Preliminary models of upper mantle P and S wave velocity structure in the western South America region (original) (raw)
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Geophysical Journal International, 2011
We present models for the upper-mantle velocity structure beneath SE and Central Brazil using independent tomographic inversions of P-and S-wave relative arrival-time residuals (including core phases) from teleseismic earthquakes. The events were recorded by a total of 92 stations deployed through different projects, institutions and time periods during the years 1992-2004. Our results show correlations with the main tectonic structures and reveal new anomalies not yet observed in previous works. All interpretations are based on robust anomalies, which appear in the different inversions for P-and S-waves. The resolution is variable through our study volume and has been analyzed through different theoretical test inversions. High-velocity anomalies are observed in the western portion of the São Francisco Craton, supporting the hypothesis that this Craton was part of a major Neoproterozoic plate (San Franciscan Plate). Low-velocity anomalies beneath the Tocantins Province (mainly fold belts between the Amazon and São Francisco Cratons) are interpreted as due to lithospheric thinning, which is consistent with the good correlation between intraplate seismicity and lowvelocity anomalies in this region. Our results show that the basement of the Paraná Basin is formed by several blocks, separated by suture zones, according to model of Milani & Ramos. The slab of the Nazca Plate can be observed as a high-velocity anomaly beneath the Paraná Basin, between the depths of 700 and 1200 km. Further, we confirm the low-velocity anomaly in the NE area of the Paraná Basin which has been interpreted by VanDecar et al. as a fossil conduct of the Tristan da Cunha Plume related to the Paraná flood basalt eruptions during the opening of the South Atlantic.
Proceedings of the National Academy of Sciences, 2007
Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling.
Geophysical Journal International, 1986
Fundamental-mode Rayleigh and Love waves generated by several earthquakes situated along great-circle paths between pairs of seismograph stations have been analysed to obtain coefficients of attenuation, group velocities, phase velocities, and specific quality factors in the period range 18-80s in two regions of the South American continent. One set of paths crosses the shield region which lies on the eastern coast and another set traverses the mountainous region inland. The average attenuation coefficient values are clearly higher in the tectonically active western region throughout the entire period range than in the eastern or shield region. Inversion of the attenuation data yielded shear wave internal friction (8') models as a function of depth in the crust and upper mantle in both regions. A low-Q zone below the lithosphere is prominent in both regions. The results show that substantial variations of Qp occur in the two regions of South America. The Qp values were found to be inversely related to the heat flow values or to the temperature.
Seismic Wave Propagation in South America
1996
below the oceanic trench off the coast from Iquique. For the model of the structure below the oceanic trench off the coast of Iquique, the modes of the shorter periods travel practically entirely in the sediments in the trench (assumed to be of thickness 1 km), while the mode of period 10 is travels predominantly in the low velocity zone at a depth of approximately 110 km. The phase velocities of these modes in the model of the structure below Iquique are between 3.06 and 4.14 km/s; the phase velocities of the Love modes of shorter period (1.9 s to 3.7 s) in the model of the structure below the trench are between 0.51 s and 0.57 s. There is practically no coupling between the Love modes below the oceanic trench and the modes below Iquique. At present we are considering the relation between the Love and Rayleigh modes of short period in our various models and the L9 and the R. phases. For Colombia, we have constructed models of the regions below Quibd6, below Barranquilla and below the Caribbean Sea northwest of Barranquilla. The Caribe plate motion near M~rida, western Venezuela, suggests, together with right hand strike-slip motion, a substantial portion of thrust. We are at present analyzing by the finite element method the propagation of Love and Rayleigh waves across these regions.
P- and S-wave 3D velocity models for the South American platform
Brazilian Journal of Geophysics
ABSTRACT. With the installation and growth of the Brazilian Seismographic Network (RSBR) since 2011, it is now possible to detect and locate events with magnitudes lower than 3.5 mb for Brazil, and better study the local and regional seismicity. Nevertheless, most of these events are located using 1D velocity models that lead to larger uncertainties precluding any correlation with the known geological structures. The use of heterogeneous 3D velocity models better predicts the travel times and guarantees the accuracy of hypocenter location; however, there is not yet a calibrated 3D velocity model for the South American Platform (SAP). In this study, integrating multiple local and regional geological and geophysical knowledge and average velocities of the crust and mantle, we have elaborated 3D P- and S-wave velocity models for the SAP, including the Central Andes area (down to 900 km). We tested the model locating the well-known 1998 Andean Aiquile earthquake (Bolivia), 6.6 Mw, obtai...
