Sergei Lebedev

Diffraction tomography using multimode surface waves

A new method is described that makes it feasible to include scattered and converted surface waves into waveform inversions for the three-dimensional (3-D) structure of the Earth. The single scattering (Born) approximation forms the basis of the method. In order to minimize the amplitude of the scattered wave field, the background model is first adapted to correct for nonconverted, forward-scattered wave energy. We then perform Born inversion of the difference between the measured and synthetic waveforms, including a suite of Love and Rayleigh modes. The Born approximation yields linear equations of the form Aδγ = δu Bor n, which allow the determination of the three-dimensional perturbations γ to the background model from the scattered wave field δu Born . This procedure is followed separately for each source-receiver pair to allow for optimized background models for each signal, as well as to minimize the computational burden. We winnow the data vector for each path by performing singular value decomposition using a diagonalization of AA T . In a realistic example we found that each vertical component seismogram yields 30-40 linear constraints on the 3-D Earth, significantly more than with conventional pure-path (WKBJ) inversions. In a synthetic test, one seismogram is shown to be able to image a simple model of a point scatterer off the great circle. As a spin-off of the formulation of the multimode inverse scattering problem, we not only obtain a series of eigenvectors that rank the sensitivity of a seismogram to Earth structure in a series of geometrical patterns, we also can compute the surface wave equivalent of a Fresnel zone.

Lithospheric structure in the Baikal–central Mongolia region from integrated geophysical‐petrological inversion of surface‐wave data and topographic elevation

[1] Recent advances in computational petrological modeling provide accurate methods for computing seismic velocities and density within the lithospheric and sub-lithospheric mantle, given the bulk composition, temperature, and pressure within them. Here, we test an integrated geophysical-petrological inversion of Rayleigh- and Love-wave phase-velocity curves for fine-scale lithospheric structure. The main parameters of the grid-search inversion are the lithospheric and crustal thicknesses, mantle composition, and bulk density and seismic velocities within the crust. Conductive lithospheric geotherms are computed using P-T-dependent thermal conductivity. Radial anisotropy and seismic attenuation have a substantial effect on the results and are modeled explicitly. Surface topography provides information on the integrated density of the crust, poorly constrained by surface waves alone. Investigating parameter inter-dependencies, we show that accurate surface-wave data and topography can constrain robust lithospheric models. We apply the inversion to central Mongolia, south of the Baikal Rift Zone, a key area of deformation in Asia with debated lithosphere-asthenosphere structure and rifting mechanism, and detect an 80–90 km thick lithosphere with a dense, mafic lower crust and a relatively fertile mantle composition (Mg# < 90.2). Published measurements on crustal and mantle Miocene and Pleistocene xenoliths are consistent with both the geotherms and the crustal and lithospheric mantle composition derived from our inversion. Topography can be fully accounted for by local isostasy, with no dynamic support required. The mantle structure constrained by the inversion indicates no major thermal anomalies in the shallow sub-lithospheric mantle, consistent with passive rifting in the Baikal Rift Zone.

Correlation between the shear-speed structure and thickness of the mantle transition zone

The 410 and 660 km seismic discontinuities that bound the mantle transition zone (TZ) are attributed to phase transformations in olivine structure. This implies that variations in TZ thickness ( HTZ) should correlate with those in TZ temperature. Pertinent seismic evidence has so far been ambiguous, however. We measure converted-wave (P ds) differential times tdiff = tP 660s − tP 410s in SE Asia and Australia and compare them with S-velocity (βTZ) estimates from regional tomographic models. Both tdiff and βTZ vary on a scale of a few hundred kilometers. Inferred variations in HTZ are up to ±30 km over length scales larger than 500 km, implying ±200 K thermal heterogeneity if the effect of composition can be neglected. tdiff and βTZ correlate strongly; the linear dependence of HTZ on the average temperature within the TZ is consistent with olivine Clapeyron slopes. We also show that this relationship holds on a global-scale as well, provided that the scalelengths and uncertainties of the variations in tdiff and βTZ are taken into account. These results confirm that the transformations in olivine structure give rise to the 410 and 660 km discontinuities globally.

