Length scales, patterns and origin of azimuthal seismic anisotropy in the upper mantle as mapped by Rayleigh waves (original) (raw)
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A simple method for inverting the azimuthal anisotropy of surface waves
Journal of Geophysical Research, 1986
We investigate the problem of oriented in preferential directions, observations retrieving anisotropy as a function of depth in of seismic anisotropy can provide information the mantle, from the observed azimuthal about the mineralogy and the deep structure of variations of Love and Rayleigh wave velocities. the mantle. Over the last few years, there has Following the approach of Smith and Dahlen, this been a large increase in the number of azimuthal dependence is expressed in terms of a observations implying anisotropy in the earth Fourier series of the azimuth e. For the most (for a review, see Crampin et al. [1984]). The general case of anisotropy (provided it is small early evidence was the discrepancy between enough), some simple linear combinations of the Rayleigh and Love wave dispersion [Anderson, elastic tensor coefficients are shown to describe 1961, 1966; Aki and Kaminuma, 1963], and the the total effect of anisotropy (both polarization azimuthal dependence of Pn velocities [Hesi, anisotropy and azimuthal anisotropy) on the 1964; Morris et al., 1969; Raitt et al., 1971 .
Upper mantle Anisotropy from Surface Wave studies
2008
Major advances in Structural Seismology during the last twenty years, are related to the emergence and development of more and more sophisticated 3D imaging techniques, usually named seismic tomography, at different scales from local to global. Progress has been made possible by the rapid developments in seismic instrumentation and by the extensive use of massive computation facilities. The scope of this chapter is limited to the tomographic elastic structure of the upper mantle. In order to obtain a good spatial coverage of this part of the Earth, it is necessary to make use of dispersive properties of surface waves. Most global tomographic models are still suffering severe limitations in lateral resolution, due to the imperfect data coverage, and to crude theoretical approximations. It is usually assumed that the propagating elastic medium is isotropic, which is a poor approximation. It is shown in this chapter how to take account of anisotropy of Earth’s materials and a complete ...
Global Tomography of Seismic Anisotropy and Interpretations
Seismic anisotropy, in spite of its inherent complexity is becoming an important ingredient for explaining various kinds of seismic data. Global tomographic models have been improved over years not only by an increase in the number of data but more importantly by using more general parameterizations, now including general anisotropy (both radial and azimuthal anisotropies). Different physical processes (lattice preferred orientation of crystals, cracks or fluid inclusions, fine layering...) related to strain field and/or stress field, give rise to observable seismic anisotropy (S-wave splitting, surface wave radial and azimuthal anisotropies), which makes its interpretation sometimes difficult and non-unique. Surface waves are well suited for imaging large scale (>1000km) lateral heterogeneities of velocity and anisotropy in the mantle by using fundamental and higher modes, since they provide an almost uniform lateral and azimuthal coverages, particularly below oceanic areas. The...
Weak elastic anisotropy in global seismology
Geological Society of America Special Papers
It has been known for over 50 years that seismic anisotropy must be included in a realistic analysis of most seismic data. The evidence for this consists of the observed dependency in many contexts (reviewed briefly here) of seismic velocity upon angle of propagation, and upon angle of S-wave polarization. Despite this well-established understanding, many current investigations continue to employ less realistic isotropic assumptions. One result is the appearance of artefacts which can be interpreted in terms of details of Earth structure, rather than of the restrictive assumptions in the analysis. The reason for this neglect of anisotropy is presumably the greater algebraic complexity, and the larger number of free parameters, of anisotropic seismics. However, the seismic anisotropy in the Earth is usually weak, and the equations for weak anisotropy are only marginally more complex than for isotropy. Further, the additional parameters are commonly required to describe the data. Moreover, the parameters of weak anisotropy defined below (combinations of the anisotropic elastic moduli) are less subject to compounding of uncertainty, and to spatial resolution issues, than are the individual anisotropic moduli themselves. Hence inversions should seek to fit data with these parameters, rather than with those individual moduli. We briefly review the theory for weak anisotropy, and present new equations for the weakly anisotropic velocities of surface waves. The analysis offers new insights on some well-known results found by previous investigations, for example the "Rayleigh wave-Love wave inconsistency", including the facts that Raleigh wave velocities depend not only on the horizontal SV velocity, but also on the anisotropy, and Love wave velocities depend not only on the horizontal SH velocity, but also on the anisotropy.
