φ, indicating transtensional regime in the pull-apart basins of Western Greece and transpressional tectonics in north-western Greece and south of Crete. The latter is characterized by stable N-S contraction, SW-NE sinistral strike-slip and E-W reverse faulting in the vicinity of the subduction zone. A low Φ, E-W oriented zone was identified along the active volcanic arc, where a remarkable 90° rotation occurs in the stress field. This rotation is related to the transition from E-W (in the north) to N-S (in the south) normal faulting in Peloponnese and Dodecanese Islands, as well as rearrangement from dextral to sinistral SW-NE strike-slip faulting in North and South Aegean, respectively.

A new prestack three-parameter amplitude variation with offset inversion method

Prestack seismic inversion is an important method in reservoir prediction and fluid identification. In this paper, we develop a new strategy to carry out a prestack three-parameter AVO inversion to obtain P-wave velocity, S-wave velocity, and density. Using the Aki and Richards equation to calculate the PP reflection coefficients, which is the approximation to the Zoeppritz PP equation, we re-derive the relationship between reflection coefficients and their partial derivatives with respect to P-wave velocity, S-wave velocity, and density. We transform the objective function into the form of matrix multiplication and solve it using a damped least-squares method and Tikhonov regularization method. Then we directly obtain the inversion results, including P-wave velocity, S-wave velocity and density. To demonstrate the validity of the proposed prestack inversion method, we set up a 1D forward simulation model and add uniform noise into the synthetic seismic records. Then we test the proposed method on the overthrust model and field data. In order to demonstrate its high accuracy and the efficiency of the Tikhonov regularization method, we carry out a damped least-squares inversion method using the same models as a comparison. We find that the proposed prestack seismic data AVO inversion method has higher anti-noise capability, accuracy, and efficiency, both in synthetic records and field data using the Tikhonov regularization method.

Estimating P- and S-wave inverse quality factors from observed seismic data using an attenuative elastic impedance

P- and S-wave inverse quality factors quantify seismic wave attenuation, which is related to several key reservoir parameters (porosity, saturation, and viscosity). Estimating the inverse quality factors from observed seismic data provides additional and useful information during gas-bearing reservoir prediction. First, we have developed an approximate reflection coefficient and attenuative elastic impedance (QEI) in terms of the inverse quality factors, and then we established an approach to estimate elastic properties (P- and S-wave impedances, and density) and attenuation (P- and S-wave inverse quality factors) from seismic data at different incidence angles and frequencies. The approach is implemented as a two-step inversion: a model-based and damped least-squares inversion for QEI, and a Bayesian Markov chain Monte Carlo inversion for the inverse quality factors. Synthetic data tests confirm that P- and S-wave impedances and inverse quality factors are reasonably estimated in the case of moderate data error or noise. Applying the established approach to a real data set is suggestive of the robustness of the approach, and furthermore that physically meaningful inverse quality factors can be estimated from seismic data acquired over a gas-bearing reservoir.

Rayleigh waves from correlation of seismic noise in Great Island of Tierra del Fuego, Argentina: Constraints on upper crustal structure

In this study, the ambient seismic noise cross-correlation technique is applied to estimate the upper structure of the crust beneath Great Island of Tierra del Fuego (TdF), Argentina, by the analysis of short-period Rayleigh wave group velocities. The island, situated in the southernmost South America, is a key area of investigation among the interaction between the South American and Scotia plates and is considered as a very seismically active one. Through cross-correlating the vertical components of ambient seismic noise registered at four broadband stations in TdF, we were able to extract Rayleigh waves which were used to estimate group velocities in the period band of 2.5–16 s using a time-frequency analysis. Although ambient noise sources are distributed inhomogeneously, robust empirical Green's functions could be recovered from the cross-correlation of 12 months of ambient noise. The observed group velocities were inverted considering a non-linear iterative damped least-squares inversion procedure and several 1-D shear wave velocity models of the upper crust were obtained. According to the inversion results, the S-wave velocity ranges between 1.75 and 3.7 km/s in the first 10 km of crust, depending on the pair of stations considered. These results are in agreement to the major known surface and sub-surface geological and tectonic features known in the area. This study represents the first ambient seismic noise analysis in TdF in order to constraint the upper crust beneath this region. It can also be considered as a successful feasibility study for future analyses with a denser station deployment for a more detailed imaging of structure.

