Joaquim Mendes Ferreira

The 1986-1988 intraplate earthquake sequence near João Câmara, northeast Brazil ― evolution of seismicity

Abstract The 1986–1988 intraplate earthquake sequence near Joao Câmara (5.5°S, 35.7° W) has been among the most significant in northeast Brazil this century. Some 14,000 events were recorded in the 18 months from August 1986, including fifteen above magnitude 4.0 and one of 5.1; 4000 buildings were damaged. A local network of four portable seismographs was operated during most of the period, augmented by a nine station telemetric network between May 1987 and April 1988. The temporal and spatial distribution of this seismicity is examined and determination of unusually good hypocentres is made possible by the simple velocity structure, good signal transmission and impulsive P- and S-arrivals on most seismograms. Activity has been closely confined to a 28 km linear zone with a strike of 040 °, except for some small events along a parallel zone displaced laterally by 12 km. Neither zone has been related to any surface feature. Some unusual characteristics have raised concern about possible future seismicity: the largest event occurred four months after the onset of activity and after several weeks of relative quiescence. A 5 km intermediate section of the inferred fault has so far experienced only a few very small events.

Stress magnitude and orientation in the Potiguar Basin, Brazil: Implications on faulting style and reactivation

[1]xa0We analyzed borehole breakout data and drilling-induced tensile fractures derived from resistivity image logs run at 10 oil wells to derive the orientation of the maximum horizontal stress SHmax from the Potiguar Basin in the continental margin of Brazil. Stress magnitudes are derived from density logs for the vertical stress, mini-frac tests for the minimum horizontal stress Shmin, and rock strength laboratory analysis to estimate the SHmax magnitudes. We compared these results with the stress regime and SHmax orientation derived from nine earthquake series located in the crystalline basement, where seismicity is concentrated, and previous breakout data from the basin. In the basin, the SHmax gradient is 20.0u2009MPa/km, and the SHmax/Shmin ratio is 1.154, indicating a normal tectonic stress regime from 0.5 to 2.0u2009km, whereas the SHmax gradient of 24.5u2009MPa/km and SHmax/Shmin ratio of 1.396 indicate a transition from a normal to strike-slip stress regime at 2.5 to 4.0u2009km. The deeper stress regime in the basin is similar to that in the basement at 1–12u2009km depth. This transition of the tectonic stress regime is consistent with an incipient tectonic inversion in the basin. We note that the SHmax orientation rotates from NW–SE in the western part of the Potiguar Basin to E–W in its central and eastern parts, roughly following the shoreline geometry, indicating that local features such as flexural stresses influence the local (scaleu2009<u2009100u2009km) stress pattern. We also conclude that the basement is critically stressed, but not the basin.

Detection of Subtle Hydromechanical Medium Changes Caused By a Small-Magnitude Earthquake Swarm in NE Brazil

Ambient noise correlation analyses are largely used in seismology to map heterogeneities and to monitor the temporal evolution of seismic velocity changes associated mostly with stress field variations and/or fluid movements. Here we analyse a small earthquake swarm related to a main mR 3.7 intraplate earthquake in North-East of Brazil to study the corresponding post-seismic effects on the medium. So far, post-seismic effects have been observed mainly for large magnitude events. In our study, we show that we were able to detect localized structural changes even for a small earthquake swarm in an intraplate setting. Different correlation strategies are presented and their performances are also shown. We compare the classical auto-correlation with and without pre-processing, including 1-bit normalization and spectral whitening, and the phase auto-correlation. The worst results were obtained for the pre-processed data due to the loss of waveform details. The best results were achieved with the phase cross-correlation which is amplitude unbiased and sensitive to small amplitude changes as long as there exist waveform coherence superior to other unrelated signals and noise. The analysis of 6xa0months of data using phase auto-correlation and cross-correlation resulted in the observation of a progressive medium change after the major recorded event. The progressive medium change is likely related to the swarm activity through opening new path ways for pore fluid diffusion. We further observed for the auto-correlations a lag time frequency-dependent change which likely indicates that the medium change is localized in depth. As expected, the main change is observed along the fault.

Seismic activity in Senador Sá-CE, Brazil, 1997-1998

O Acude Tucunduba esta localizado no municipio de Senador Sa, a aproximadamente 290 km a oeste de Fortaleza - CE. O monitoramento sismico da regiao foi realizado com uma rede sismografica local e teve inicio em 11 de junho de 1997, logo apos a ocorrencia de um evento com magnitude 3,2 mb, no dia 09 de junho de 1997. A rede era composta de uma estacao analogica (utilizada na determinacao da magnitude e no controle estatistico da atividade) e sete estacoes digitais. As estacoes digitais, com tres componentes cada, operaram no periodo de junho a novembro de 1997. Neste trabalho foram analisados os dados coletados pelas estacoes digitais objetivando a determinacao de hipocentros e mecanismos focais. Para determinacao hipocentral, usamos um modelo de semi-espaco, de parâmetros iguais a 5,95 km/s, para a velocidade da onda P, e 1,69, para a razao entre as velocidades das ondas P e S. A zona ativa era de aproximadamente 1 km de extensao e a profundidade variando de 4,5 a 5,2 km. Com um conjunto de 16 sismos, registrados na mesmas seis estacoes, foi determinado um plano de falha (NE-SW) com um mecanismo focal transcorrente e uma pequena componente normal. O mergulho foi estimado para esta entre 65oSE, usando o mecanismo focal composto e 80oSE, atraves da razao das amplitudes P/S. Estimativas preliminares do esforco maximo compressivo horizontal, a partir da direcao do eixo-P, estao de acordo com Ferreira et al. (1998). A pequena diferenca pode ser devido a influencia da bacia sedimentar no esforco regional. O comportamento da atividade esta de acordo com a sismicidade descrita por Assumpcao (1998).

Seismogenic faulting in the Meruoca granite, NE Brazil, consistent with a local weak fracture zone

A sequence of earthquakes occurred in 2008 in the Meruoca granitic pluton, located in the northwestern part of the Borborema Province, NE Brazil. A seismological study defined the seismic activity occurring along the seismically-defined Riacho Fundo fault, a 081° striking, 8 km deep structure. The objective of this study was to analyze the correlation between this seismic activity and geological structures in the Meruoca granite. We carried out geological mapping in the epicentral area, analyzed the mineralogy of fault rocks, and compared the seismically-defined Riacho Fundo fault with geological data. We concluded that the seismically-defined fault coincides with ∼E-W-striking faults observed at outcrop scale and a swarm of Mesozoic basalt dikes. We propose that seismicity reactivated brittle structures in the Meruoca granite. Our study highlights the importance of geological mapping and mineralogical analysis in order to establish the relationships between geological structures and seismicity at a given area.