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Previous issue date: 1992-09 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior (CAPES) / From May 1987 to April 1988, a 9 station, seismic network was operated near the town of Jo?o C?mara (5?33'S, 35?51'W) in Rio Grande do Norte state, Brazil. The network was installed 6 months after the occurrence of a mb =5.1 earthquake. This thesis is concerned with the study of microearthquakes recorded by this network, which is in the Precambrian Borborema Province of northeast Brazilian shield.
The study revealed a remarkably well-defined distribution of seismicity associated with a simple fault structure. Hypocentre location was done using the HYPO71 program. The results of the analysis show that the fault is divided into two main segments with the same N37?E strike. The north segment is dipping 76?NW. The south segment is more nearly vertical, dipping about 82?NW and composed of three or more closely spaced almost parallel faults. A small gap and a low seismicity zone were found to separate the north and south segments. No events deeper than 9 km were found in the entire region studied. Examination of the epicentral map and corresponding vertical cross sections also revealed that events are not random but are clustered. Right lateral strike slip with a small normal component was inferred as the fault mechanism from composite fault plane solutions.
Off-fault events distant from the main fault were also observed, their pattern showing a classical example explained by the model of Das & Scholz as a consequence of off-fault shear stress increase after the occurrence of the main earthquake. Magnitude determinations for the events recorded by the telemetric network have been performed by developing a specific earthquake magnitude scale based on the duration of the observed seismic signal. A maximum likelihood estimate of b-value shows no significant variation during the recording period.
Shear wave splitting was observed in the J.C?mara region in all the selected events recorded by the three component station JCAZ. The polarization direction of the first split shear-wave arrival for those events lay in the north-south direction which does not agree with the premise of extensive dilatancy anisotropy for a maximum compressive stress in the east-west direction as inferred by the fault plane solutions. / Uma rede s?smica telem?trica composta por 9 esta??es, operou em torno da cidade de Jo?o C?mara (5?33'S, 35?51'W) no Estado do Rio Grande do norte, Brasil, entre os meses de maio de 1987 a abril de 1988. A rede foi instalada 6 meses depois da ocorr?ncia de um terremoto de magnitude mb=5.1. A presente tese analisa os dados de micro tremores registrados por esta rede, e que ocorreram na Prov?ncia Pr? cambriana da Borborema do nordeste brasileiro.
O estudo revelou uma distribui??o de sismos bem definida, associada a uma falha geol?gica de estrutura simples. A determina??o dos hipocentros foi feita usando-se o programa HYPO71. Os resultados das an?lise mostram que a falha ? dividida em dois grandes segmentos com a mesma dire??o de N37?E. O segmento norte da falha mergulha num ?ngulo de 76?NW. O segmento sul mergulha na mesma dire??o, num ?ngulo de 82?NW, e ? composto de tr?s ou mais falhas quase paralelas. Um pequeno vazio de epicentros, acompanhado de uma zona de baixa sismicidade, separam os dois grandes segmentos. N?o foi encontrado evento a profundidades maiores que 9 km. As an?lises dos mapas de epicentros, e das respectivas proje??es verticais, revelaram que os eventos est?o distribu?dos em v?rios aglomerados. O mecanismo de falha foi determinado como sendo do tipo transcorrente, dextr?giro com uma pequena componente normal.
Foram detectados micro tremores distantes, v?rios quil?metros da falha principal. Estes eventos s?o explicados pelo modelo cl?ssico de Das & Scholz, como conseq??ncia do aumento das tens?es de cisalhamento em regi?es fora da falha principal ap?s a ocorr?ncia do sismo principal. As magnitude dos micro tremores foram calculadas atrav?s da determina??o de uma escala espec?fica de magnitudes, baseadas na dura??o do sinal s?smico. Os valores determinados para o par?metro tect?nico b, da lei de Gutenberg e Richter, n?o mostraram varia??es significativas durante o per?odo de observa??es.
O fen?meno da dupla refra??o, nas ondas s?smicas de cisalhamento, foi observada na regi?o de Jo?o C?mara em todos os eventos selecionados e registrados pela esta??o de tr?s componentes JCAZ. As an?lises mostraram a dire??o norte-sul como sendo a dire??o de polariza??o da primeira onda separada, em desacordo com as previs?es da te?ricas da extensive dilatancy anisotropy.
| Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufrn.br:123456789/21695 |
| Date | 09 1900 |
| Creators | Takeya, Mario Koechi |
| Contributors | Douglas, Ross Allan, Main, Ian, Pearce, Robert G. |
| Publisher | Science and Engineering, Gra-bretanha, Faculty os Science and Engineering |
| Source Sets | IBICT Brazilian ETDs |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/doctoralThesis |
| Source | reponame:Repositório Institucional da UFRN, instname:Universidade Federal do Rio Grande do Norte, instacron:UFRN |
| Rights | info:eu-repo/semantics/openAccess |
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