Geophysical Journal International, 2002
We present an azimuthally anisotropic 3-D shear-wave speed model of the Australian upper mantle obtained from the dispersion of fundamental and higher modes of Rayleigh waves. We compare two tomographic techniques to map path-average earth models into a 3-D model for heterogeneity and azimuthal anisotropy. Method I uses a rectangular surface cell parametrization and depth basis functions that represent independently constrained estimates of radial earth structure. It performs an iterative inversion with norm damping and gradient regularization. Method II uses a direct inversion of individual depth layers constrained by Bayesian assumptions about the model covariance. We recall that Bayesian inversions and discrete regularization approaches are theoretically equivalent, and with a synthetic example we show that they can give similar results. The model we present here uses the discrete regularized inversion of independent path constraints of Method I, on an equal-area grid. With the exception of westernmost Australia, we can retrieve structure on length scales of about 250 km laterally and 50 km in the radial direction, to within 0.8 per cent for the velocity, 20 per cent for the anisotropic magnitude and 20 • for its direction. On length scales of 1000 km and longer, down to about 200 km, there is a good correlation between velocity heterogeneity and geologic age. At shorter length scales and at depths below 200 km, however, this relationship breaks down. The observed magnitude and direction of maximum anisotropy do not, in general, appear to be correlated to surface geology. The pattern of anisotropy appears to be rather complex in the upper 150 km, whereas a smoother pattern of fast axes is obtained at larger depth. If some of the deeper directions of anisotropy are aligned with the approximately N-S direction of absolute plate motion, this correspondence is not everywhere obvious, despite the fast (7 cm yr −1 ) northward motion of the Australian plate. More research is needed to interpret our observations in terms of continental deformation. Predictions of SKS splitting times and directions, an integrated measure of anisotropy, are poorly matched by observations of shear-wave birefringence.
Physics of the Earth and Planetary Interiors, 2009
We compare a global compilation of shear-wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface-wave tomography. The currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online. The comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model. Comparing these predicted splitting parameters with the observed ones, we find a considerable correlation between the two datasets at global scale. This result is noteworthy, since such correlation did not seem to exist in previous studies. The spatial resolution associated with the two types of methods is rather different. While surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers, SKS splitting measurements have good lateral, but poor vertical resolution. The correlation can be understood in light of recent propositions that anisotropy seen by SKS splitting constrains mostly the upper mantle, and therefore a similar depth region as surface waves. The correlation also confirms the generally good quality of the shear-wave measurements, as well as that of the anisotropic tomography model. Crown
Statistical properties of seismic anisotropy predicted by upper mantle geodynamic models
Journal of Geophysical Research, 2006
1] We study how numerically predicted seismic anisotropy in the upper mantle is affected by several common assumptions about the rheology of the convecting mantle and deformation-induced lattice preferred orientations (LPO) of minerals. We also use these global circulation and texturing models to investigate what bias may be introduced by assumptions about the symmetry of the elastic tensor for anisotropic mineral assemblages. Maps of elasticity tensor statistics are computed to evaluate symmetry simplifications commonly employed in seismological and geodynamic models. We show that most of the anisotropy predicted by our convection-LPO models is captured by estimates based on a best fitting hexagonal symmetry tensor derived from the full elastic tensors for the computed olivine:enstatite LPOs. However, the commonly employed elliptical approximation does not hold in general. The orientations of the best fitting hexagonal symmetry axes are generally very close to those predicted for finite strain axes. Correlations between hexagonal anisotropy parameters for P and S waves show simple, bilinear relationships. Such relationships can reduce the number of free parameters for seismic inversions if this information is included a priori. The match between our model predictions and observed patterns of anisotropy supports earlier, more idealized studies that assumed laboratory-derived mineral physics theories and seismic measurements of anisotropy could be applied to study mantle dynamics. The match is evident both in agreement between predicted LPO at selected model sites and that measured in natural samples, and in the global pattern of fast seismic wave propagation directions.
