Navegando por Autor "SANTA ROSA, Antonio Nuno de Castro"

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Dispersão das ondas de Rayleigh na Plataforma Sulamericana
(Universidade Federal do Pará, 1989-05-10) SANTA ROSA, Antonio Nuno de Castro; LEITE, Lourenildo Williame Barbosa; http://lattes.cnpq.br/8588738536047617
This work represents a dispersion study of the vertical component of the Rayleigh wave with trajectories on the South American platform. The records used are from stations located in the Brazilian territory such as the ones from Rio de Janeiro (RDJ), Caicó (CAI), Brasília (BDF) and Belém (BEB). These are the only seismological stations in Brazil with long period sensors that can be useful for the dispersion studies in the interval 4 to 50 seconds. The earthquakes used are localized along the eastern part of Andean mountain belt and inside the South American platform, with tipically continental trajectories. 34 events were selected using the following pratical criteria: the localization, the magnitude mb, the depth, and that they occured in the period of January 1978 to June 1987. The dispersion study done here means the determination of group velocities and spectral amplitudes, corresponding to the fundamental and first higher modes. Normally the modes of second order or more are rarely observed. Two types of measurements were made: (i) group velocity versus period and (ii) spectral amplitude versus period. Dispersion studies are important for determining the structure of the crust and upper mantle which are directly related to gelogical phenomena. In this work, regionalization is defined as the identification of different forms of dispersion curves and their relation to the epicenter-station trajectory on the South American platform. We also correlate them geologicaly, as described in item 4.3 of this work. The distribution of epicenters occurs from the extreme South of Argentina to the extreme North of Venezuela, with the objective of iniciating with this work a sistematic study of the sub-Andean area in our institution. Three distinct types of curves were observed in 27 trajectories and grouped in three families (1, 2 and 3). Their forms were compared with the regional geological features of South American platform. The multiple filter technique was used to obtain the dispersion curves (Dziewonski et al 1969). This filter has the property of separating the modes through their group velocities for each frequency selected and also recovers the espectral amplitudes characteristic for each harmonic (Herrmann, 1973). The theorectical development of the filter properties as well as the limitations and aplicabilities are treated by Dziewonski et al (1972). As part of this work there was the implementation, adaptions and the layout of fluxograms of the multiple filter software, as well as the digitalization procedure for data processing and non-automatic interpretation of the results.
Acesso aberto (Open Access)
Inversão de dados de ondas de superfície Rayleigh
(Universidade Federal do Pará, 1996-02-26) SANTA ROSA, Antonio Nuno de Castro; ROSA, João Willy Corrêa; http://lattes.cnpq.br/3486252051195441
We have compiled a large set of group velocity measurements for fundamental mode Rayleigh waves with period smaller than 100 seconds. These data are composed of worldwide information gathered from the geophysical literature. Part of the data was compiled in previous works, and a second part of these data was measured in this thesis. For the South American plate, we have selected the main sets of measurements for such waves and gathered them in several profiles, for which the depth distribution of the shear wave velocity was determined from the inversion of the group velocity dispersion curves. These depth profiles were useful to have an idea of the internal structure of the Earth underneath the South American plate. Using the global group velocity data set, it was possible to determine the maps showing the lateral variation of group velocity for reference period values ranging from 20 to 100 seconds. These maps were produced in the same way that ROSA (1986) used to generate the corresponding phase velocity maps, using blocks measuring 10x10 degrees around the earth and considering the mercator projection. The group velocity value at each block was obtained, for each reference period value, from the stochastic inversion of the travel time delays from the JORDAN (1981) each model, considering the average group velocity values determined by ROSA et al. (1992) for this model. The group velocity maps obtained here were then jointly used with the phase velocity values determined by ROSA (1986) for the determination of the earth’s internal distribution of shear wave velocity as well as the density distribution, from the depth inversion of both phase and group velocity dispersion curves. The results enabled us to construct the first map showing the possible depth of the Moho discontinuity (upper mantle depth) for South America.