Monte-Carlo inversion for a global shear-velocity model of the crust and upper mantle (original) (raw)

A global shear velocity model of the upper mantle from fundamental and higher Rayleigh mode measurements

Journal of Geophysical Research, 2012

1] We present DR2012, a global SV-wave tomographic model of the upper mantle. We use an extension of the automated waveform inversion approach of which improves our mapping of the transition zone with extraction of fundamental and higher-mode information. The new approach is fully automated and has been successfully used to match approximately 375,000 Rayleigh waveforms. For each seismogram, we obtain a path average shear velocity and quality factor model, and a set of fundamental and higher-mode dispersion and attenuation curves. We incorporate the resulting set of path average shear velocity models into a tomographic inversion. In the uppermost 200 km of the mantle, SV wave heterogeneities correlate with surface tectonics. The high velocity signature of cratons is slightly shallower (≈200 km) than in other seismic models. Thicker continental roots are not required by our data, but can be produced by imposing a priori a smoother model in the vertical direction. Regions deeper than 200 km show no velocity contrasts larger than AE1% at large scale, except for high velocity slabs within the transition zone. Comparisons with other seismic models show that current surface wave datasets allow to build consistent models up to degrees 40 in the upper 200 km of the mantle. The agreement is poorer in the transition zone and confined to low harmonic degrees (≤10).

Savani: a variable-resolution whole-mantle model of anisotropic shear-velocity variations based on multiple datasets

We present a new global whole-mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE-rani) based on~43,000,000 surface wave and~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short-period group-velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high-resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts. Preliminary Reference Earth Model (PREM) [Dziewoński and Anderson, 1981], one of the most widely used 1-D reference models, incorporates radial anisotropy from the Moho (24.4 km depth) to the 220 km discontinuity. It is mainly constrained by joint analysis of Rayleigh and Love wave data. Montagner and Anderson [1989] and Montagner and Kennett [1996] modified PREM using additional normal-mode and travel time data, respectively. Moreover, Beghein et al. [2006] also modified PREM using normal-mode data in a wide frequency range. Fully 3-D radially anisotropic models for both the upper and whole mantle have been developed since the 1980s [e.g.,

Anisotropic shear‐wave velocity structure of the Earth's mantle: A global model

Journal of Geophysical Research: Solid Earth, 2008

We combine a new, large data set of surface wave phase anomalies, long‐period waveforms, and body wave travel times to construct a three‐dimensional model of the anisotropic shear wave velocity in the Earth's mantle. Our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model, simultaneous inversion for velocity and anisotropy, as well as discontinuity topographies, and implementation of nonlinear crustal corrections for waveforms. A comparison of our new three‐dimensional model, S362ANI, with two other models derived from comparable data sets but using different techniques reveals persistent features: (1) strong, ∼200‐km‐thick, high‐velocity anomalies beneath cratons, likely representing the continental lithosphere, underlain by weaker, fast anomalies extending below 250 km, which may represent continental roots, (2) weak velocity heterogeneity between 250...

Multimode Rayleigh wave inversion for shear wave speed heterogeneity and azimuthal anisotropy of the Australian upper mantle

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.

Upper-mantle shear-wave structure under East and Southeast Asia from Automated Multimode Inversion of waveforms

Geophysical Journal International, 2015

We present a new Sv-velocity model of the upper mantle under East and Southeast Asia constrained by the inversion of seismic waveforms recorded by broad-band stations. Seismograms from earthquakes occurred between 1977 and 2012 are collected from about 4786 permanent and temporary stations in the region whenever and wherever available. Automated Multimode Inversion of surface and multipleS waveforms is applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. The equations are then solved for the seismic velocity perturbations in the crust and upper mantle with respect to a three-dimensional (3-D) reference model and a realistic crust. Major features of the lithosphere-asthenosphere system in East and Southeast Asia are identified in the resulting model. At lithospheric depth, low velocities can be seen beneath Tibet, whereas high velocities are found beneath cratonic regions, such as the Siberian, North China, Yangtze,) Tarim, and Dharwarand cratons. A number of microplates are mapped and the interaction with neighbouring plates is discussed. Slabs from the Pacific and Indian Oceans can be seen in the upper mantle. Passive marginal basins and subduction zones are also properly resolved.

Global upper‐mantle tomography with the automated multimode inversion of surface and S‐wave forms

