HR: 1400h
AN: S33A-02    [Abstracts]
TI: Regional Seismic Tomography in Brazil and Uncertainty Evaluation Through Jackknife Re- Sampling Method
AU: * Rocha, M P
EM: rocha@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao, 1226 Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AU: Schimmel, M
EM: schimmel@ija.csic.es
AF: Institute of Earth Sciences Jaume Almera, Carrer Soles i Sabaris, S/N, Barcelona, 08028, Spain
AU: Assumpcao, M
EM: marcelo@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao, 1226 Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AB: We used the regional seismic tomography to study the upper mantle beneath SE and Central Brazil. This method is based on the inversion of P- and S-wave relative travel time residuals (VanDecar, 1991) obtained from more than 80 stations in an area of 20 x 20 degrees. The ~11000 P and PKP residuals and ~8000 S, ScS, SKS, and SKKS residuals have been obtained from waveform cross-correlations for up to 12 simultaneous stations. Our results show correlations of seismic anomalies with the main tectonic structures and reveal new anomalies not yet observed in previous works. High velocity anomalies in the western portion of the Sao Francisco Craton support the hypothesis that this craton was part of a major Neoproterozoic plate. Low velocity anomalies beneath the Tocantins Province (mainly fold belts between the Amazon and Sao Francisco cratons) are interpreted as due to lithospheric thinning. Assumpcao et al. (2004) showed a good correlation between intraplate seismicity and low velocity anomalies in this region. The slab of the Nazca Plate is observed as a high velocity anomaly beneath the Parana basin (at 700-1200 km depths). At these depths, large low velocity anomalies appear accompanying the slab. Synthetic tests show that these anomalies are artifacts of the inversion generated by the presence of the slab. We use the Jackknife re-sampling method to evaluate the robustness of the tomographic results with respect to the data. The main advantage is that it is not necessary to assume a particular error distribution, since the model variability is accessed directly from the data variability. The approach is based on a random removal of a small percentage of the data (1%) to generate various new subsets, which are inverted to evaluate the model variability. These local estimates include inherently the highly variable ray coverage and measurement errors and can provide confidence in the interpretation of anomalies. This measure should not be interpreted as the resolution. As expected, the Jackknife approach shows that the inversions are less robust at shallow depth and at the margins of the study volume. The model variability is also used as an additional criterion to determine the optimum number of iterations in the inversion.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7218 Lithosphere (1236)
DE: 7270 Tomography (6982, 8180)
DE: 8103 Continental cratons
SC: Seismology [S]
MN: 2007 Joint Assembly