Anatoli L. Levshin

Eurasian surface wave tomography: Group velocities

This paper presents the results of a study of the dispersion characteristics of broadband fundamental surface waves propagating across Eurasia. The study is broader band, displays denser and more uniform data coverage, and demonstrates higher resolution than previous studies of Eurasia performed on this scale. In addition, the estimated group velocity maps reveal the signatures of geological and tectonic features never before displayed in similar surface wave studies. We present group velocity maps from 20 s to 200 s period for Rayleigh waves and from 20 s to 125 s for Love waves. Broadband waveform data from about 600 events from 1988 through 1995 recorded at 83 individual stations across Eurasia have produced about 9000 paths for which individual dispersion curves have been estimated. Dispersion curves from similar paths are clustered to reduce redundancy, to identify outliers for rejection, and to assign uncertainty estimates. On average, measurement uncertainty is about 0.030–0.040 km/s and is not a strong function of frequency. Resolution is estimated from “checker-board” tests, and we show that average resolutions across Eurasia range from 5° to 7.5° but degrade at periods above about 100 s and near the periphery of the maps. The estimated maps produce a variance reduction relative to the Preliminary Reference Earth Model (PREM) of more than 90% for Rayleigh waves below 60 s period but reduce to about 70% between 80 and 200 s period. For Love waves, variance reductions are similar, being above 90% for most periods below 100 s and falling to 70% at 150 s. Synthetic experiments are presented to estimate the biases that theoretical approximations should impart to the group velocity maps, in particular source group time shifts, azimuthal anisotropy, and systematic event mislocations near subducting slabs. The most significant problems are probably caused by azimuthal anisotropy, but above 100 s the effect of source group time shifts may also be appreciable. These effects are probably below the signal levels that we interpret here, however. Many known geological and tectonic structures are observed in the group velocity maps. Of particular note are the signatures of sedimentary basins, continental flood basalts, variations in crustal thickness, backarc spreading, downgoing slabs, and continental roots. Comparison of the estimated group velocity maps with those predicted by CRUST5.1/S16B30 is qualitatively good, but there are significant differences in detail which provide new information that should help to calibrate future crustal and upper mantle models of Eurasia.

A Fast and Reliable Method for Surface Wave Tomography

Abstract — We describe a method to invert regional or global scale surface-wave group or phase-velocity measurements to estimate 2-D models of the distribution and strength of isotropic and azimuthally anisotropic velocity variations. Such maps have at least two purposes in monitoring the nuclear Comprehensive Test-Ban Treaty (CTBT): (1) They can be used as data to estimate the shear velocity of the crust and uppermost mantle and topography on internal interfaces which are important in event location, and (2) they can be used to estimate surface-wave travel-time correction surfaces to be used in phase-matched filters designed to extract low signal-to-noise surface-wave packets.¶The purpose of this paper is to describe one useful path through the large number of options available in an inversion of surface-wave data. Our method appears to provide robust and reliable dispersion maps on both global and regional scales. The technique we describe has a number of features that have motivated its development and commend its use: (1) It is developed in a spherical geometry; (2) the region of inference is defined by an arbitrary simple closed curve so that the method works equally well on local, regional, or global scales; (3) spatial smoothness and model amplitude constraints can be applied simultaneously; (4) the selection of model regularization and the smoothing parameters is highly flexible which allows for the assessment of the effect of variations in these parameters; (5) the method allows for the simultaneous estimation of spatial resolution and amplitude bias of the images; and (6) the method optionally allows for the estimation of azimuthal anisotropy.¶We present examples of the application of this technique to observed surface-wave group and phase velocities globally and regionally across Eurasia and Antarctica.

