Mikhail Barmin

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.

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.

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.

Tutorial review of seismic surface waves’ phenomenology

In recent years, surface wave seismology has become one of the leading directions in seismological investigations of the Earth’s structure and seismic sources. Various applications cover a wide spectrum of goals, dealing with differences in sources of seismic excitation, penetration depths, frequency ranges, and interpretation techniques. Observed seismic data demonstrates the great variability of phenomenology which can produce difficulties in interpretation for beginners. This tutorial review is based on the many years’ experience of authors in processing and interpretation of seismic surface wave observations and the lectures of one of the authors (ALL) at Workshops on Seismic Wave Excitation, Propagation and Interpretation held at the Abdus Salam International Center for Theoretical Physics (Trieste, Italy) in 1990–2012. We present some typical examples of wave patterns which could be encountered in different applications and which can serve as a guide to analysis of observed seismograms.