Crustal anisotropy beneath Hudson Bay from ambient noise tomography: Evidence for post-orogenic lower-crustal flow?

The crust underlying Hudson Bay, Canada records a long and complex tectonic history. In this study, we investigate this region using tomographic inversion based on continuous ambient noise recordings from 37 broadband seismograph stations that encircle Hudson Bay. The ambient noise data were processed to obtain group-velocity dispersion measurements from 10–35 s period, which were inverted using an algorithm that incorporates the effects of anisotropy. This work is among the first in which ambient noise data have been used to investigate azimuthal anisotropy. The inversion method uses smoothing and damping to regularize the solution; due to the significantly increased number of model parameters relative to the isotropic case, we performed a careful analysis for parameter selection to determine whether “leakage” occurs between isotropic and anisotropic model parameters. We observe a robust pattern of anisotropic fast directions in the mid-crust that are consistent with large-scale tectonic trends based on magnetic-anomaly patterns. In particular, a distinctive double-indentor shape for the Superior craton is clearly expressed in both data sets. This pattern breaks down deeper in the crust, suggesting that some degree of lithospheric decoupling in the lower crust, such as channel flow, occurred during orogenesis. Given regional evidence for vertically coherent deformation in the crust and underlying mantle, we interpret this pattern in the lower crust as a tectonic overprint that post-dates the main phase of Trans-Hudson deformation. At most levels in the crust, we observe a profound change in direction of anisotropic fast direction across an inferred suture beneath Hudson Bay.

Toward a generalized plate motion reference frame

An absolute plate motion (APM) model is required to address issues such as the thermochemical evolution of Earths mantle. All APM models have to rely on indirect inferences, including those based on hot spots and seismic anisotropy, each with their own set of uncertainties. Here, we explore a seafloor spreading-aligned reference frame. We show that this reference frame fits azimuthal seismic anisotropy in the uppermost mantle very well. The corresponding Euler pole is close to those of hot spot reference frames, ridge motion minimizing models, and geodynamic estimates of net rotation and predicts clear trench motion patterns. We conclude that a net rotation pole guided by the spreading-aligned model (at 64°E, 61°S, with moderate rotation of ∼ 0.2 … 0.3°/Myr) could indeed represent a standard, comprehensive reference frame for present-day plate motions with respect to the deep mantle.

A plate tectonics oddity: Caterpillar-walk exhumation of subducted continental crust

Since plate tectonics began on Earth, grandiose “subduction factories” have continually shaped the continents, accreting continental blocks and new crust at the convergent plate boundaries. An enigmatic product of subduction factories is the high-pressure to ultrahigh-pressure (HP-UHP) metamorphic crustal rocks, regurgitated to Earth’s surface, sometimes from depths as great as 200 km. The Aegean backarc domain comprises two continental blocks that underwent HP metamorphism during the subduction of the African plate. Here, we use thermomechanical numerical simulations to show that subduction of small continental-lithosphere blocks separated by oceanic domains induces variations in the slab buoyancy, giving rise to episodic rollback-exhumation cycles. The single, self-consistent numerical model successfully reproduces the major structural patterns and pressure-temperature-time paths of HP rocks across the Aegean. We suggest that the “caterpillar walk” of exhuming HP rock units, revealed by our simulations, is a fundamental mechanism behind HP exhumation globally.