Estimation of modified fluid factor and dry fracture weaknesses using azimuthal elastic impedance

We consider the problem of fluid identification and fracture detection in unconventional reservoir (tight gas sand and shale gas) characterization. We begin with a simplification of the stiffness parameters and the derivation of a linearized reflection coefficient and azimuthal elastic impedance (EI). The accuracy of the simplification is confirmed in application to gas-bearing fractured rocks with low porosity and small fracture density. We have developed a modified fluid factor that is more sensitive to fluid type and less influenced by porosity. A two-step inversion workflow is evaluated based on the derived linearized reflection coefficient and azimuthal EI, including (1) a damped least-squares inversion for azimuthal EI, constrained by an initial model, and (2) a Bayesian Markov chain Monte Carlo inversion for the modified fluid factor and dry fracture weaknesses. Stability and accuracy are examined with synthetic data, from which we conclude that the modified fluid factor and dry fracture weaknesses can be stably determined in the presence of moderate data error/noise. The stability of our approach is further confirmed on a fractured tight gas sand field data set, within which we observe that geologically reasonable parameters (Lamé constants, the modified fluid factor, and dry fracture weaknesses) are determined. We conclude that our inversion workflow and its underlying assumptions form realistic predictions/discriminations of reservoir fracture and fluid parameters.

Source-Type-Specific Inversion of Moment Tensors

The mapping of the fit of seismic moment tensor (MT) solutions in source-type space helps to characterize uncertainty and solution uniqueness. Current practice relies on the forward testing of a distribution of randomly generated MTs in source-type space, which is slow and does not necessarily recover the true maximum fit surface. We design an iterative damped least-squares inversion scheme to invert waveforms and/or P -wave first motions for best-fitting MT solutions for specific source types. An event associated with the sinkhole at the Napoleonville salt dome, Louisiana, an industrial quarry explosion, and an earthquake at The Geysers geothermal field, northern California, are presented as examples. We find that the inversion method is more accurate and successful than the random-search approach in recovering the region of best-fitting MT solutions or source types and is substantially faster. The approach also enables the determination of the best-fitting MT for specified source types such as pure double couples, tensile cracks, or explosions, as well as compound mechanisms in a single numerical framework. Online Material: Description and figures comparing inversion performance using spherical and Cartesian eigenvector parameterizations; figures showing the revised full moment tensor solution of event TE1 and additional network sensitivity solutions of event TE3.

Impedance inversion based on L1 norm regularization

Impedance is an important parameter in geophysics which is greatly relevant to petroleum reservoir. Traditional iterative inversion methods, such as damped least-squares inversion method, often lead to unreliable inversion results when there is strong noise in the seismic records. In this paper, we propose a new impedance inversion method called L1 norm regularization inversion method which has high anti-noise ability. The accuracy of impedances derived from the proposed method is much higher than those derived from damped least-squares inversion method using synthetic seismic record and field data. We set up a forward simulation model and add different intensities of uniform and Gaussian noise into the synthetic seismic records respectively. We find that when there is strong noise in the synthetic record, L1 norm regularization inversion method can obtain the inverted impedance with higher accuracy compared with damped least-squares inversion method. At last, it is found that impedance derived from L1 norm regularization method has a higher accuracy than impedance derived from damped-squares inversion method when there is strong noise both in synthetic record and field data. We conclude that the proposed inversion method shows higher anti-noise ability and can provide impedances with higher accuracy.

Crustal structure of Damavand volcano, Iran, from ambient noise and earthquake tomography

We investigated the shear wave velocity structure beneath the Damavand volcano, Iran, using analysis of Rayleigh wave empirical Green’s functions obtained from the cross-correlation of 1-year seismic ambient noise and 26 local earthquakes. After the observed phase, velocity dispersion curves were inverted to obtain phase velocity tomographic maps; the synthetic phase velocity dispersion curves were then estimated for each evenly spaced geographic grid point. Estimated synthetic phase velocities were inverted using the nonlinear damped least-squares inversion method to obtain a quasi-3D shear wave velocity for the study area. Analysis of obtained quasi-3D shear wave velocity model reveals the presence of three distinct low or high anomalies in the upper 5 km. A low-velocity layer (VS ~ 2.8 km/s) was observed in the upper ~2 km of the crust that includes the sedimentary sequence of carbonate, siliciclastic, and volcanic rocks. According to our results, there is an indication of a high-velocity layer (VS ≥ 3.0 km/s) at the depth range of 2 to 5 km, which may indicate the presence of dense, cooled magma. There is a clear indication of intrusion of the low-velocity anomaly/body into the higher velocity layer, in the depth range of approximately 3.0 to 4.5 km. The corresponding low-velocity body is interpreted as a hot magma chamber associated with the young eruption of Damavand. The chamber is located southwest of the crater, which stretches from the west to beneath the Damavand cone.