Crustal and upper mantle structure beneath Antarctica and surrounding oceans

We present and discuss a new model of the crust and upper mantle at high southern latitudes that is produced from a large, new data set of fundamental mode surface wave dispersion measurements. The inversion for a 2°×2° shear velocity model breaks into two principal steps: first, surface wave tomography in which dispersion maps are produced for a discrete set of periods for each wave type (Rayleigh group velocity, 18–175 s; Love group velocity, 20–150 s; Rayleigh and Love phase velocity, 40–150 s) and, second, inversion for a shear velocity model. In the first step, we estimate average resolution at high southern latitudes to be about 600 km for Rayleigh waves and 700 km for Love waves. The second step is a multistage process that culminates in a Monte Carlo inversion yielding an ensemble of acceptable models at each spatial node. The middle of the ensemble (median model) together with the half width of the corridor defined by the ensemble summarize the results of the inversion. The median model fits the dispersion maps at about the measurement error (group velocities, 20–25 m/s; phase velocities, 10–15 m/s) and the dispersion data themselves at about twice the measurement error. We refer to the features that appear in every member of the ensemble as “persistent.” Some of persistent features are the following: (1) Crustal thickness averages ∼27 km in West Antarctica and ∼40 km in East Antarctica, with maximum thicknesses approaching 45 km. (2) Although the East Antarctic craton displays variations in both maximum velocity and thickness, it appears to be a more or less average craton. (3) The upper mantle beneath much of West Antarctica is slow and beneath the West Antarctic Rift is nearly indistinguishable from currently dormant extensional regions such as the western Mediterranean and the Sea of Japan. Our model is therefore consistent with evidence of active volcanism underlying the West Antarctic ice sheet, and we hypothesize that the West Antarctic Rift is the remnant of events of lithospheric rejuvenation in the recent past that are now quiescent. (4) The Australian-Antarctic Discordance is characterized by a moderately high velocity lid to a depth of 70–80 km with low velocities wrapping around the discordance to the south. There is a weak trend of relatively high velocities dipping to the west at greater depths that requires further concentrated efforts to resolve. (5) The strength of radial anisotropy (vsh − vsv)/vsv in the uppermost mantle across the Southern Hemisphere averages ∼4%, similar to the Preliminary Reference Earth Model. Radial anisotropy appears to be slightly stronger in West Antarctica than in East Antarctica and in the thinner rather than the thicker regions of the East Antarctic craton.

A synoptic view of the distribution and connectivity of the mid-crustal low velocity zone beneath Tibet

[1] Based on 1–2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around Tibet, Rayleigh wave phase velocity maps are constructed from 10 s to 60 s period. A 3-D Vsv model of the crust and uppermost mantle is derived from these maps. The 3-D model exhibits significant apparently inter-connected low shear velocity features across most of the Tibetan middle crust at depths between 20 and 40 km. These low velocity zones (LVZs) do not conform to surface faults and, significantly, are most prominent near the periphery of Tibet. The observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones, rather than being localized horizontally near the edges of rigid blocks. The prominent LVZs are coincident with strong mid-crustal radial anisotropy in western and central Tibet and probably result at least partially from anisotropic minerals aligned by deformation, which mitigates the need to invoke partial melt to explain the observations. Irrespective of their cause in partial melt or mineral alignment, mid-crustal LVZs reflect deformation and their amplification near the periphery of Tibet provides new information about the mode of deformation across Tibet.

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

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 vSH between Moho and 220 km exists across most of Eurasia, but is absent for vSV. 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.

Automated detection, extraction, and measurement of regional surface waves

Our goal is to develop and test an effective method to detect, identify, extract, and quantify surface wave signals for weak events observed at regional stations. We describe an automated surface wave detector and extractor designed to work on weak surface wave signals across Eurasia at intermediate periods (8 s-40 s). The method is based on phase-matched filters defined by the Rayleigh wave group travel-time predictions from the broadband group velocity maps presented by RITZWOLLER and LEVSHIN (1998) and RITZWOLLER et al. (1998) and proceeds in three steps: Signal compression, signal extraction or cleaning, and measurement. First, the dispersed surface wave signals are compressed in time by applying an anti-dispersion or phase-matched filter defined from the group velocity maps. We refer to this as the ‘compressed signal.’ Second, the surface wave is then extracted by filtering ‘noise’ temporally isolated from the time-compressed signal. This filtered signal is then redispersed by applying the inverse of the phase-matched filter. Finally, we adaptively estimate spectral amplitude as well as group and phase velocity on the filtered signal. The method is naturally used as a detector by allowing origin time to slide along the time axis. We describe preliminary results of the application of this method to a set of nuclear explosions and earthquakes that occurred on or near the Chinese Lop Nor test site from 1992 through 1996 and one explosion on the Indian Rajasthan test site that occurred in May of 1998.

Surface-wave group velocity tomography of East Asia

Abstract : Group velocities of both Rayleigh and Love waves are used in a tomographic inversion to obtain group velocity maps of East Asia (60 deg E-140 deg E and 20 deg N-50 deg N). The period range studied is 30-70 seconds. For periods longer than 40 seconds, a high group velocity gradient clearly exists along longitude 105 deg E; the velocities are noticeably higher east of this longitude than west of this longitude. The Tibetan Plateau appears as a prominent low velocity (about 15%) structure in this area; central Tibet appears as the area with the lowest velocity. The North China Plain is an area of high velocities, probably as a result of thin crust. The variability of deep crustal and upper mantle structures underneath the different tectonic provinces in the study can clearly be seen. In a separate study, using the dataset above and that from the former Soviet Union, we have derived the Rayleigh tomographic images of a larger area (40 deg E-160 deg E and 20 deg N-70 deg N). While the Tibetan plateau still remains to be the most prominent low velocity features, two other features are also clear, a very high velocity Siberian platform and a high velocity ridge extending from Lake Baikal to Central Mongolia. These studies are useful in delineating tectonics.