Joint inversion of long-period magnetotelluric data and surface-wave dispersion curves for anisotropic structure: Application to data from Central Germany

Geophysical datasets sensitive to different physical parameters can be used to improve resolution of Earths internal structure. Herein, we jointly invert long-period magnetotelluric (MT) data and surface-wave dispersion curves. Our approach is based on a joint inversion using a genetic algorithm for a one-dimensional (1-D) isotropic structure, which we extend to 1-D anisotropic media. We apply our new anisotropic joint inversion to datasets from Central Germany demonstrating the capacity of our joint inversion algorithm to establish a 1-D anisotropic model that fits MT and seismic datasets simultaneously and providing new information regarding the deep structure in Central Germany. The lithosphere/asthenosphere boundary is found at approx. 84 km depth and two main anisotropic layers with coincident most conductive/seismic fast-axis direction are resolved at lower crustal and asthenospheric depths. We also quantify the amount of seismic and electrical anisotropy in the asthenosphere showing an emerging agreement between the two anisotropic coefficients.

The upper mantle beneath the Philippine Sea region from waveform inversions

We present a three-dimensional S-velocity model for the upper mantle beneath the Philippine Sea region. It was derived from inversions of 281 broad band vertical-component seismograms recorded in the area at the Global Seismological Network (GSN) and SKIPPY portable array stations. We have been able to obtain high-resolution tomographic images spanning the depths down to 200–300 km and locally down to the upper transition zone. High-velocity subducting slabs and low-velocity volcanic arc regions are the dominant features of the model. Fast, thin lithosphere of back-arc basins is underlain by a prominent low-velocity zone. Low velocities at lithospheric depths are observed beneath the extinct Central Basin Ridge in the West Philippine Basin and close to the Eauripik Ridge that separates the East and West Caroline basins. High upper-mantle heterogeneity and resulting scattering presents a difficulty and limits the resolution, especially below 200–300 km. Explicit modeling of seismic wave diffraction may be necessary for a significant improvement in resolution.

Global Love wave overtone measurements

[1] Love wave phase velocities for fundamental and higher modes are difficult to measure because the different modes cannot easily be separated. Following Yoshizawa and Kennett (2002), we generate suites of path specific onedimensional shear wave velocity profiles using the Neighbourhood Algorithm of Sambridge (1999a). From this family of O(10 4 ) models both fundamental and higher mode phase velocities with mutually consistent uncertainties are calculated. We have fully automated the method and analysed over forty thousand Love wave seismograms from the GDSN and GEOSCOPE global networks from 1994–2004. Our phase velocity measurements agree remarkably well with previous studies, but we have been able to enlarge the available dataset dramatically. We present global Love wave phase velocity maps (up to the fifth overtone) with unprecedented resolution due to the improved path coverage. Comparing these maps to existing tomographic models, we discern evidence of significant anisotropy inthe lower mantlearound a depth of 1000 km in the Pacific. Citation: Visser, K., S. Lebedev, J. Trampert, and B. L. N. Kennett (2007), Global Love wave overtone measurements, Geophys. Res. Lett., 34, L03302,

The 660-km discontinuity within the subducting NW-Pacific lithospheric slab

Abstract The 660-km seismic discontinuity ( 660 ) in Earth’s mantle is generally attributed to the breakdown of the ringwoodite phase of olivine, but other mineral reactions are also thought to occur near 660-km depth. Recently, complex arrivals of P 660 s waves (converted from P to s at the 660 ) in active and recently active subduction zones have been interpreted as evidence for additional seismic discontinuities caused by the garnet–perovskite and garnet–ilmenite–perovskite phase transformations ( gt → pv , gt → il → pv ) at relatively low temperatures. Here we show that the P 660 s phases converting at the 660 within the subducting NW-Pacific slab beneath the station MDJ in Northeast China are clear and coherent, with no additional arrivals in the vicinity. P 660 s waves that convert near the boundaries of the area where the 660 occurs within the slab produce distinctly more complex, multiple arrivals, but they are more likely to be caused by small-scale topography rather than ‘multiplicity’ of the 660 . Our observations suggest that the gt → pv transformation and the gt → il → pv , if it occurs in the mantle, are spread over tens of kilometers and do not have sharp onsets visible to short-period seismic waves.