1,721,138 research outputs found
Mantle structure and dynamic topography in the Mediterranean Basin
No full text[1] We study the contribution of mantle flow to surface deformation within the Mediterranean Basin. Flow is modeled numerically based on lateral changes in mantle temperature estimated from tomography models. We find that modeling results are significantly affected by the properties of the selected tomography models. Shear-velocity models based on surface-wave observations achieve the highest resolution of upper-mantle structure, and, as a result, are most successful in predicting microplate motion and dynamic topography. Citation: Boschi, L., C. Faccenna, and T. W. Becker (2010), Mantle structure and dynamic topography in the Mediterranean Basin, Geophys. Res. Lett., 37, L20303, doi:10.1029/2010GL045001. RI Becker, Thorsten/A-6665-201
Vertical coherence in mantle heterogeneity from global seismic data
No full textThe vertical coherence of mantle structure is of importance for a range of dynamic issues including convective mass transport and the geochemical evolution of Earth. Here, we use seismic data to infer the most likely depth ranges of strong, global changes in the horizontal pattern of mantle heterogeneity. We apply our algorithm to a comprehensive set of measurements, including various shear- and compressional-wave delay times and Love- and Rayleigh-wave fundamental mode and overtone dispersion, so that tomography resolution is as high as possible at all mantle depths. We find that vertical coherence is minimum at similar to 100 km and similar to 800 km depths, corresponding to the base of the lithosphere and the transition between upper and lower mantle, respectively. The D '' layer is visible, but not as prominent as the shallower features. The rest of the lower mantle is, essentially, vertically coherent. These findings are consistent with slab stagnation at depths around, and perhaps below, the 660-km phase transition, and inconsistent with global, chemically distinct, mid-mantle layering. Citation: Boschi, L., and T. W. Becker (2011), Vertical coherence in mantle heterogeneity from global seismic data, Geophys. Res. Lett., 38, L20306, doi:10.1029/2011GL049281
On the statistical significance of correlations between synthetic mantle plumes and tomographic models
No full textIn a recent article, [Boschi, L., Becker, T.W., Stemberger, B., 2007. Mantle plumes: dynamic models and seismic images. Geochem. Geophys. Geosyst. 8, Q10006. doi:10.1029/2007GC001733] (BBS07) have reevaluated the degree to which slow seismic tomography anomalies correlate with the possible locations of plume-like mantle upwellings connected to surface hotspots. They showed that several, but not all, hotspots are likely to have a deep mantle origin. importantly, they found that when advection of plume conduits in mantle flow is considered, such correlations are significantly higher than when conduits are assumed to be vertical under hotspots. The validity of these statements depends, however, on the definition of statistical significance. BBS07 evaluated the significance of correlation through simple Student's t tests. Anderson (personal communication, July 2007) questioned this approach, given that the true information content of published tomography models is generally unknown, and proposed, instead, to evaluate the significance of correlation by comparing tomographic results with Monte Carlo simulations of randomly located plumes. Following this approach, we show here that the correlation found by BBS07 between advected plumes and slow anomalies in S-velocity tomography is less significant than previously stated, but still significant (at the 99.7% confidence level). We also find an indication that the seismic/geodynamic correlation observed by BBS07 does not only reflect the natural tendency of plumes to cluster in slow/hot regions of the mantle: although realistically advected, and thereby biased towards such regions, our random plumes correlate with slow tomographic anomalies significantly less than the plume models of BBS07. A less significant correlation with plume models characterizes P-velocity tomography; the correlation is, however, enhanced, if flow is computed from tomographic models with amplified heterogeneity, possibly accounting for the known resolution limits of global seismic data. In summary, the conclusions of BBS07 are confirmed: even at relatively long wavelengths, tomographic models are consistent with the presence of a number of tilted, whole-mantle plume-shaped slow anomalies, connected to surface hotspots. (C) 2008 Elsevier B.V. All rights reserved. RI Becker, Thorsten/A-6665-201
Stationary-phase integrals in the cross correlation of ambient noise
