Anelasticity of the Crust and Upper Mantle of South America From the Inversion of Observed Surface Wave Attenuation (original) (raw)
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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.
Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle
Geophysical Journal International, 2004
Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, Q −1 s , and seismic velocity, V s , we compare global maps of the thermal structure of the continental upper mantle with global Q −1 s and V s maps as determined from Rayleigh waves at periods between 40 and 150 s. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high.
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, 1994
A combined inversion/fonvard modelling procedure, in which the frequency dependence of shear-wave internal friction (Q;') is allowed to vary with depth, was developed and applied t o selected Rayleigh wave and Lg attenuation data in the Basin and Range Province of the southwestern United States. Both Q and the frequency dependence of Lg waves were used to constrain the models. Many models can explain Rayleigh wave and Lg data sets within their uncertainties, but at 1 Hz most have low values of Q, (50-80) in the upper 8 km of the crust, rapidly increasing values to about 1000 at mid-crustal depths, and decreasing values at greater depths. Models which include a layer of higher Q values (80-150) in the upper few kilometres of the crust, overlying a region of lower Q values, cannot be precluded by the attenuation data of this study. Assuming that Q, varies with frequency as f <, models for which the frequency dependence is low (5 = 0.0-0.1) in the upper crust best explain the data of this study. In the lower crust that frequency dependence is not well determined, but the models which best explain both the fundamental-mode and Lg data and produce realistic models of Q, are characterized at 1 Hz by high values of both Q, and its frequency dependence. Because of that frequency dependence Q, may be an order of magnitude lower at a period of 100 s (~1 0 0) than it is at 1 s (=lOOO). Investigations of the effects of changing crustal velocity on values for Lg Q and its frequency dependence indicate that realistic velocity changes cause only small changes in those values and thus are inconsequential to our results. The low Q, values, and their constancy with frequency, in the uppermost crust can be explained by fluid flow in a network of cracks in brittle rock. Increasing Q, with depth to 10-15 km can be explained by the closing of those cracks due to pressure and their enhanced healing with increasing temperature. Plastic flow at greater depths may contribute further to the dissipation of cracks and to further increases in Q,. Decreasing Q, values at greater depths can be explained as being the result of increasing temperature, increasing content of partial melt, enhanced dislocation motion or some combination of these effects.
Journal of Geophysical Research, 1961
Phase and group velocities of mantle Love and Rayleigh waves obtained frown strain seismograph records of the Chilean earthquake are presented. The velocities of mantle Rayleigh waves of period from 300 to 550 seconds agree with those predicted from periods of free spheroidal oscillation of the earth and do not show a flattening of the group velocity curve for periods greater than 380 seconds. Group velocities for mantle Rayleigh waves reach a maximum of 7.8 km/sec at a period of about 1000 sec. Study of initial phases of Rayleigh waves indicates a difference of phase of •r between the azimuth to Isabella and the azimuths to •afia and Ogdensburg. Determinations of phase and group velocities of Love waves have been extended to periods of 700 seconds. The phase velocity data of Sat• [1958] has been corrected for the polar phase shift. The correct curve has been identified from the numerous possible curves which result from a 2•r ambiguity in the phase correlation made by Sat8. Values of phase velocities are presented for periods in the range of 60 to 700 seconds. The group and phase velocities of both Love waves and Rayleigh waves agree well with those predicted for the Gutenberg-Bullen A model of the earth. It is verified that analysis of seismograms in terms of progressive wave trains is equivalent to analysis in terms of standing waves. In the presence of absorption, as for the earth, the analysis in terms of progressive wave trains has many advantages.
