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43.
Lucas Vieira Barros Marcelo Assumpção Ronnie Quintero Vinicius Martins Ferreira 《Journal of Seismology》2011,15(2):391-409
Small local earthquakes from two aftershock sequences in Porto dos Gaúchos, Amazon craton—Brazil, were used to estimate the
coda wave attenuation in the frequency band of 1 to 24 Hz. The time-domain coda-decay method of a single backscattering model
is employed to estimate frequency dependence of the quality factor (Q
c) of coda waves modeled using Qc = Q0 fhQ_{\rm c} =Q_{\rm 0} f^\eta , where Q
0 is the coda quality factor at frequency of 1 Hz and η is the frequency parameter. We also used the independent frequency model approach (Morozov, Geophys J Int, 175:239–252, 2008), based in the temporal attenuation coefficient, χ(f) instead of Q(f), given by the equation
c(f)=g+\fracpfQe \chi (f)\!=\!\gamma \!+\!\frac{\pi f}{Q_{\rm e} }, for the calculation of the geometrical attenuation (γ) and effective attenuation (Qe-1 )(Q_{\rm e}^{-1} ). Q
c values have been computed at central frequencies (and band) of 1.5 (1–2), 3.0 (2–4), 6.0 (4–8), 9.0 (6–12), 12 (8–16), and
18 (12–24) Hz for five different datasets selected according to the geotectonic environment as well as the ability to sample
shallow or deeper structures, particularly the sediments of the Parecis basin and the crystalline basement of the Amazon craton.
For the Parecis basin Qc = (98±12)f(1.14±0.08)Q_{\rm c} =(98\pm 12)f^{(1.14\pm 0.08)}, for the surrounding shield Qc = (167±46)f(1.03±0.04)Q_{\rm c} =(167\pm 46)f^{(1.03\pm 0.04)}, and for the whole region of Porto dos Gaúchos Qc = (99±19)f(1.17±0.02)Q_{\rm c} =(99\pm 19)f^{(1.17\pm 0.02)}. Using the independent frequency model, we found: for the cratonic zone, γ = 0.014 s − 1, Qe-1 = 0.0001Q_{\rm e}^{-1} =0.0001, ν ≈ 1.12; for the basin zone with sediments of ~500 m, γ = 0.031 s − 1, Qe-1 = 0.0003Q_{\rm e}^{-1} =0.0003, ν ≈ 1.27; and for the Parecis basin with sediments of ~1,000 m, γ = 0.047 s − 1, Qe-1 = 0.0005Q_{\rm e}^{-1} =0.0005, ν ≈ 1.42. Analysis of the attenuation factor (Q
c) for different values of the geometrical spreading parameter (ν) indicated that an increase of ν generally causes an increase in Q
c, both in the basin as well as in the craton. But the differences in the attenuation between different geological environments
are maintained for different models of geometrical spreading. It was shown that the energy of coda waves is attenuated more
strongly in the sediments, Qc = (78±23)f(1.17±0.14)Q_{\rm c} =(78\pm 23)f^{(1.17\pm 0.14)} (in the deepest part of the basin), than in the basement, Qc = (167±46)f(1.03±0.04)Q_{\rm c} =(167\pm 46)f^{(1.03\pm 0.04)} (in the craton). Thus, the coda wave analysis can contribute to studies of geological structures in the upper crust, as the
average coda quality factor is dependent on the thickness of sedimentary layer. 相似文献
44.
Daniel A. Gregori Jose Kostadinoff Guillermina Alvarez Ariel Raniolo Leonardo Strazzere Juan C. Martinez Mercedes Barros 《地学前缘(英文版)》2013,4(6):693-708
The Preandean geological configuration of the eastern North Patagonian Massif is established through the use of geological and geophysical analysis.The positive gravity anomalies located near the Atlantic coast are due to 535 and 540 Ma old rocks belonging to the Pampean Orogeny (Precambrian-middle Cambrian),which are widely recognized in central and northern Argentina.The Famatinian Cycle (Ordovician-Devonian) is represented by a Silurian-Devonian marine basin equivalent to those of eastern-central Argentina and South Africa,and which was deformed at the end of the Devonian byan~E-W to WNW-ESE compressional event,part of the Famatinian Orogeny.Containing strong gravity gradients,the NW-SE belt is coincident with fault zones which were originated during the Gondwanide Orogeny.This event also produced NW-SE overthrusting of the Silurian-Devonian sequences and strike-slip faults that displaced blocks in the same direction.This deformation event belongs to the Gondwanide Orogeny that includes movements related to a counterclockwise rotation of blocks in northern Patagonia.The strong negative anomalies located in the western part of the area stem from the presence of rocks of the Jurassic Ca(n)adón Asfalto basin interbedded in the Marifil Complex.These volcaniclastic sequences show mild deformation of accommodation zones in a pre-Jurassic paleorelief. 相似文献
45.
