Banca de QUALIFICAÇÃO: SANDRO GIOVANI DE FARIAS ALVES GOMES

Uma banca de QUALIFICAÇÃO de DOUTORADO foi cadastrada pelo programa.
STUDENT : SANDRO GIOVANI DE FARIAS ALVES GOMES
DATE: 06/12/2019
TIME: 10:00
LOCAL: Labsis
TITLE:

Hypocentral inversion using inter-source distances constraints from coda wave interferometry: a deterministic approach

KEY WORDS:

Hypocentral inversion; coda wave interferometry; intraplate seismicity

PAGES: 50
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

This work uses coda wave interferometry (CWI) to estimate inter-source distances for nearby events with the same focal mechanism. The CWI relative distances estimates are measured from the maximum cross-correlation coefficient between the coda waves of each source and yields coherent results even using a single station. Because of the probabilistic nature of these estimates early works have employed a Bayesian approach to describe the results. That approach accounts for the bias in underestimate the relative distances as the real separation increases. Furthermore, it accounts for the fact that there is a breakdown point for real distance from which estimates become ambiguous and unreliable. In this work it was performed a modification in the CWI methodology and explored the known CWI property to provide better estimates for higher relative distances when the waveforms are filtered to lower frequency contents, or equivalently, larger wavelengths. In order to evaluate this feature, were generated synthetic seismograms by a finite-difference algorithm that simulates acoustic sources propagating in a heterogeneous bi-dimensional medium. A wide range of real distances were simulated at different depths. Before performing the cross-correlation of the coda signals, the waveforms were filtered to different frequency bands: (1-2 Hz, 2-3 Hz, 3-4 Hz, 4-5 Hz and 5-6 Hz). As result, distinct estimates for relative separation were acquired for each source pair. A posterior analysis was performed in order to verify which frequency bands provides the more reliable and representative estimates for each source separations considered. The CWI results were regarded in a deterministic meaning. This allows to perform the analysis in a simpler and more efficient way, rather than the Bayesian approach. Our results show that for frequency bands between 5-6 Hz, 4-5 Hz and 3-4 Hz the estimates are equally effective for estimates lower than 200 m. For estimated values between 200 and 300 m the frequency bands of 1-2 Hz and 2-3 Hz yields more accurate results. And for values greater than 300 m the 1-2 Hz band provides the better estimates with a growing tendency to underestimate the real distance values. Since the CWI yields relative distance estimates between two seismic events, the main objective of this work is employ them as constraints on relative location in a hypocentral inversion algorithm along with double-difference arrival time data. Although simple double-difference based algorithms have proved to be very effective to provide high resolution relocation results, they are still very dependent to a good station coverage. Dense and well distributed station networks are fairly rare on intraplate regions. In Northeast of Brazil, an intraplate region where seismic activity is commo, it is usual to observe seismic sequences without a proper station coverage. This precludes an accurate hypocentral localization and a deeper analysis. Whereas CWI methodology has the ability to provide constraints on events separations, even using a single station, its employment in the inversion algorithm proposed by this work seeks the improvement of the quality and reliability of the location procedure in areas with poor station coverage and hence contributes to an advance in the knowledge of intraplate seismicity.

BANKING MEMBERS:
Presidente - 1451214 - ADERSON FARIAS DO NASCIMENTO
Externo ao Programa - 026.534.074-85 - FLAVIO LEMOS DE SANTANA
Interno - 1863578 - JORDI JULIA CASAS
Externo ao Programa - 346468 - JOSE ANTONIO DE MORAIS MOREIRA