Jean-Paul Montagner

Global upper mantle tomography of seismic velocities and anisotropies

A data set of 2600 paths for Rayleigh waves and 2170 paths for Love waves enabled us to retrieve three-dimensional distributions of different seismic parameters. Shallow layer corrections have been carefully performed on phase velocity data before regionalization and inversion at depth. The different seismic parameters include the five parameters of a radially anisotropic medium and the eight azimuthal anisotropic parameters as defined by Montagner and Nataf. It is found that the lateral heterogeneities of velocities and anisotropies in the upper mantle are dominated down to 250–30 km by plate tectonics with slow velocities below ridges, high velocities below continents and a velocity increasing with the age of the seafloor. Anisotropy is present in this whole depth range and the directions of maximum velocities are in good agreement with absolute plate velocities. Below 300 km, there is a sharp decreasing of the amplitude of lateral heterogeneities of seismic velocities and anisotropies. Below 450 km, lateral heterogeneities display a degree 2 and to a less extent a degree 6 pattern. Therefore, between 250 km and 450 km, there is a transition region where vertical circulation of matter is possible as shown by subducted slabs and “plumes” of slow velocities but which probably separates two types of convection. The first one is closely related to plate tectonics and to the distribution of continents. The second one dominates below 450 km and is characterized by two downgoing and two upgoing flows.

Global‐scale analysis of the mantle Pds phases

A global-scale analysis of travel times of the mantle Pds phases (converted from P to S from the discontinuity at a depth d beneath the receiver) is presented. Most of the data are related to P′410′s and P′660′s, recorded by the broadband stations of Geoscope, Incorporated Research Institutions for Seismology (IRIS) and other networks. In order to provide a good global-scale coverage, published measurements at 37 stations are complemented by our measurements at 45 stations. Lateral resolution of our data is many times higher than that of SS and its precursors, previously used for similar purposes, but we have fewer data in the oceans. Lateral variations of travel times of the major Pds phases are strongly correlated with the lateral P and S velocity variations in the uppermost mantle, as revealed by tomographic studies. The scaling coefficient between the teleseismic S and P residuals, as found from the variations of travel times of the major Pds phases, is around 3.7 for North America and around 3.3 for the rest of the world. Almost all the differential travel times t(P′660′s) – t(P′410′s) are in the range of ±1 s, relative to the average value of 24.0 s. If the lateral variations of the velocities within the mantle transition zone (MTZ) can be neglected, the corresponding variations of thickness of the MTZ are in the range of ±10 km, relative to the standard depth of 250 km, and do not show significant correlation with either velocities in the upper mantle above the MTZ or with the variations of thickness of the MTZ, as found in some recent studies of precursors to SS. There is a weak but marginally significant positive correlation between the variations of thickness of the MTZ inferred from our data and the S velocity variations in the MTZ, as given by Su et al. [1994]. This correlation suggests that both variables contain temperature-dependent components. Lateral variability of the MTZ is also manifested by extreme weakness of P′410′s at several locations. This may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity. At some stations there are arrivals that can be interpreted as P′520′s. This interpretation implies that the strengh and/or sharpness of 520 km discontinuity, as well as its depth, are laterally variable in a broad range.

Petrological constraints on seismic anisotropy

The purpose of this paper is to investigate the correlations between anisotropic parameters for different orientations and mineralogies for realistic mineralogical and petrological models of the upper mantle. Such correlations make it possible to reduce the number of independent parameters required in seismic modelling of the upper mantle. The variation with depth and with degree of random orientation is also investigated. It is shown that the anisotropic combinations involved in an equivalent transversely isotropic medium (azumithally averaged velocities) with a vertical symmetry axis are strongly correlated but the combinations involved in azimuthal anisotropy are more weakly correlated. Comparison with available seismic data does not yet allow discrimination, based on anisotropy, between different petrological models. It is shown that, even if a proportion of random orientation is introduced, it should be possible to achieve petrological discrimination from seismic data for some orientation configurations, if the inverse problem is well-posed, i.e., takes account of the correlations between anisotropic parameters. These correlations are incorporated in an inversion process to show that it is possible to explain the dataset used to derive the PREM, by alternative anisotropic models for the uppermost 220 km.

