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Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations
Full text linkOn constraining 3D seismic anisotropy in subduction, mid-ocean-ridge, and plume environments with teleseismic body wave data
Full text linkConventional seismic tomography studies consider the Earth's interior as mechanically isotropic, despite seismic anisotropy being widely observed. This current standard approach to seismic imaging is likely to lead to significant artefacts in tomographic images with first-order effects on interpretations and hinders the quantitative integration of seismology with geodynamic flow models. Although a few methodologies have been proposed for carrying out anisotropic tomography, their ability in simultaneously recovering isotropic and anisotropic structures has not been rigorously tested. In this contribution we use geodynamic and seismological modeling to predict the elastic properties and synthetic teleseismic P- and S-wave travel-time datasets for three different tectonic settings: a plume rising in an intraplate setting, a divergent margin, and a subduction zone. Subsequently, we perform seismic anisotropy tomography testing a recently developed methodology that allows for the inversion of an arbitrarily oriented weakly anisotropic hexagonally symmetric medium using multiple bodywave datasets. The tomography experiments indicate that anisotropic inversions of separate and joint P- and Swave travel-times are capable of recovering the first order isotropic velocity anomalies and anisotropic patterns. In particular, joint P- and S-wave anisotropic inversions show that by leveraging both phases it is possible to greatly mitigate issues related to imperfect data coverage common in seismology and reduce parameter tradeoffs. In contrast, by neglecting seismic anisotropy, isotropic tomographic models provide no information on the mantle fabrics and in all cases are contaminated by strong velocity artifacts. In the inversions the magnitude of anisotropy (as well as that of seismic anomalies) is always underestimated owing to regularization procedures and smearing effects. It follows that the true seismic anisotropy of mantle rocks is likely higher than estimated from anisotropic tomographies, and more consistent with predictions from laboratory and numerical micromechanical experiments. Altogether, these results suggest that anisotropic body-wave tomography could provide unprecedented information about the Earth's deep geological structure, and that the latter could be better recovered by complementing teleseismic body-wave travel-times with other geophysical datasets
Imaging upper mantle anisotropy with traveltime and splitting intensity observations from teleseismic shear waves: insights from tomographic reconstructions of subduction simulations
Full text linkTeleseismic traveltime tomography remains one of the most popular methods for obtaining images of Earth's upper mantle. However, despite extensive evidence for an elastically anisotropic mantle, the isotropic assumption remains commonplace in such imaging studies. This can result in significant model artefacts which in turn may yield misguided inferences regarding mantle dynamics. The nature of anisotropy-induced apparent velocity anomalies has been well-documented in P-wave imaging and various strategies have been proposed to constrain both isotropic and anisotropic heterogeneity from these data. In contrast, few studies have explored the consequences for shear wave tomography and no practical framework for the anisotropic inversion of S-wave delays exists. Here, we propose a new method for constraining arbitrarily oriented hexagonal anisotropy using both traveltime and splitting intensity observations from direct S phases. Our approach accounts for polarization and finite-frequency effects and allows for isotropic starting models. The imaging method is validated through the tomographic analysis of a realistic synthetic dataset produced from waveform simulations through a geodynamic model of subduction. Results illustrate that neglecting anisotropy produces distortions in slab geometry and the appearance of sub- and supraslab low-velocity zones. Anisotropic inversions remove these artefacts while also constraining geodynamically relevant fabric properties including dip
Imaging Upper-Mantle Anisotropy with Transdimensional Bayesian Monte Carlo Sampling
No full textUnderdetermination is a condition affecting all problems in seismic imaging. It manifests mainly in the nonuniqueness of the models inferred from the data. This condition is exacerbated if simplifying hypotheses like isotropy are discarded in favor of more realistic anisotropic models that, although supported by seismological evidence, require more free parameters. Investigating the connections between underdetermination and anisotropy requires the implementation of solvers which explore the whole family of possibilities behind nonuniqueness and allow for more informed conclusions about the interpretation of the seismic models. Because these aspects cannot be investigated using traditional iterative linearized inversion schemes with regularization constraints that collapse the infinite possible models into a unique solution, we explore the application of transdimensional Bayesian Monte Carlo sampling to address the consequences of underdetermination in anisotropic seismic imaging. We show how teleseismic waves of P and S phases can constrain upper-mantle anisotropy and the amount of additional information these data provide in terms of uncertainty and trade-offs among multiple fields
A study of upper mantle anisotropy in Cascadia using teleseismic shear wave delays
Full text linkopenSeismic tomography is one of the most powerful tools available when attempting to characterise the Earth’s interior. Often an underlying assumption of these inversions is one of elastic mantle isotropy, when it is known that anisotropy is present within the upper mantle. Simple shear deformation of the mantle creates lattice-preferred orientation (LPO) of minerals, principally olivine which is an anisotropic mineral. Therefore where an LPO forms we expect a seismically anisotropic upper mantle. When a linearly polarised shear wave (S-wave) encounters this anisotropy it splits into two orthogonal waves with one component polarised along the plane of greatest seismic velocity. Determining the direction of anisotropy can reveal information about deformational history, flow and other properties in the upper mantle. Accounting for anisotropy will lead to more accurate seismic tomography models whilst providing indications of convective flow and structural fabrics within the upper mantle.
