10 research outputs found
Near-Surface Model Prediction and Refinement by Full Waveform Surface Wave Inversion
No full textGenetic algorithm full waveform inversion (GA-FWI) is able to predict complex shear-wave velocity (Vs) models fairly from surface waves, even in the case when very limited or null a-priori information is available (Xing and Mazzotti, 2017a). Out of consideration for computing time reduction, a two-grid approach (Sajeva et al., 2016; Aleardi and Mazzotti, 2017; Mazzotti et al., 2017), one coarse grid for the inversion and one small grid for the modeling, is recommended for the method. Thus, we generally obtain smooth velocity models whose wavelengths are dependent on the coarse grid spacing. In this paper, we show that these models are suitable starting models for FWI approaches with local optimization methods and that, in general, significant details of the depth model can be retrieved. Instead, we do not discuss the influences caused by different surface wave modeling strategies (Thorbecke and Draganov, 2011; Groos, 2013; Xing and Mazzotti, 2016) and by assumptions in wave modeling (Xing and Mazzotti, 2017b), thus we focus on model prediction and refinement
A New Workflow for Surface Wave FWI Combining Genetic Algorithm and Gradient-Based Optimization Algorithms
No full textWe propose a new workflow for surface wave (SW) inversion by combining the two-grid genetic algorithm (GA) and gradient-based full waveform inversion (FWI). The workflow circumvents the notorious requirement of having a “good enough” starting model” in gradient-based SW FWI. At the 1st step of the workflow, without any a-priori information, by employing a coarse inversion grid and by inverting the lower frequencies only of the observed data, GA SW FWI reconstructs the long-wavelength structures of the subsurface. Then, in the next step of the workflow, the GA predicted model is used as the initial model for gradient-based SW FWI. In virtue of the higher efficiency of the gradient-based method, finer inversion grids are adopted and data with higher frequencies can be inverted yielding refined predicted models. We discuss our approach making use of two synthetic examples that reproduce complex near-surface models and we show that fair inversion outcomes are obtained. Models predicted by GA SW FWI are proved to be adequate initial models for gradient-based SW FWI. In addition, the examples confirm the extremely strong impacts that initial models have on gradient-based SW FWI results
Tomographic Image Interpretation and Central-Western Mediterranean-Like Upper Mantle Dynamics From Coupled Seismological and Geodynamic Modeling Approach
No full textThe Central-Western Mediterranean (CWM) is one of the most complex tectonic setting on Earth. Episodes of slab rollback, break-off and tearing, the opening of back-arc extensional basins (i.e., Liguro-Provencal, Alborean, Algerian and Tyrrhenian basins), the presence of large mountain ranges, active volcanoes and violent earthquakes have made the Mediterranean an ideal environment to study a wide range of geodynamic processes and an important target for seismological studies (e.g, seismic tomography). Here we build a geodynamic model which, although it does not reproduce its exact tectonic structure (e.g., due to the limits of the numerical method, approximations in the initial setup, etc), presents multiple and geometrically complex subduction systems analogous to those found in the CWM. The tectonic evolution of this model is estimated with petrological-thermo-mechanical 3D simulations, then, we dynamically compute the upper mantle fabrics and seismic anisotropy as a function of the strain history and local P-T conditions. After comparing the model with SKS splitting observations in order to quantify the discrepancies with the true Central-Western Mediterranean, we use the elastic tensors predicted for the modeled configuration to perform 3D P-wave anisotropic tomography by inverting synthetic P-wave delay times. Using the geodynamic model as reference, we evaluate the capabilities of a recently developed seismic tomography technique to recover the isotropic anomalies and anisotropy patterns related to a complex subduction environment in different conditions, such as poor data coverage and bad data quality. We observe that, although P-wave tomography still remains a powerful tool to investigate the upper mantle, the reliability of the retrieved structures strongly depends on data quality and data density. Furthermore, the recovered anisotropic patterns are consistent with those of the target model, but in general an underestimation of the anisotropy magnitude in the upper mantle is observed. In the light of future developments, our study suggests that by combining micro- and macro-scale geodynamic simulations and seismological modeling of seismic anisotropy it will be possible to reproduce, at least to a first order, the tectonic evolution of real study regions (e.g., the Mediterranean) thus providing fundamental constraints on the processes that have contributed in shaping their current geological scenario
Artificial age-independent seismic anisotropy, slab thickening and shallowing due to limited resolving power of (an)isotropic tomography