2008

We apply the Automated Multimode Inversion of surface and S-wave forms to a large global data set, verify the accuracy of the method and assumptions behind it, and compute an S vvelocity model of the upper mantle (crust-660 km). The model is constrained with ∼51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations. Structure of the mantle and crust is constrained by waveform information both from the fundamentalmode Rayleigh waves (periods from 20 to 400 s) and from S and multiple S waves (higher modes). In order to enhance the validity of the path-average approximation, we implement the automated inversion of surface-and S-wave forms with a three-dimensional (3-D) reference model. Linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3-D reference, in this study composed of a 3-D model of the crust and a one-dimensional (1-D), global-average depth profile in the mantle below. Waveform information is related to shear-and compressional-velocity structure within approximate waveform sensitivity areas. We use two global triangular grids of knots with approximately equal interknot spacing within each: a finely spaced grid for integration over sensitivity areas and a rougher-spaced one for the model parametrization. For the tomographic inversion we use LSQR with horizontal and vertical smoothing and norm damping. We invert for isotropic variations in Sand P-wave velocities but also allow for S-wave azimuthal anisotropy-in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity. The lateral resolution of the resulting isotropic upper-mantle images is a few hundred kilometres, varying with data sampling. We validate the imaging technique with a 'spectral-element' resolution test: inverting a published global synthetic data set computed with the spectral-element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately. This test confirms both the accuracy of the implementation of the method and the validity of the JWKB and path-average approximations as applied in it. Reviewing the tomographic model, we observe that lowS v-velocity anomalies beneath mid-ocean ridges and backarc basins extend down to ∼100 km depth only, shallower than according to some previous tomographic models; this presents a close match to published estimates of primary melt production depth ranges there. In the seismic lithosphere beneath cratons, unambiguous high velocity anomalies extend to ∼200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South America; Tanzania) they are neighboured by volcanic areas near cratonic boundaries. The images of these low-velocity zones may indicate hot material-possibly of mantle-plume origin-trapped or spreading beneath the thick cratonic lithosphere.

Joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations

Journal of Geophysical Research: Solid Earth, 2015

We present a new global whole-mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE-rani) based on~43,000,000 surface wave and~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short-period group-velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high-resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts. Preliminary Reference Earth Model (PREM) [Dziewoński and Anderson, 1981], one of the most widely used 1-D reference models, incorporates radial anisotropy from the Moho (24.4 km depth) to the 220 km discontinuity. It is mainly constrained by joint analysis of Rayleigh and Love wave data. Montagner and Anderson [1989] and Montagner and Kennett [1996] modified PREM using additional normal-mode and travel time data, respectively. Moreover, Beghein et al. [2006] also modified PREM using normal-mode data in a wide frequency range. Fully 3-D radially anisotropic models for both the upper and whole mantle have been developed since the 1980s [e.g.,

Shear velocity structure of central Eurasia from inversion of surface wave velocities

Physics of the Earth and Planetary Interiors, 2001

We present a shear velocity model of the crust and upper mantle beneath central Eurasia by simultaneous inversion of broadband group and phase velocity maps of fundamental-mode Love and Rayleigh waves. The model is parameterized in terms of velocity depth profiles on a discrete 2 • × 2 • grid. The model is isotropic for the crust and for the upper mantle below 220 km but, to fit simultaneously long period Love and Rayleigh waves, the model is transversely isotropic in the uppermost mantle, from the Moho discontinuity to 220 km depth. We have used newly available a priori models for the crust and sedimentary cover as starting models for the inversion. Therefore, the crustal part of the estimated model shows good correlation with known surface features such as sedimentary basins and mountain ranges. The velocity anomalies in the upper mantle are related to differences between tectonic and stable regions. Old, stable regions such as the East European, Siberian, and Indian cratons are characterized by high upper-mantle shear velocities. Other large high velocity anomalies occur beneath the Persian Gulf and the Tarim block. Slow shear velocity anomalies are related to regions of current extension (Red Sea and Andaman ridges) and are also found beneath the Tibetan and Turkish-Iranian Plateaus, structures originated by continent-continent collision. A large low velocity anomaly beneath western Mongolia corresponds to the location of a hypothesized mantle plume. A clear low velocity zone in v SH between Moho and 220 km exists across most of Eurasia, but is absent for v SV . The character and magnitude of anisotropy in the model is on average similar to PREM, with the most prominent anisotropic region occurring beneath the Tibetan Plateau. (A. Villaseñor).

Multimode Rayleigh wave inversion for heterogeneity and azimuthal anisotropy of the Australian upper mantle

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.

Waveform inversion of surface wave data: test of a new tool for systematic investigation of upper mantle structures

Geophysical Journal International, 2007

In most tomographic inversion of surface wave data, the long-period seismograms are first interpreted in terms of dispersion and/or attenuation curves before performing an inversion in terms of laterally and depth-varying properties. An alternative to this approach is to perform a direct waveform inversion or, as in Cara & LCvSque (1987), to use another set of secondary observables built up from the seismograms. In this paper, we systematically test with actual Rayleigh wave records this latter technique by considering laterally homogeneous models. We use for this purpose a set of recent long-period digital records from the Geoscope station Dumont d'urville, Antarctica, for surficial events in the south Indian Ocean and the southeast Pacific Ocean, and for intermediate-depth events in the Vanuatu and Kermadec trenches. In addition to the obvious advantages of being able to deal with situations where overtones are present in the seismogram, it is found that the waveform inversion procedure allows us to reach a better depth resolution than in classical inversion of dispersion curves, even when only the fundamental mode is present in the seismogram. Quite good resolution is obtained to depth as large as 300km for S velocity when using surficial events located at a few thousand kilometres from the station, while classical surface wave studies do not allow us to resolve S velocity at depth larger than 150 km for these events. When intermediatedepth events are used at distances of about 5000 km, the presence of overtones in the signal allows us to get resolution to depth as large as 600 km for S velocity. Poorer resolution is obtained in both situations for the attenuation factor. S velocity appears furthermore to be more robust than attenuation at depth where good resolution is achieved. Due to the great sensitivity of surface wave amplitude to departure from the assumption of lateral homogeneity, more sophisticated direct modelling would be required to get more confidence in the inverted attenuation models.