Variations ofPwave speeds in the mantle transition zone beneath the northern Philippine Sea
Journal of Geophysical Research, 1997
Using waveforms and travel times from deep earthquakes, we constructed 16 seismic profiles, each of which constrains the radial variation in Vp over a small area beneath the northern Philippine Sea. Taken together, the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high Vp (up to 3% faster than average Earth models) originally suggested by travel time tomography. Unlike travel time tomography, which relies heavily on arrival times of the direct P phase, we utilize the waveforms and move-out of later arrivals that mainly sample the mantle transition zone of interest. Our results identify three important characteristics of the northern Philippine Sea anomaly that are distinct from previous results. First, being approximately a subhorizontal, laterally uniform feature, the anomaly is localized beneath the northwestern comer of the Philippine Sea, within a region of approximately 500 x 500 km 2 immediately east of the Ryukyu arc. Second, the anomaly is well constrained to occur in the lower portion of the transition zone, extending all the way down to the 660-km discontinuity. Third, the presence of such a distinct anomaly reduces the contrast in V, across the 660-km discontinuity from approximately 6% to 3%. Such a configuration •s consistent wath the interpretanon that the anomaly is caused by a remnant of subducted slab, as negative buoyancy should rest the slab just above the 660-km discontinuity where resistance to subduction is expected from a negative Clapeyron slope during the spinel-Mg-Fe-perovskite transition. between the upper and the lower mantle [e.g., Anderson, 1989; Lay, 1994; Silver et at., 1988]. Several recent studies showed evidence that the extent of slab penetration into the lower mantle seems to vary laterally along subduction zones such that at least some subducted slabs remain stagnant in the upper mantle [e.g., Glennon and Chen, 1995a; Ding and Grand, 1994], while others plunge steeply into the lower mantle [e.g., Jordan, 1977; Creager and Jordan, 1984, 1986]. For instance, on the basis of long-period (-20 s) converted and reflected body waves, Shearer and Masters [ 1992] and Shearer [ 1991 ] contended that the topography of the 660-km discontinuity shows
Journal of Geophysical Research, 2003
1] We determine the crustal and upper mantle structure of southern South America by inverting regional seismograms recorded by the Seismic Experiment in Patagonia and Antarctica. We present a waveform inversion method that utilizes a niching genetic algorithm. The niching genetic algorithm differs from the traditional genetic algorithm in that the inversion is performed in multiple subpopulations, thus generating a broader search of the model space and enabling us to examine alternative local error minima. The vertical and transverse waveforms were used, extending from the P arrival to the surface waves, and the inversion was performed at either 0.005-0.06 Hz (larger events) or 0.02-0.06 Hz (smaller events). The inversion included anisotropy by solving for separate SV and SH structures in the upper mantle. Results indicate that crustal thickness varies from 26 to 36 km with thicker values toward the northeast, suggesting that there is little crustal thickening beneath the austral Andes. The average upper mantle velocities are similar to the preliminary reference Earth model (PREM) except that the southernmost region shows velocities of 5% slower than PREM. The upper mantle has up to 5% polarization anisotropy between the Moho and 120 km depth. The strongest anisotropy is localized in a lithospheric lid shallower than 65 km depth, which overlies a pronounced low-velocity zone. This shallow limit to anisotropy is consistent with the relatively small shear wave splitting values found in this region. These results suggest that the anisotropy is limited to lithospheric depths and may imply the absence of a strong mantle flow pattern in the asthenosphere. Crustal and upper mantle structure of southernmost South America inferred from regional waveform inversion,
Geophysical Journal International, 2002
We investigate the correlation of large-scale P-and S-velocity heterogeneity in the mantle by determining how well 106,000 compressional P, PP, PPP, and PKPab traveltimes can be explained by S-wave velocity model S20RTS (scaled using a depth dependent factor) and by a model in which the lateral P-velocity variations are different. We first assess the assumption that P-wave traveltimes can be explained by a model in which lateral P-velocity variations (δv P) are identical to S-velocity variations (δv S) in model S20RTS. For a given depth, we project δv S from S20RTS into model S2P using a depth-dependent scaling factor R defined as: δv S = R(z) × δv P. We find, by grid search, that the highest reduction of data variance is obtained when R increases linearly from 1.25 at the surface to 3.0 at the core-mantle boundary. A comparison of S-wave (+SS) and P-wave (+P P) traveltimes for identical source-receiver pairs also indicates that R increases with depth. Significantly higher variance reduction is not obtained when R is parametrized with an additional degree of freedom. Therefore, the precise shape of R cannot be constrained by our data. P-and PP-wave traveltime anomalies with respect to the scaled model S2P yield coherent geographic variations. This indicates that there are large-scale lateral P-velocity variations in the mantle that are different from those in model S2P. We estimate these variations by inverting P-wave traveltime anomalies with respect to model S2P for a degree 12 model of P-velocity heterogeneity. This model, P12 s2p , indicates where in the well-sampled mantle regions we need to modify model S2P to further improve the fit to the traveltime data. Anomalies in P12 s2p exist throughout the mantle. It is, therefore, not obvious that compositional heterogeneity is prominent in the lower 1000 km of the mantle only, as suggested previously. Low P-wave velocities in the upper mantle beneath oceans are the strongest anomalies in P12 s2p and explain better the delayed traveltimes of PP-wave phases with oceanic surface refection points. Lower mantle anomalies include high and low P-velocity structures beneath eastern Asia and North America, respectively. The high P-velocity anomaly in the lower mantle beneath the central Pacific is consistent with the assertion made by other researchers that large-scale lower mantle upwellings are not purely thermal in origin.