Impacts of forest conversion and agriculture practices on water pathways in Southern Brazil 下载免费PDF全文
Jérémy Robinet Jean P. G. Minella Cláudia A. P. de Barros Alexandre Schlesner Andreas Lücke Yolanda Ameijeiras-Mariño Sophie Opfergelt Jan Vanderborght Gerard Govers 《水文研究》2018,32(15):2304-2317
Land‐use/cover change (LUCC), and more specifically deforestation and multidecadal agriculture, is one of the various controlling factors of water fluxes at the hillslope or catchment scale. We investigated the impact of LUCC on water pathways and stream stormflow generation processes in a subtropical region in southern Brazil. We monitored, sampled and analysed stream water, pore water, subsurface water, and rainwater for dissolved silicon concentration (DSi) and 18O/16O (δ18O) signature to identify contributing sources to the streamflow under forest and under agriculture. Both forested and agricultural catchments were highly responsive to rainfall events in terms of discharge and shallow groundwater level. DSi versus δ18O scatter plots indicated that for both land‐use types, two run‐off components contributed to the stream discharge. The presence of a dense macropore network, combined with the presence of a compact and impeding B‐horizon, led to rapid subsurface flow in the forested catchment. In the agricultural catchment, the rapid response to rainfall was mostly due to surface run‐off. A 2‐component isotopic hydrograph separation indicated a larger contribution of rainfall water to run‐off during rainfall event in the agricultural catchments. We attributed this higher contribution to a decrease in topsoil hydraulic conductivity associated with agricultural practices. The chemical signature of the old water component in the forested catchment was very similar to that of the shallow groundwater and the pore soil water: It is therefore likely that the shallow groundwater was the main source of old water. This is not the case in the agricultural catchments where the old water component had a much higher DSi concentration than the shallow groundwater and the soil pore water. As the agricultural catchments were larger, this may to some extent simply be a scale effect. However, the higher water yields under agriculture and the high DSi concentration observed in the old water under agriculture suggest a significant contribution of deep groundwater to catchment run‐off under agriculture, suggesting that LUCC may have significant effects on weathering rates and patterns. 相似文献
46.
Research in aquatic ecology requires adequate sampling designs if the underlying processes that govern population and community dynamics are to be understood. Adequate sampling design is particularly important for organisms inhabiting environments such as mangroves. Thus, collecting quantitative ecological data remains a major challenge. Here, we analysed the structural and temporal dynamics of a tidal cycle and mangrove fish assemblages at 15 sites along the Brazilian central coast. Specifically, we used two non-extractive sampling techniques, which included stationary underwater visual census (s-UVC) and stationary underwater wide-camera census (s-UWC). Our results showed that by reducing potential observer effects, the s-UWC protocol maximized fish counts, yielding more accurate and precise estimates. In contrast, although the s-UVC technique was a useful approach, this technique did not yield precise density and richness estimates. Furthermore, the negative effect associated with the presence of an observer (i.e., diver) was a significant constraint to the observer-based technique. Furthermore, the methods were more similar for the high tide samples and more divergent for the flood tide and ebb tide samples. These pronounced divergences among tidal periods allowed the application of a temporal mixed-sampling protocol (i.e., unbalanced temporal samples) based on s-UWC without the loss of statistical power. Methodological gains also were felt with a reduction in the costs associated with the presence of a trained observer in water and overcoming of constraints positively related to depth limitations. Overall, the s-UWC protocol adequately captures species using different stages of the tide to enter the mangroves. 相似文献
47.