Crustal structure of the northern margin of the North China Craton and adjacent region from SinoProbe02 North China seismic WAR/R experiment

Abstract The SinoProbe02 North China seismic experiment consisted of three seismic recording efforts along a 450 km long profile. The profile extended from northwest of Beijing, across the northern margin of the North China Craton (NCC), and across the southeastern portion of the Central Asian Orogenic Belt (CAOB) to the Solonker suture zone. We processed, analyzed, and modeled the data collected during a wide-angle reflection and refraction effort. The recording of seismic waves from 8 explosions was conducted in 4 deployments of 300 Texan recorders with station spacings of 1–1.5 km. The P-wave field provided good quality data for most of the profile. Arrivals of refracted and reflected waves from sediments and basement (Pg), intracrustal phases (PcP, PlP), and the Moho (PmP, Pn) were typically observed. Travel time modeling was done layer-by-layer using the top to bottom approach and was constrained by a coincident deep reflection seismic profile. The initial velocity model was altered by trial and error, and the forward model was updated by damped least-squares inversion. In our modeling, calculated travel times fit observed arrivals for all 1439 traces with an average RMS error of 0.113 s. The final 2-D P-wave velocity model contains many features of tectonic significance: (1) the velocity structure varies significantly from the NCC to the CAOB; (2) thicker crust appears beneath the Yinshan–Yanshan belt and was probably generated by compression in the early Jurassic and modified during the craton's destabilization and extension; the flat and relatively shallow Moho in the CAOB may be attributed to the extension; (3) strong velocity variations beneath the Bainaimiao arc and Ondor Sum subduction accretion complex indicate that multiple pulses of magmatism occurred during the complex tectonic evolution of this area.

Tomographic upper-mantle velocity structure beneath the Iranian Plateau

The Iranian plateau is one of the most structurally complex and tectonically inhomogeneous regions in the world. In this study, we analyze Pn arrival-times from regional seismicity in order to resolve lateral velocity variations within the uppermost-mantle under the Iranian Plateau. More than 48,000 Pn first arrival times selected from the EHB catalog were used with epicentral distances of 200 to 1600km. We used regularized isotropic and anisotropic damped least-squares inversion to image lateral velocity variations in the upper mantle. Our velocity model, with high lateral resolution, shows positive anomalies in the Zagros mountain belt with a distinct transition approximately along the Main Zagros Thrust to the lower mantle velocity zone of Central Iran. Anomalously low velocities are observed predominantly beneath NW Iran and eastern Turkey, suggesting a zone of relatively weak mantle. Low velocity region under the Damavand volcano reveals the hot upper mantle beneath the central Alborz mountains.

Modeling of Rayleigh wave dispersion in Iberia

Phase and group velocities of 15–70 s Rayleigh waves propagating across the Iberian Peninsula have been transformed into local dispersion curves by linear inversion of travel times. The procedure permits that the waveform dispersion to be obtained as a continuous period-dependent velocity function at grid points belonging to the area probed by the waves, thus providing phase- and group-velocity contour maps for several periods within the interval of interest. The regionalization process rests on a homogeneous initial data set in which the number of observations remains almost constant for all periods of reference. Damped least-squares inversion of the local dispersion curves for shear-wave velocity structure is performed to obtain depth-dependent S-wave velocity profiles at the grid points covering the model region. The reliability of the results should improve significantly owing to the use of phase and group velocities simultaneously. On this basis, we have built horizontal depth sections that give an updated view of the seismic velocity structure of the peninsula at lithospheric and upper mantle depths (20–200 km). After averaging all the pure-path S-wave velocities previously determined at each grid point, the velocity-depth models so obtained for major tectonic units allow the comparison between the Hercynian basement and other areas of Mesozoic folding and Tertiary basins.