Time‐reversal imaging of seismic sources and application to the great Sumatra earthquake

The increasing power of computers and numerical methods (like spectral element methods) allows continuously improving modelization of the propagation of seismic waves in heterogeneous media and the development of new applications in particular time reversal in the three-dimensional Earth. The concept of time-reversal (hereafter referred to as TR) was previously successfully applied for acoustic waves in many fields like medical imaging, underwater acoustics and non destructive testing. We present here the first application at the global scale of TR with associated reverse movies of seismic waves propagation by sending back long period time-reversed seismograms. We show that seismic wave energy is refocused at the right location and the right time of the earthquake. When TR is applied to the Sumatra-Andaman earthquake (26 Dec. 2004), the migration of the rupture from the south towards the north is retrieved. Therefore, TR is potentially interesting for constraining the spatio-temporal history of complex earthquakes.

The unique dynamics of the Pacific Hemisphere mantle and its signature on seismic anisotropy

Abstract The portion of Earth’s mantle lying below the hemispherical region occupied by the Pacific Ocean basin is characterized by a unique style of convection. The unique dynamics below the ‘Pacific Hemisphere’ are revealed by analyzing the mantle flow predicted on the basis of high-resolution seismic images of mantle structure. In the Pacific Hemisphere we find a dome-like upwelling, originating in the lower mantle, which actively transports material into the upper mantle. A unique aspect of this flow, found only in the Pacific Hemisphere, is a ‘demi-tour’ or U-turn of the circulation which occurs in the shallow mantle, west of the East Pacific Ridge. This ‘demi-tour’ connects the westward movement of the Pacific plate to the dome-like upwelling below the central Pacific Ocean and it also provides an explanation for the asymmetric distribution of seismic anisotropy on either side of the ridge axis. In a global analysis of the correlation between mantle flow and seismic anisotropy, we find the strongest agreement occurs in the Pacific Hemisphere mantle.

Phase velocity structure from Rayleigh and Love waves in Tibet and its neighboring regions

Deep geodynamic processes involved in the continental collision between India and Asia are still controversial. To address this issue, phase velocities of 619 Rayleigh waves and 254 Love waves have been inverted by using an anisotropic tomographic technique. Such a technique enables us not only to retrieve phase velocity distributions but also to map azimuthal anisotropy on a large scale. Only phase velocity anomalies, in the period range 50–200 s, are dealt with here, but the azimuthal anisotropy has been taken into account in the inversion process. Moreover, because of the important influence of shallow layers on the inversion results, an inversion of phase velocities corrected for shallow layers has been performed. To correct raw phase velocities, the 3SMAC crust (heterogeneous reference Earth model) has been used. The comparison of phase velocity distributions for Love and Rayleigh waves with and without corrections of surficial layers implies that much of the low-velocity zone, visible at short periods, in central Tibet, can be explained by unusually thick crust. At periods longer than 100 s, Tibet is, overall, characterized by high-velocity anomalies. Such anomalies seem to converge toward the center of the plateau as period increases. We conclude that the phase velocity structure beneath Tibet results, at short periods, from thick crust and possible partial melting in the uppermost lithosphere and, at long periods, from the inward plunge of the cold lithospheric mantle surrounding the high plateau.

Seismic anisotropy beneath Tibet: evidence for eastward extrusion of the Tibetan lithosphere?

Strong seismic anisotropy beneath Tibet has recently been reported from the study of SKS shear wave splitting. The fast split waves are generally polarized in an easterly direction, close to the present day direction of motion of the Tibetan crust relative to stable Eurasia, as deduced from Holocene slip rates on the major active faults in and around Tibet. This correlation may be taken to suggest that the whole Tibetan lithosphere is being extruded in front of indenting India and that the anisotropic layer is the deforming asthenosphere, that accommodates the motion of the Tibetan lithosphere relative to the fixed mantle at depth. Uncertainties about this motion are at present too large to bring unambiguous support to that view. Assuming that this view is correct however, a simple forward model is used to compute theoretical delay times as a function of the thickness of the anisotropic layer. The observed delay times would require a 50–100 km thick anisotropic layer beneath south-central Tibet and an over 200 km thick layer beneath north-central Tibet, where particularly hot asthenosphere has been inferred. This study suggests that the asthenospheric anisotropy due to present absolute block motion might be dominant under actively deforming continents.