Full text linkThe cross correlation of ambient signal allows seismologists to collect data even in the absence of seismic events. “Seismic interferometry” shows that the cross correlation of simultaneous recordings of a random wavefield made at two locations is formally related to the impulse response between those locations. This idea has found many applications in seismology, as a growing number of dense seismic networks become available: cross-correlating long seismic records, the Green's function between instrument pairs is “reconstructed” and used, just like the seismic recording of an explosion, in tomography, monitoring, etc. These applications have been accompanied by theoretical investigations of the relationship between noise cross correlation and the Green's function; numerous formulations of “ambient noise” theory have emerged, each based on different hypotheses and/or analytical approaches. The purpose of this study is to present most of those approaches together, providing a comprehensive overview of the theory. Understanding the specific hypotheses behind each Green's function recipe is critical to its correct application. Hoping to guide nonspecialists who approach ambient noise theory for the first time, we treat the simplest formulation (the stationary-phase approximation applied to smooth unbounded media) in detail. We then move on to more general treatments, illustrating that the “stationary-phase” and “reciprocity theorem” approaches lead to the same formulae when applied to the same scenario. We show that a formal cross correlation/Green's function relationship can be found in complex, bounded media and for nonuniform source distributions. We finally provide the bases for understanding how the Green's function is reconstructed in the presence of scattering obstacles.Geoscience & EngineeringCivil Engineering and Geoscience
Global multiresolution models of surface wave propagation: comparing equivalently regularized Born and ray theoretical solutions
No full textI invert a large set of teleseismic phase-anomaly observations, to derive tomographic maps of fundamental-mode surface wave phase velocity, first via ray theory, then accounting for finite-frequency effects through scattering theory, in the far-field approximation and neglecting mode coupling. I make use of a multiple-resolution pixel parametrization which, in the assumption of sufficient data coverage, should be adequate to represent strongly oscillatory Frechet kernels. The parametrization is finer over North America, a region particularly well covered by the data. For each surface-wave mode where phase-anomaly observations are available, I derive a wide spectrum of plausible, differently damped solutions; I then conduct a trade-off analysis, and select as optimal solution model the one associated with the point of maximum curvature on the trade-off curve. I repeat this exercise in both theoretical frameworks, to find that selected scattering and ray theoretical phase-velocity maps are coincident in pattern, and differ only slightly in amplitude
Measures of resolution in global body wave tomography
No full text[1] The resolution of tomographic images is most often evaluated through synthetic tests: the inversion algorithm used to derive the image itself is applied to a synthetic data set having the same source-station geometry of the real one, but theoretical travel times computed from a chosen "input model'' (e.g., a checkerboard). The similarity between input model and solution of the synthetic test, used as a measure of resolution, has the major shortcoming of depending on the choice of the input model. Conversely, the similarity of the "model resolution matrix'' (R) to the identity matrix is a rigorous measure of resolution that does not depend on any input model, but has the drawback of being computationally heavy. In the past decade, several authors have devised complicated algorithms for the approximate or iterative derivation of R. I show here that parallel Cholesky factorization of A(T) . A ( A being the matrix that identifies the linear inverse problem), feasible on shared-memory multiprocessor servers, provides an efficient way of determining both least squares solutions and resolution matrices in global tomography. I apply this procedure in an evaluation of the resolution of mantle structure from a global P-wave travel time data set
Using the Post-Widder formula to compute the Earth's viscoelastic Love numbers
No full textThe post-glacial or post-seismic relaxation of a Maxwell viscoelastic earth, 1-D or slightly laterally heterogeneous, can be calculated in a normal-mode approach, based on an application of the propagator technique. This semi-analytical approach, widely documented in the literature, allows to compute the response of an earth model whose rheological parameters vary quite strongly with depth, at least as accurately and efficiently as by 1-D numerical integration (Runge-Kutta). Its main drawback resides in the need to identify the roots of a secular polynomial, introduced after reformulating the problem in the Laplace domain, and required to transform the solution back to the time domain. Root finding becomes increasingly difficult, and ultimately unaffordable, as the complexity of rheological profiles grows: the secular polynomial gradually gets more ill behaved, and a larger number of more and more closely spaced roots is to be found. Here, we apply the propagator method to solve the Earth's viscoelastic momentum equation, like in the above-mentioned normal-mode framework, but bypass root finding, using, instead, the Post-Widder formula to transform the solution, found again in the Laplace domain, back to the time domain. We test our method against earlier normal-mode results, and prove its effectiveness in modelling the relaxation of earth models with extremely complex rheological profiles
