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Knowledge of the mantle reflectivity structure is highly dependent on our ability to efficiently extract, and properly interpret, small seismic arrivals. Among the various data types and techniques, long-period SS/PP precursors and high-frequency receiver functions are routinely utilized to increase the confidence of the recovered mantle stratifications at distinct spatial scales. However, low resolution and a complex Fresnel zone are glaring weaknesses of SS precursors, while over-reliance on receiver distribution is a formidable challenge for the analysis of converted waves from oceanic regions. A promising high frequency alternative to receiver functions is P′P′ precursors, which are capable of resolving mantle structures at vertical and lateral resolution of ∼5 and ∼200 km, respectively, owing to their spectral content, shallow angle of incidence and near-symmetric Fresnel zones. This study presents a novel processing method for both SS (or PP) and P′P′ precursors based on deconvolution, stacking, Radon transform and depth migration. A suite of synthetic tests is performed to quantify the fidelity and stability of this method under different data conditions. Our multiresolution survey of the mantle at targeted areas near Nazca-South America subduction zone reveal both olivine and garnet related transitions at depths below 400 km. We attribute a depressed 660 to thermal variations, whereas compositional variations atop the upper-mantle transition zone are needed to explain the diminished or highly complex reflected/scattered signals from the 410 km discontinuity. We also observe prominent P′P′ reflections within the transition zone, and the anomalous amplitudes near the plate boundary zone indicate a sharp (∼10 km thick) transition that likely resonates with the frequency content of P′P′ precursors. The migration of SS precursors in this study shows no evidence of split 660 reflections, but potential majorite–ilmenite (590–640 km) and ilmenite–perovskite transitions (740–750 km) are identified based on similarly processed high-frequency P′P′ precursors. Additional findings of severely scattered energy in the lithosphere and distinct lower mantle reflections at ∼800 km could be potentially important but require further verifications. Overall, our improved imaging methods and the strong sensitivity of P′P′ precursors to the existence, depth, sharpness and strength of reflective structures offer significant future promise for the understanding of mantle mineralogy and dynamics. [ABSTRACT FROM AUTHOR]

SUMMARY Simultaneous estimation of origin time, location and moment tensor of seismic events is critical for automatic, continuous, real-time monitoring systems. Recent studies have shown that such systems can be implemented via waveform fitting methods based on pre-computed catalogues of Green's functions. However, limitations exist in the number and length of the recorded traces, and the size of the monitored volume that these methods can handle without compromising real-time response. This study presents numerical tests using a novel waveform fitting method based on compressive sensing, a field of applied mathematics that provides conditions for sampling and recovery of signals that admit a sparse representation under a known base or dictionary. Compressive sensing techniques enable us to determine source parameters in a compressed space, where the dimensions of the variables involved in the inversion are significantly reduced. Results using a hypothetical monitoring network with a dense number of recording stations show that a compressed catalogue of Green's functions with 0.004 per cent of its original size recovers the exact source parameters in more than 50 per cent of the tests. The gains in processing time in this case drop from an estimated 90 days to browse a solution in the uncompressed catalogue to 41.57 s to obtain an estimation using the compressed catalogue. For simultaneous events, the compressive sensing approach does not appear to influence the estimation results beyond the limitations presented by the uncompressed case. The main concern in the use of compressive sensing is detectability issues observed when the amount of compression is beyond a minimum value that is identifiable through numerical experiments. Tests using real data from the 2002 June 18 Caborn Indiana earthquake show that the presence of noise and inaccurate Green's functions require a smaller amount of compression to reproduce the solution obtained with the uncompressed catalogue. In this case, numerical simulation enables the assessment of the amount of compression that provides a reasonable rate of detectability. Overall, the numerical experiments demonstrate the effectiveness of our compressed domain inversion method in the real-time monitoring of seismic sources with dense networks of receivers. As an added benefit of the compression process, the size of the monitored volume can also be increased under specific restrictions while maintaining the real-time response. [ABSTRACT FROM AUTHOR]

SUMMARY Using a proper parametrization, the source displacement field of a seismic event can be efficiently reconstructed by a redundant dictionary of Green's functions based on sparse representation theory. Then, by subjecting the pre-existing event records and pre-computed dictionary of Green's functions into a sparsity-promoting algorithm, it is possible to simultaneously evaluate the origin time, hypocentre coordinates and seismic moment tensor. The proposed method is applicable to single- or multiple-source scenarios and, with minor adjustments, can be a valuable tool for real-time, automatic monitoring systems. This study demonstrates the effectiveness and accuracy of the dictionary-based approach via (1) detection of microseismic events produced during the hydraulic fracturing of oil and gas wells and (2) inversion of a small-magnitude, regional earthquake (2002 June 18 in Caborn, Indiana) data. Our experiments based on numerical simulations and earthquake observations underscore the largely untapped potential of dictionary-based approaches and sparse representation theory in continuous source parameter recovery. [ABSTRACT FROM AUTHOR]

In this paper, we present a new approach to estimate high-resolution teleseismic receiver functions using a simultaneous iterative time-domain sparse deconvolution. This technique improves the deconvolution by using reweighting strategies based on a Cauchy criterion. The resulting sparse receiver functions enhance the primary converted phases and its multiples. To test its functionality and reliability, we applied this approach to synthetic experiments and to seismic data recorded at station ABU, in Japan. Our results show Ps conversions at approximately 4.0 s after the primary P onset, which are consistent with other seismological studies in this area. We demonstrate that the sparse deconvolution is a simple, efficient technique in computing receiver functions with significantly greater resolution than conventional approaches. [ABSTRACT FROM AUTHOR]

Until now, modelling of threee-dimensional (3-D) velocity variations in the mantle and topography of the transition zone discontinuities have been considered separately. Velocity models were obtained assuming that the radii of the discontinuities are constant. Then, the travel time data sensitive to the topography, such as the SS precursors, were corrected for the effect of 3-D seismic structure and interted for depth variations of a discontinuity. Such a procedure is unsatisfactory, as it may introduce artefacts that could significantly affect the topographic results; the opposite trade-off is less likely to introduce conceptually important changes in the velocity distribution but should also be considered. In this study we bring together the same set of S-velocity sensitive sata as used by Gu st al. and combine it with a large set of differential travel times of SS-S400S, S-S670S, and direct measurements of S400S-S670S. We formulate the inverse problem in terms of the volumetric (3-D) and topographic (2-D) perturbations for both the 400- and 670-km discontinuities. The best-fitting model of the joint inversion significanlty improves the variance reduction of SS-S400S and SS-S670S residuals. The velocity distribution in the resulting model, TOPOS362D1, is very similar to that in model S362D1 (with correlation coefficients >0.9 throughout the mantle), which indicates that lateral variations of discontinuity depths have only minor influence on global modelling of velocity. Important changes, however, have been made to the topography of 400- and 670-km discontinuities with respect to those obtained earlier assuming an existing velocity model. The overall undulation of the 400-km discontinuity is considerably less than that reported by earlier global studies; in TOPOS362D1 its maximum variation does not exceed 12 km. The strong degree-1 component before the joint inversion has decreased, such that the coreelation between the velocities above the discontinuity... [ABSTRACT FROM AUTHOR]

Examines results of a joint inversion for compressional and shear velocity in the mantle using a data set consisting of travel times, wave forms and surface wave dispersion measurements. Peak variations in the model; Ratio of velocity perturbations; Correlation between bulk-sound and shear velocity.