The area of this study is the Cascadia Subduction Zone in the Western USA. The relatively young and small Juan de Fuca (JDF) plate is subducting beneath the North American plate at the Cascadia trench. It is a complex geodynamic setting which comprises: JDF ocean ridge, Cascadia trench, forearc between the trench and Cascadia volcanic chain, a back arc region. This is bound to the North and South by triple junctions. This study utilises 593 seismic stations on 25 different networks and data from September 2011 to August 2015. Previous anisotropic S-wave tomography studies have focused mainly on the SKS phase. Conversion from P to S-wave at the core mantle boundary ensured that only anisotropy within the mantle will influence the S-wave arrival. This study instead focuses on 4 different S-wave phases: S, sS, Sdiff and sSdiff.
Teleseismic S-wave arrivals for 383 earthquake events were identified on the transverse channel of the seismic station. Initially the S-wave arrivals were analysed with reference to the AK135 earth reference model. Alignment of the S-waves was undertaken using multichannel cross correlation (MCC). A stacked version of all the S-wave arrivals was created to identify the waveform. The measured delay times and the stacked traces arrival time were combined to calculate the travel time. The stacked traces were then rotated to find the azimuth of the polarisation angle, the angle of maximum S-wave energy arrival. Subsequently a second MCC was completed in the polarisation direction to determine the delay time of the polarised wave, containing information about both isotropic and anisotropic velocity variations along the ray path.
Inverse tomographic modelling was completed using the updated travel times from both the transverse channel and the polarisation channel. Initially a deterministic inversion was utilised to produce 700km deep, 3D S-wave velocity models. For anisotropic inversions, a vector aligned with the axis of symmetry of the hexagonal velocity model was also calculated, allowing a visual representation of anisotropy. Following these results, a set of stochastic inversions based on the reversible jump Markov Chain Monte Carlo (rjMCMC) methodology were completed with the intention of comparing the two inversion methods for the same dataset.
From initial interpretations from both the isotropic and anisotropic deterministic inversion results, the subducting JDF plate can be observed as a steeply dipping fast velocity anomaly. Both models also display evidence of a possible gap within the subducting plate. A roughly horizontal circular pattern in the anisotropy vector at the southern edge of the JDF slab could be interpreted as toroidal mantle return flow caused by slab rollback of the subducting plate. Initial results from the isotropic stochastic inversion are comparable to the deterministic inversions. A stochastic anisotropic inversion is currently being undertaken
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Anisotropic seismic tomography with the reversible jump Markov chain Monte Carlo
Full text linkThe established isotropic tomographic models show the features of subduction zones in terms
of seismic velocity anomalies, but they are generally subjected to the generation of artifacts
due to the lack of anisotropy in forward modelling. There is evidence for the significant
influence of seismic anisotropy in the mid-upper mantle, especially for boundary layers like
subducting slabs. As consequence, in isotropic models artifacts may be misinterpreted as
compositional or thermal heterogeneities.
In this thesis project the application of a trans-dimensional Metropolis-Hastings method is
investigated in the context of anisotropic seismic tomography. This choice arises as a response
to the important limitations introduced by traditional inversion methods which use iterative
procedures of optimization of a function object of the inversion.
On the basis of a first implementation of the Bayesian sampling algorithm, the code is tested
with some cartesian two-dimensional models, and then extended to polar coordinates and
dimensions typical of subduction zones, the main focus proposed for this method. Synthetic
experiments with increasing complexity are realized to test the performance of the method
and the precautions for multiple contexts, taking into account also the possibility to apply
seismic ray-tracing iteratively.
The code developed is tested mainly for 2D inversions, future extensions will allow the
anisotropic inversion of seismological data to provide more realistic imaging of real subduction zones, less subjected to generation of artifacts
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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