Source effects on surface wave group travel times and group velocity maps

Abstract In most seismic surface wave studies observed group travel times are interpreted as time delays due entirely to the wave propagation along the wave path, and source effects are considered as negligibly small. This is in contrast with observed phase times where correction for the source phase is generally acknowledged to be mandatory. An important, yet unanswered, question is how neglecting source group time (SGT) in broadband surface wave studies will affect the accuracy of the measured group velocity curves and the tomographic maps constructed from these measurements. We consider here the effect of SGT on group velocity measurements for fundamental Rayleigh waves and report on its dependence on period (10–200 s), source mechanism, and source depth. Varying these parameters strongly affects the magnitude and azimuthal pattern of SGT shifts and we present statistics of certain salient functionals that characterize this dependence. SGT is negligible for periods less than about 75 s and for earthquake shallower than about 25 km. At longer periods and for deeper events, average SGT corrections are greater than 10 s in magnitude, which for continental scale studies translates into group velocity perturbations of 1–2%. We estimate the bias caused by uncorrected SGT in inversions for Rayleigh wave group velocity maps across the Eurasian continent. The largest perturbations to these maps (up to 1–2% for the 50-s period and up to 5% for the 100-s period) are found near the periphery of the continent where ray coverage is poor. From these results, some statistical estimates for adjacent wave paths (clusters), and the fact that SGT corrections display considerable sensitivities to earthquake depths, we conclude that the effects of SGT on group velocity tomographic images may safely be ignored at periods less than about 75 s and for shallow sources. Although such corrections are appreciable at longer periods for events deeper than about 25 km and should in principle be applied, the inherent inaccuracy of present day CMT solutions and group velocity measurements make these corrections practically non-essential for current group velocity tomographic studies.

Pn and Sn tomography across Eurasia to improve regional seismic event locations

Abstract This paper has three motivations: first, to map P n and S n velocities beneath most of Eurasia to reveal information on a length scale relevant to regional tectonics, second, to test recently constructed 3-D mantle models and, third, to develop and test a method to produce P n and S n travel time correction surfaces which are the 3-D analogue of travel time curves for a 1-D model. Our third motive is inspired by the need to improve regional location capabilities in monitoring nuclear treaties such as the nuclear Comprehensive Test Ban Treaty (CTBT). To a groomed version of the ISC/NEIC data, we apply the tomographic method of Barmin et al. [Pure Appl. Geophys. (2001)], augmented to include station and event corrections and an epicentral distance correction. The P n and S n maps are estimated on a 1°×1° grid throughout Eurasia. We define the phases P n and S n as arriving between epicentral distances of 3° and 15°. After selection, the resulting data set consists of about 1,250,000 P n and 420,000 S n travel times distributed inhomogeneously across Eurasia. The rms misfit to the entire Eurasian data set from the P n and S n model increases nearly linearly with distance and averages about 1.6 s for P n and 3.2 s for S n , but is better for events that occurred on several nuclear test sites and for selected high-quality data subsets. The P n and S n maps compare favorably with recent 3-D models of P and S in the uppermost mantle and with recently compiled teleseismic station corrections across the region. The most intriguing features on the maps are the low-velocity anomalies that characterize most tectonically deformed regions such as the anomaly across central and southern Asia and the Middle East that extends along a tortuous path from Turkey in the west to Lake Baikal in the east. These anomalies are related to the closing of the Neo-Tethys Ocean and the collision of India with Asia. The uppermost mantle beneath the Pacific Rim back-arc is also very slow, presumably due to upwelling that results from back-arc spreading, as is the Red Sea rift, the Tyrrhenian Sea and other regions undergoing active extension.

New constraints on the arctic crust and uppermost mantle: surface wave group velocities, Pn, and Sn

We present the results of a study of surface wave dispersion across the Arctic region (>60N) and compare the estimating group velocity maps with new maps of the body wave phases Pn and Sn. Data recorded at about 250 broadband digital stations from several global and regional networks were used to obtain Rayleigh and Love wave group velocity measurements following more than 1100 events with magnitudesM s > 5:0 that occurred in the northern hemisphere from 1977 to 1998. These measurements were used to construct both isotropic and 2 azimuthally anisotropic group velocity maps from 15 to 200 s period. As elsewhere in the world, the observed maps display the signatures of sedimentary and oceanic basins, crustal thickness variations, and upper mantle anomalies under both continents and oceans. We also present Pn and Sn maps produced from a groomed data set of travel times from the ISC and NEIC bulletins. The long period group velocity maps correlate well with Pn and Sn velocities. Finally, at long wavelengths, the estimated 2 azimuthal anisotropy in Rayleigh wave group velocity correlates well with the azimuthal anisotropy in phase velocity obtained in a global scale study of Trampert and Woodhouse. Because attempts to improve the resolution to regional scales change both the amplitude and the pattern of the inferred azimuthal anisotropy, caution should be exercised in interpreting the anisotropy maps.