Surface-Wave Attenuation From Seismic Ambient Noise: Numerical Validation and Application
No full textWe evaluate, by numerical tests, whether surface-wave attenuation can be determined from ambient-noise data. We generate synthetic recordings of numerically simulated ambient seismic noise in several experimental setups, characterized by different source distributions and different values of attenuation coefficient. We use them to verify that the source spectrum can be reconstructed from ambient recordings (provided that the density of sources and the attenuation coefficient are known) and that true attenuation can be retrieved from normalized cross correlations of synthetic signals. We then apply the so validated method to real continuous recordings from 33 broadband receivers distributed within the Colorado Plateau and Great Basin. A preliminary analysis of the signal-to-noise ratio as a function of azimuth reveals a SW-NE preferential directionality of the noise sources within the secondary microseism band (6–8 s), consistent with previous studies. By nonlinear inversion of noise data we find the attenuation coefficient in the area of interest to range from ∼ 1 × 10−5 m−1 at 0.3 Hz to ∼ 4.5 × 10−7 m−1 at 0.065 Hz, and confirm the statistical robustness of this estimate by means of a bootstrap analysis. The result is compatible with previous observations based on both earthquake-generated and ambient Rayleigh waves. In this regard, the method proves to be promising in accurately quantifying surface-wave attenuation at relatively high frequencies
Whole Earth tomography from delay times of P, PcP, and PKP phases: Lateral heterogeneities in the outer core or radial anisotropy in the mantle?
No full textWe perform tomographic inversions of compressional wave travel time measurements, in order to image the laterally varying radial anisotropy of P wave velocity in the entire mantle. In order to achieve an adequate sensitivity to both horizontal and vertical P velocity throughout the mantle, we include in our data set travel time measurements corresponding to ray paths of very different geometries. Specifically, we use data associated with the phases P, PcP(core-reflected) and PKPbc, PKPdf (core-refracted), all extracted from the bulletins of the International Seismological Centre, 1964-1993, PcP, PKPbc, and PKPdf data are also sensitive to the topography of the core-mantle boundary (CMB) and (PKP only) to the velocity structure of the Earth's core. These circumstances require that we also allow for CMB topographic anomalies as free parameters in our inversions and investigate the possibility of the existence of lateral heterogeneities in the outer core [e.g., Ritzwoller et al., 1986; Stevenson, 1987; Wahr and de Vries, 1989; Vasco and Johnson, 1998]. The solution models that we propose include alternatively a laterally heterogeneous outer core, a radially anisotropic mantle, or both, so that trade-offs between those features can also be evaluated. We conclude that only minor, and very localized, radially anisotropic anomalies are present in the middle and lower mantle, the most prominent being located in the deepest 400 km, under the western Pacific. We find a discrepancy, particularly evident by a comparison between different models of the CMB topography, between the travel time measurements that are sensitive td the Earth's core and those that are not; we show that it is not possible to explain this discrepancy only in terms of radial anisotropy of the mantle and/or lateral heterogeneity of the outer core
High- and low-resolution images of the Earth's mantle: Implications of different approaches to tomographic modeling
No full textRecently published images of the Earth's mantle are characterized by a nominal resolution much higher than that used in previous studies, where substantially different techniques were employed. The agreement between such "high-resolution" and "low-resolution" images often seems very poor. In an attempt to determine the reason for this discrepancy, we analyze how the choice of inversion algorithm (exact or iterative), regularization (norm or roughness minimization), and parameterization (spherical harmonics up to a variable degree, blocks) affects a tomographic model. We also investigate the effects of the varying density of the data coverage on the final solution. In our experiments we employ two seismic data sets: Rayleigh wave phase velocity at 75 s period and P wave travel times. We construct a new model of P velocity in the mantle (BDP98) based on the International Seismological Center bulletins 1964-1992. We use our findings in an evaluation of recent mantle models, including our own, focusing on similarities and discrepancies between models of different nominal resolution. In all the models the long-wavelength component is the most stable. However, consistent high-resolution details, probably corresponding to features of the real Earth, are also seen. In general, we conclude that most of the differences between existing tomographic models derive from the arbitrary choices made in the process of defining and solving the inverse problem, rather than from actual errors or approximations
- …