We present an azimuthally anisotropic 3-D shear-wave speed model of the Australian upper mantle obtained from the dispersion of fundamental and higher modes of Rayleigh waves. We compare two tomographic techniques to map path-average earth models into a 3-D model for heterogeneity and azimuthal anisotropy. Method I uses a rectangular surface cell parametrization and depth basis functions that represent independently constrained estimates of radial earth structure. It performs an iterative inversion with norm damping and gradient regularization. Method II uses a direct inversion of individual depth layers constrained by Bayesian assumptions about the model covariance. We recall that Bayesian inversions and discrete regularization approaches are theoretically equivalent, and with a synthetic example we show that they can give similar results. The model we present here uses the discrete regularized inversion of independent path constraints of Method I, on an equal-area grid. With the exception of westernmost Australia, we can retrieve structure on length scales of about 250 km laterally and 50 km in the radial direction, to within 0.8 per cent for the velocity, 20 per cent for the anisotropic magnitude and 20 • for its direction. On length scales of 1000 km and longer, down to about 200 km, there is a good correlation between velocity heterogeneity and geologic age. At shorter length scales and at depths below 200 km, however, this relationship breaks down. The observed magnitude and direction of maximum anisotropy do not, in general, appear to be correlated to surface geology. The pattern of anisotropy appears to be rather complex in the upper 150 km, whereas a smoother pattern of fast axes is obtained at larger depth. If some of the deeper directions of anisotropy are aligned with the approximately N-S direction of absolute plate motion, this correspondence is not everywhere obvious, despite the fast (7 cm yr −1 ) northward motion of the Australian plate. More research is needed to interpret our observations in terms of continental deformation. Predictions of SKS splitting times and directions, an integrated measure of anisotropy, are poorly matched by observations of shear-wave birefringence.
Seismic anisotropy in the upper oceanic crust
Journal of Geophysical Research, 1985
Seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at Deep Sea Drilling Project (DSDP) site 504B on DSDP leg 92. Particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis. There are four diagnostic patterns'(1) Along symmetry axes, where vertically polarized shear waves (SV) are generated but horizontally polarized shear waves (SH) are not generated, the particle motions are purely vertical, (2) for azimuths at which both S V and SH are generated and the SH velocity is significantly faster than SV, a cruciform pattern with horizontal first motion is observed, (3) for azimuths at which both are generated and the SV velocity is significantly faster than SH, a cruciform pattern with vertical first motion is observed, and (4) for azimuths at which both are generated and SV and SH velocities are similar elliptical particle motions are observed. The shear wave particle motions and compressional wave travel times (from a 2-km radius circle) are consistent with an anisotropic model with hexagonal symmetry. The compressional wave velocity has a two theta azimuthal variation between 4.0 and 5.0 km/s. The symmetry axis is horizontal with an azimuth of N20øW + 10 ø. The spreading direction at the site (6 m.y. age) is north-south. The observed seismic anisotropy is most probably caused by the preferred orientation of large-scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting. oceanic crust have too much scatter in arrival time to unambiguously demonstrate anisotropy. Furthermore, the observation of Stephen [1981] of SH arriving after SV could be attributed to a scattering phenomenon rather than anisotropy. Consequently, althohgh seismic anisotropy has been a preferred explanation for a number of observations of this type, it has not been specifically required by the data. In this paper we present shear wave particle motion data which do specifically require anisotropic structure in the upper crust. The anisotropy indicated by the shear wave observations is consistent with the anisotropy indicated by compressional wave arrival times for a short-range (2 km) circle which
Variable Azimuthal Anisotropy in Earth's Lowermost Mantle
Science, 2004
A persistent reversal in the expected polarity of the initiation of vertically polarized shear waves that graze the Dµ layer (the layer at the boundary between the outer core and the lower mantle of Earth) in some regions starts at the arrival time of horizontally polarized shear waves. Full waveform modeling of the split shear waves for paths beneath the Caribbean requires azimuthal anisotropy at the base of the mantle. Models with laterally coherent patterns of transverse isotropy with the hexagonal symmetry axis of the mineral phases tilted from the vertical by as much as 20-are consistent with the data. Small-scale convection cells within the mantle above the Dµ layer may cause the observed variations by inducing laterally variable crystallographic or shapepreferred orientation in minerals in the Dµ layer.