No full textSeismic anisotropy is key to constrain mantle flow, but it is challenging to image and interpret it. Existing large-scale tomography models of seismic anisotropy typically show large discrepancies, which can lead to completely distinct geodynamical interpretations. To better quantify the robustness of anisotropy tomography, we create a 2-D ridge-to-slab geodynamic model and compute the associated fabrics. Using the resulting 21 elastic constants, we compute seismic full waveforms, which are inverted for isotropic and radially anisotropic structure. We test the effects of different data coverage and levels of regularization on the resulting images and on their geodynamical interpretation. Within the context of our specific imposed conditions and source-receiver configuration, the retrieved isotropic images exhibit substantial artificial slab thickening and loss of the slab's high-velocity signature below similar to 100 km depth. Our results also show that the first-order features of radial anisotropy are well retrieved despite strong azimuthal anisotropy (up to 2.7 per cent) in the input model. On the other hand, regularization and data coverage strongly control the detailed characteristics of the retrieved anisotropy, notably the depth-age dependency of anisotropy, leading to an artificial flat depth-age trend shown in existing anisotropy tomography models. Greater data coverage and additional complementary data types are needed to improve the resolution of (an)isotropic tomography models
Slab Geometry and Upper Mantle Flow Patterns in the Central Mediterranean From 3D Anisotropic P‐Wave Tomography
No full textWe present the first three‐dimensional (3D) anisotropic teleseismic P‐wave tomography model of the upper mantle covering the entire Central Mediterranean. Compared to isotropic tomography, it is found that including the magnitude, azimuth, and, importantly, dip of seismic anisotropy in our inversions simplifies isotropic heterogeneity by reducing the magnitude of slow anomalies while yielding anisotropy patterns that are consistent with regional tectonics. The isotropic component of our preferred tomography model is dominated by numerous fast anomalies associated with retreating, stagnant, and detached slab segments. In contrast, relatively slower mantle structure is related to slab windows and the opening of back‐arc basins. To better understand the complexities in slab geometry and their relationship to surface geological phenomenon, we present a 3D reconstruction of the main Central Mediterranean slabs down to 700 km based on our anisotropic model. P‐wave seismic anisotropy is widespread in the Central Mediterranean upper mantle and is strongest at 200–300 km depth. The anisotropy patterns are interpreted as the result of asthenospheric material flowing primarily horizontally around the main slabs in response to pressure exerted by their mid‐to‐late Cenezoic horizontal motion, while sub‐vertical anisotropy possibly reflects asthenospheric entrainment by descending lithosphere. Our results highlight the importance of anisotropic P‐wave imaging for better constraining regional upper mantle geodynamics
Model CWM evolution
No full textModel CWM evolution
Supplementary Movie S1 from "Lo Bue R, Rappisi F, Vanderbeek BP and Faccenda M (2022) Tomographic Image Interpretation and Central-Western Mediterranean-Like Upper Mantle Dynamics From Coupled Seismological and Geodynamic Modeling Approach.
Front. Earth Sci. 10:884100.
doi: 10.3389/feart.2022.884100"</p
Crustal Structure of Etna Volcano (Italy) From P‐Wave Anisotropic Tomography
No full textSeveral seismic tomographic studies have been carried out to outline the intricate interplay between tectonics and magma uprising at Etna volcano. Most of these studies assume a seismically isotropic crust. Here we employ a novel methodology that accounts for the anisotropic structure of the crust. Anisotropy patterns are consistent with the Etna structural trends, unveiling the depth extent of fault segments. A high-velocity volume, deepening toward the northwest, identifies the subducting foreland units that appear to confine a low-velocity anomaly, interpreted as the expression of magmatic fluids within the crust. A discontinuity, likely tectonic in origin, affects the subducting units and allows magma transfer from depth to the surface. This structural configuration may explain the presence of such a very active basaltic strato-volcano within an atypical collisional geodynamic context.[In our study, we investigate the complex relationship between tectonics and magma ascent at Etna volcano. Unlike previous research, we consider the anisotropic nature of the crust, meaning its properties vary depending on direction. Using our innovative methodology, we have uncovered patterns aligned with Etna fault segments, revealing their depth and distribution. We have identified the presence of magma fluids within the crust and potential pathways for its uprising. Our results provides an explanation for the emplacement of the very active Etna basaltic volcano within an unusual collisional geodynamic setting.]This study conducts the first P-wave anisotropic tomography using local earthquakes recorded in the area of Etna volcano The tomography resolves three-dimensional (3D) isotropic and anisotropic structures beneath the volcano Anisotropy reveals geological features providing valuable insights into the interplay between tectonic deformation and volcanic activit
3‐D Mantle Flow and Structure of the Mediterranean From Combined P‐Wave and Splitting Intensity Anisotropic Tomography