A Damped Least-Squares Inversion Program for the Interpretation of Schlumberger Sounding Curves

A Damped Least-Squares Inversion Program for the Interpretation of Schlumberger Sounding Curves

Resolving three-dimensional anisotropic structure with shear wave splitting tomography

SUMMARY Shear wave splitting observations are a commonly used tool for inferring anisotropy and flow within the Earth's interior. Here we present the development and validation of a new technique for imaging anisotropy in the upper mantle using local events—shear wave splitting tomography (SWST). The mantle is parametrized as a 3-D block model of crystallographic orientations with the elastic properties of olivine and orthopyroxene, and both orthorhombic and hexagonal symmetries are tested. To efficiently forward calculate splitting, the Christoffel equation is used to progressively split the horizontal components of a synthetic wavelet in each block of the model, and predicted shear wave splitting parameters are obtained with an eigenvalue minimization technique. Numerically calculated partial derivatives are utilized in a linearized, damped least-squares inversion to solve for a best-fitting model of crystallographic orientations. To account for the non-linear properties of shear wave splitting, the inversion is applied iteratively and partial derivatives are recalculated after each iteration. A starting model that incorporates information from predicted splitting parameters is found by spatially averaging fast directions and the ratio of observed-to-predicted splitting times. Models from inversions utilizing this average starting model reach lower misfit levels than do inversions with a random or uniform starting model. Modelling results using synthetic data from several anisotropic structures (i.e. sharp lateral and vertical variations in anisotropy) both within an idealized and a real (Nicaragua–Costa Rica) subduction zone illustrate the capabilities and limitations of SWST. With a station spacing of 25 km in an idealized subduction zone containing uniformly spaced events down to 225 km, both the azimuth and dip of crystallographic axes are resolvable to a depth of 100–150 km and lateral heterogeneities in anisotropy on a scale of 50 km at arc and forearc distances from the trench are retrieved. Spatial resolution of anisotropy at scales of 75 km is possible further into the backarc above 150 km depth. The geometry of stations and observed seismicity in the Nicaragua–Costa Rica subduction zone yields partial to good resolution at scales of 50–75 km beneath the forearc, arc and limited regions of the backarc down to 100 km, and resolution at coarser scales is possible in wider regions beneath the backarc. Given the distributions of seismic sources within many subduction zones and the advances in broad-band seismic array deployments, this new method offers a powerful means with which to constrain the orientation of anisotropic fabric in the upper mantle.

Joint inversion of gravity and magnetic data for two-layer models

Gravity and magnetic data are inverted jointly in terms of a model consisting of an interface separating two layers having a constant density and magnetization contrast. A damped least-squares inversion is used to determine the topography of the interface. The inversion requires knowledge of the physical property contrasts across the interface and its average depth. Since the relationship between model parameters and data is weakly nonlinear, a constant damped least-squares inverse is used during the iterative solution search. The effect of this inverse is closely related to a downward continuation of the field to the average interface depth. The method is used to map the base of the Sept-Iles mafic intrusion, Quebec, Canada, and the shape of the central uplift at the Chicxulub impact crater, Yucatan, Mexico. At Sept-Iles, the intrusion reaches a thickness of [Formula: see text], coincident with the maximum gravity anomaly, south of the intrusion center. At Chicxulub, the top of the central uplift is modeled to be [Formula: see text] deep and has a single peak form.

Three-dimensional upper-mantleS-velocity model for the Eurasia-Africa plate boundary region

SUMMARY A new regional S-velocity study resolving the Eurasia‐Africa plate boundary region from the Azores to the eastern Mediterranean Sea is presented. The resolution of existing velocity models has been complemented by using new seismic broad-band data recorded by the temporary MIDSEA network and at permanent European seismic stations. Following the partitioned waveform inversion method, we interactively fitted the waveforms of S and Rayleigh wave trains of more than 1100 seismograms. The linear constraints on upper-mantle S velocity provided by the waveform fits have been combined with independent estimates of Moho depth in a linear damped least-squares inversion for S velocity and crustal thickness. The resulting S-velocity structure for the Mediterranean Sea shows strong lateral variations, confirming the complex evolution of this plate boundary region. The upper mantle along the Eurasia‐Africa suture zone is characterized by high-velocity material representing subducted oceanic lithosphere. This signature can be followed to depths of 300‐500 km, depending on the region and resolution. A high-velocity body, possibly representing a fragment of subducted lithosphere, has been imaged beneath eastern Spain at a depth between 250 and 500 km. Not only convergence has been recorded in the upper mantle, but extension also has its own signature beneath the