Antarctica II: Upper-mantle structure from velocities and anisotropy

Abstract To improve the lateral resolution of three-dimensional seismic wave velocity models in Antarctica and the surrounding oceans, we have analysed direct earthquake-to-station Rayleigh-wave data observed on the vertical high-gain long-period and the very long period components of seven GEOSCOPE stations located in the southern hemisphere and three other stations at equatorial latitudes. The phase velocities of Rayleigh waves along 400 well-distributed paths are obtained in the period range 60–300 s, by fitting the data with synthetic seismograms computed with known source parameters in a reference earth model represented by the Preliminary Reference Earth Model (PREM). Corrections for shallow layers have been carefully applied to the observed phase velocities. The geographical distributions of phase velocities and azimuthal anisotropy are then computed with the tomographic method without any a priori regionalization developed by Montagner (Ann. Geophys., 4(B3): 283–294, 1986). The results show some new and important features of Antarctica and the southern hemisphere. The locations of velocity anomalies are well resolved. The eastern part of Antarctica corresponds to a craton-like structure down to depths of about 250 km, and the highest velocities are observed in Enderby Land, where some of the oldest rocks in the world have been sampled. The low velocities are located along the ridges encircling the Antarctic continent. The lowest velocities appear in some areas corresponding to hotspots (Crozet, Kerguelen, Macquarie and Balleny Islands). Also, an elongated low velocity is found on the western flank of the Transantarctic Mountains, which might be related to the existence of a rift zone similar to the African rift. The Australia-Antarctica Discordance (AAD) presents slow velocities near the surface but fast velocities below the lithosphere. These main features are discussed in the framework of the Gondwana hypothesis and the earlier supercontinent. The first azimuthal anisotropy results are also discussed. Anisotropy values are smaller within the Antarctic continent than in the surrounding oceans. They are also small in the AAD but particularly large in the areas around it, suggesting an active tectonic process characterized by a downward flow at depth, a good candidate for a cold spot or a new subduction zone.

Constrained reference mantle model

A dataset of updated normal mode eigenperiods is used to develop a new average upper mantle model. The starting model takes account of recent body wave and surface wave models and of constraints derived from different petrological models. Different parameterizations with or without anisotropy down to different depths are checked. Anisotropy seems to be required at least down to 200 km and preferably down to 400 km but with a smaller amplitude than in the preliminary reference Earth model (PREM). The new models provide velocity gradients down to 400 km in good agreement with finite strain trajectories. The S-wave velocity jump at 400 km is smaller than previously found from regional studies. If the S-wave and P-wave velocity jumps at the 400-km discontinuity are considered simultaneously, this discontinuity cannot be explained by an upper mantle composed mainly of olivine.

Imaging of Complex Media with Acoustic and Seismic Waves

Time-Reversal Invariance and the Relation between Wave Chaos and Classical Chaos.- Acoustic Time-Reversal Mirrors.- Ocean Acoustics, Matched-Field Processing and Phase Conjugation.- Time Reversal, Focusing and Exact Inverse Scattering.- Detection and Imaging in Complex Media with the D.O.R.T. Method.- Ultrasound Imaging and Its Modeling.- Nondestructive Acoustic Imaging Techniques.- Seismic Anisotropy Tomography.- Elastic-Wave Propagation in Random Polycrystals: Fundamentals and Application to Nondestructive Evaluation.- Imaging the Viscoelastic Properties of Tissue.- Estimation of Complex-Valued Stiffness Using Acoustic Waves Measured with Magnetic Resonance.- A New Approach for Traveltime Tomography and Migration Without Ray Tracing.- Simple Models in the Mechanics of Earthquake Rupture.