No full textWe present a novel three-dimensional anisotropic seismic tomography model of the Mediterranean region, achieved through the simultaneous inversion of P-wave travel-times and SKS splitting intensity. This dual approach has allowed us to obtain a comprehensive tomographic model that not only delineates the primary structural features of the area but also sheds light on its tectonic evolution. Our findings reveal that the isotropic component of the model is dominated by fast anomalies associated with retreating, stagnant, and detached slab segments including the Alboran, Apennine, and Alpine slabs in the central and western Mediterranean, and the Dinaric, Carpathian, and Hellenic slabs in the east. Slower mantle structures are associated with slab windows and back-arc basin formation, such as those observed in the Tyrrhenian, Apennine and Hellenic regions. The recovered anisotropic patterns provide crucial insights into the tectonic history of the Mediterranean, highlighting periods of collision and tectonic relaxation. Notably, we observe a range of plunge angles, with both near-horizontal and steeply dipping anisotropic fabrics present in different regions, reflecting the influence of horizontal and vertical asthenospheric flow. By interpreting the high-velocity zones as subducting lithosphere, we construct a detailed 3-D model of the main slabs and analyzed the surrounding Pwave anisotropic patterns. This work represents the first comprehensive anisotropic tomography study of the entire Mediterranean region. Plain Language Summary This study presents a new 3-D model of the deep structure of the Mediterranean region, offering a clearer understanding of the tectonic evolution of the area. Unlike previous research, which assumed the Earth's mantle behaves the same in all directions, our study takes into account its anisotropic nature, where the mantle's properties vary depending on the direction seismic waves travel through it. By combining different types of seismic data, the study reveals intricate interactions between tectonic plates, including the subduction of some plates and the extension or fragmentation of others. The findings highlight significant tectonic features, such as the Alboran, Apennine, and Alpine slabs in the central and western Mediterranean, and the Dinaric, Carpathian, and Hellenic slabs in the east. The study also reveals important mantle dynamics, including horizontal and vertical flow patterns, slab detachment, and volcanic processes. This research provides a deeper insight into the Mediterranean's geological history and the forces that continue to shape the region.PublishedJCR Journa
Artificial age-independent seismic anisotropy, slab thickening and shallowing due to limited resolving power of (an)isotropic tomography
No full textSeismic anisotropy is key to constrain mantle flow, but it is challenging to image and interpret it. Existing large-scale tomography models of seismic anisotropy typically show large discrepancies, which can lead to completely distinct geodynamical interpretations. To better quantify the robustness of anisotropy tomography, we create a 2-D ridge-to-slab geodynamic model and compute the associated fabrics. Using the resulting 21 elastic constants, we compute seismic full waveforms, which are inverted for isotropic and radially anisotropic structure. We test the effects of different data coverage and levels of regularization on the resulting images and on their geodynamical interpretation. Within the context of our specific imposed conditions and source–receiver configuration, the retrieved isotropic images exhibit substantial artificial slab thickening and loss of the slab’s high-velocity signature below ∼100 km depth. Our results also show that the first-order features of radial anisotropy are well retrieved despite strong azimuthal anisotropy (up to 2.7 per cent) in the input model. On the other hand, regularization and data coverage strongly control the detailed characteristics of the retrieved anisotropy, notably the depth–age dependency of anisotropy, leading to an artificial flat depth–age trend shown in existing anisotropy tomography models. Greater data coverage and additional complementary data types are needed to improve the resolution of (an)isotropic tomography models
Artificial age-independent seismic anisotropy, slab thickening and shallowing due to limited resolving power of (an)isotropic tomography
No full textSeismic anisotropy is key to constrain mantle flow, but it is challenging to image and interpret it. Existing large-scale tomography models of seismic anisotropy typically show large discrepancies, which can lead to completely distinct geodynamical interpretations. To better quantify the robustness of anisotropy tomography, we create a 2-D ridge-to-slab geodynamic model and compute the associated fabrics. Using the resulting 21 elastic constants, we compute seismic full waveforms, which are inverted for isotropic and radially anisotropic structure. We test the effects of different data coverage and levels of regularization on the resulting images and on their geodynamical interpretation. Within the context of our specific imposed conditions and source–receiver configuration, the retrieved isotropic images exhibit substantial artificial slab thickening and loss of the slab’s high-velocity signature below ∼100 km depth. Our results also show that the first-order features of radial anisotropy are well retrieved despite strong azimuthal anisotropy (up to 2.7 per cent) in the input model. On the other hand, regularization and data coverage strongly control the detailed characteristics of the retrieved anisotropy, notably the depth–age dependency of anisotropy, leading to an artificial flat depth–age trend shown in existing anisotropy tomography models. Greater data coverage and additional complementary data types are needed to improve the resolution of (an)isotropic tomography models