Guarino Vincenza Wu Fu-Yuan Melluso Leone de Barros Gomes Celso Tassinari Colombo Celso Gaeta Ruberti Excelso Brilli Mauro 《International Journal of Earth Sciences》2017,106(6):1963-1989
International Journal of Earth Sciences - The Catalão II carbonatitic complex is part of the Alto Paranaíba Igneous Province (APIP), central Brazil, close to the Catalão I complex.... 相似文献
48.
Leonardo Strazzere Daniel A. Gregori Leonardo Benedini Paulo Marcos Mercedes V. Barros Mauro C. Geraldes Cecilia Pavon Pivetta 《地学前缘(英文版)》2019,10(1):299-314
The Marifil Volcanic Complex, exposed in the eastern North Patagonian Massif, Argentina, includes up to 550 m of red conglomerates, sandstones, black siltstones, limestones, and reworked tuff of the Puesto Piris Formation. The basal part of this unit, which was deposited in high-gradient topographic relief, is composed of conglomerates and sandstones with thin layers of reworked tuffs. The lithofacies associations of the basal part indicate that the depositional mechanisms were mantled and gravitational flows.The middle part of the unit consists of fine sandstones, limestones, and black siltstones that were deposited in low-energy fluvial and lacustrine environments. The outcrops are located along the NEe SW direction and the major thickest units represented by limestones and siltstones, occur near the southeastern border of this NEeS W depocenter. Since the rhyolitic and trachytic lava flows and tuffs of the Marifil Volcanic Complex are interbedded with the sedimentary sequences of the Puesto Piris Formation,both units are coeval. Zircon Ue Pb age was obtained for a trachytic lava flow(193.4 ±3.1 Ma) suggesting that sedimentation and volcanism are Sinemurian. This extensional episode was recorded in the eastern,western, and southwestern sectors of the North Patagonian Massif, and is possibly associated with the Gondwana supercontinent breakup. 相似文献
49.
Summary ?Some features of the climate system that can be considered predictors of the onset and end of the convective season over
the Amazon were identified using one-month lag correlations and field composites. The fields analyzed were sea surface temperature
(SST), outgoing long-wave radiation (OLR), vertical velocity and upper tropospheric winds.
Warm (cold) anomalies in the SST in the tropical North Atlantic and the Caribbean Sea tend to be associated with delayed (early)
onsets. Likewise, there is a tendency towards a delayed (early) end of the convective season with cold (warm) anomalies in
these ocean regions. In addition, the SST in the cold tongue region of the equatorial Pacific is negatively, though weakly
correlated with the onset date. The signal of this SST is more evident in the case of the end date, which is earlier with
respect to its mean date in most of El Ni?o cases.
The convective activity intensity itself conditions the onset and the end of the convective season, as it is evidenced by
the behavior of the OLR and the vertical velocity fields. The more (less) intense the convective activity over South America
during the preceding month, the earlier the onset and the later the end of the convective season on the Amazon region.
The prediction of the onset and end dates of the convective season in the Amazon region was explored using a simple multiple
regression technique based on the variables that have shown precursor signals with respect to these dates. The correlation
coefficient between the predicted and the observed onset date is 0.81, and in the case of the end date, it is 0.76. The skill
to predict early, delayed and normal categories was high, since in more than two thirds of the cases the category was successfully
predicted, and there were no predictions of categories opposed to those observed.
Received July 23, 2001; revised February 22, 2002; accepted April 26, 2002 相似文献
50.
Measurements of the response of the ¼‐scale reinforced concrete Hualien (Taiwan) containment model obtained during forced vibration tests are used to identify some of the characteristics of the superstructure and the soil. In particular, attempts are made to determine the fixed‐base modal frequencies, modal damping ratios, modal masses and participation factors associated with translation and rocking of the base. The shell superstructure appears to be softer than could have been predicted on the basis of the given geometry and of test data for the properties of concrete. Estimates of the shear‐wave velocity and damping ratio in the top layer of soil are obtained by matching the observed and theoretical system frequency and peak amplitude of the response at the top of the structure. The resulting models for the superstructure and the soil lead to theoretical results for the displacement and rotations at the base and top of the structure which closely match the observed response. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献