Journal of Geodynamics
Upper mantle P and S wave velocities in the western South America region are obtained at depths of foci from an analysis of travel time data of deep earthquakes[ The inferred velocity models for the ChileÐPeruÐ Ecuador region reveal an increase of P velocity from 7[93 km:s at 39 km to 7[17 km:s at 149 km depth\ while the S velocity remains almost constant at 3[51 km:s from 39 to 109 km depth[ A velocity discontinuity "probably corresponding to the L discontinuity in the continental upper mantle# at 119Ð149 km depth for P and 199Ð119 km depth for S waves\ with a 2Ð3) velocity increase\ is inferred from the velocityÐdepth data[ Below this discontinuity\ P velocity increases from 7[43 km:s at 149 km to 7[51 km:s at 219 km depth and S velocity increases from 3[70 km:s at 109 km to 3[88 km:s at 189 km depth[ Travel time data from deep earthquakes at depths greater than 499 km in the BoliviaÐPeru region\ reveal P velocities of about 8[54 km:s from 499 to 469 km depth[ P velocityÐdepth data further reveal a velocity discontinuity\ either as a sharp boundary at 469 km depth with 7Ð09) velocity increase or as a broad transition zone with velocity rapidly increasing from 459 to 509 km depth[ P velocity increases to 09[64 km:s at 549 km depth[ A comparison with the latest global average depth estimates of the {559 km| discontinuity reveals that this discontinuity is at a relatively shallow depth in the study region[ Further\ a velocity discontinuity at about 399 km depth with a 09) velocity increase seems to be consistent with travel time observations from deep earthquakes in this region[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
Shear wave anisotropy beneath the Andes from the BANJO, SEDA, and PISCO experiments
Journal of Geophysical Research, 2000
We present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western South America: the Broadband Andean Joint experiment (BANJO), a 1000-km-long east-west line at 20øS, and the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) and Seismic Exploration of the Deep Altiplano (SEDA), deployed several hunderd kilometers north and south of this line. We determined the splitting parameters q) (fast polarization direction) and õt (splitting delay time) for waves that sample the above-and below-slab regions: teleseismic *KS and S, ScS waves from local deep-focus events, as well as S waves from intermediate-focus events that sample only the above-slab region. All but one of the *KS stacks for the BANJO stations show E-W fast directions with •St varying between 0.4 and 1.5 s. However, for *KS recorded at most of the SEDA and PISCO stations, and for local deep-focus S events north and south of BANJO, there is a rotation of q) to a more nearly trench parallel direction. The splitting parameters for above-slab paths, determined from events around 200 km deep to western stations, yield small delay times (<0.3 s) and N-S fast polarization directions. Assuming the anisotropy is limited to the top 400 km of the mantle (olivine stability field), these data suggest the following spatial distribution of anisotropy. For the above-slab component, as one goes from east (where *KS reflects the above-slab component) to west, q) changes from E-W to N-S, and delay times are substantially reduced. This change may mark the transition from the Brazilian craton to actively deforming (E-W shortening) Andean mantle. We see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion. The small delay times for above-slab paths in the west require the existence of significant, spatially varying below-slab anisotropy to explain the *KS results. The implied anisotropic pattern below the slab is not easily explained by a simple model of slab-entrained shear flow beneath the plate. Instead, flow induced by the retrograde motion of the slab, in combination with local structural variations, may provide a better explanation.
Geophysical Journal International, 2012
The joint inversion of Rayleigh wave group velocity dispersion and receiver functions has been used to study the crust and upper mantle structure at eight seismic stations in Cuba. Receiver functions have been computed from teleseismic recordings of earthquakes at epicentral (angular) distances in the range from 30 • to 90 • and Rayleigh wave group velocity dispersion relations have been taken from earlier surface wave tomographic studies in the Caribbean area. The thickest crust (∼30 km) below Cuban stations is found at Cascorro (CCC) and Maisí (MAS) whereas the thinnest crust (∼18 km) is found at stations Río Carpintero (RCC) and Guantánamo Bay (GTBY), in the southeastern part of Cuba; this result is in agreement with the southward gradual thinning of the crust revealed by previous studies. In the crystalline crust, the S-wave velocity varies between ∼2.8 and ∼3.9 km s -1 and, at the crust-mantle transition zone, the shear wave velocity varies from ∼4.0 and ∼4.3 km s -1 . The lithospheric thickness varies from ∼65 km, in the youngest lithosphere, to ∼150 km in the northeastern part of the Cuban island, below Maisí (MAS) and Moa (MOA) stations. Evidence of a subducted slab possibly belonging to the Caribbean plate is present below the stations Las Mercedes (LMG), RCC and GTBY whereas earlier subducted slabs could explain the results obtained below the Soroa (SOR), Manicaragua (MGV) and Cascorro (CCC) station.