Tensor momento sísmico para eventos no nordeste do Brasil com amplitudes espectrais: estudos em São Caetano, PE
The study of the seismic source is important to improve our understanding of the physical processes that trigger earthquakes, and also for seismotectonics investigations of a region. In this work, we modify an existing methodology for determination of the seismic moment tensor by inversion of sp...
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| Formato: | Dissertação |
| Idioma: | por |
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Brasil
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| Endereço do item: | https://repositorio.ufrn.br/jspui/handle/123456789/21887 |
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| Resumo: | The study of the seismic source is important to improve our
understanding of the physical processes that trigger earthquakes, and also for
seismotectonics investigations of a region. In this work, we modify an existing
methodology for determination of the seismic moment tensor by inversion of
spectral amplitudes, and test it with events recorded at local distances near Sao
Caetano, Pernambuco, in northeastern Brazil. The methodology consists of
minimizing the difference between observed and predicted values of the
spectral amplitudes – with polarity attached - of P, SV and SH waves, to
determine the six independent components of the moment tensor. The original
methodology inverts spectral amplitudes of direct waves for events in deep
mines and recorded at underground seismographic networks. In our study, we
adapt this methodology to tectonic events recorded by surface networks. First,
the effects of the Earth's free surface are removed from the seismic recordings
to recover the incident P, SV and SH waves; then, the corresponding spectral
amplitudes are measured and a polarity is attached; finally, the spectral
amplitudes are inverted in a least squares sense. At pre-critical incidence
angles, the correction is easily achieved by dividing the vertical component of
the P and SV waveforms by the corresponding free-surface reflection
coefficients; at post-critical incidence angles, a more sophisticated correction
involving the Hilbert transform of the recorded waveforms is needed to account
for waveform distortion introduced by the coefficient’s phase shift. Correction of
the SH components is achieved through division by a factor of 2. The proposed
methodology is extremely advantageous since it uses the radiation patterns of both P and S waves to determine the focal mechanism of individual events. The
adapted methodology is applied to 16 earthquakes recorded at local distances
(< 10 km) by an aftershock network deployed in the locality of São Caetano, PE,
between September 15th and December 23rd, 2010. Comparison with a
composite focal mechanisms developed in previous works in the study area
from fist-motion polarities shows consistency with our moment tensor solutions,
thus demonstrating the portability of the spectral amplitude method. Additionally,
the ported methodology allows the estimation of moment magnitudes for
individual events. |
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