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Caracterização da actividade sísmica de Monchique
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Quando se fala sobre a atividade sísmica da Península Ibérica, nomeadamente, Portugal continental, o contexto intraplaca desempenha um papel inferior em relação ao regime convergente entre as placas litosféricas Eurásia e Africana. No entanto, o cluster de sismos identificado na região de Monchique em estudos anteriores está associado à sismicidade intraplaca. O complexo de Monchique é uma intrusão ígnea associada a várias outras intrusões oceânicas e continentais formadas durante a actividade magmática alcalina do Cretácico superior (70-100 Ma). A ascensão e instalação de magma na crosta provoca alterações nas propriedades físicas e químicas das rochas encaixantes, tornando a litosfera mais propícia à formação de fraturas e, consequentemente, à ocorrência de sismos. Neste trabalho, estudou-se a região de Monchique com o objetivo de caracterizar, da melhor forma, a sismicidade e associá-la ao contexto geotectónico da região. Procurou-se, também, compreender os mecanismos desencandadores dos sismos que ocorrem na região. Para esse fim, relocalizaram-se os eventos ocorridos na região de Monchique entre 2007 e 2015 através do programa NonLinLoc. Efetuou-se, também, uma análise das formas de onda apenas dos eventos localizados sobre o maciço ígneo de modo a verificar se o padrão de radiação destes eventos é semelhante e, consequentemente, o tipo de mecanismo focal. Assume-se que a litosfera da região de Monchique se assemelha a uma litosfera de uma zona tectonicamente ativa típica, visto que, o b-value calculado foi de 0.99 ≈ 1. A sismicidade observada em Monchique foi considerada profunda pois, a maior parte dos hipocentros encontram-se entre os 5 e 20 km. A relocalização permitiu reconhecer duas lineações com orientação NNE-SSW e E-W já observada em estudos anteriores, sendo que a última aparenta estar dividida em dois segmentos (ENE-WSW e WNW-ESE), separando as secções norte e sul do maciço. Estas lineações refletem, de forma grosseira, os limites reológicos que se observam à superfície e, existe a possibilidade de que as mesmas possam delimitar falhas ativas existentes no local. Outra hipótese seria que os sismos seriam provocados em pequenas fraturas indicando a presença de uma zona de fratura (shear zone). De acordo com a hidrologia desta região, é possível que a percolação de fluidos e/ou os contrastes reológicos sejam alguns dos principais fatores desencadeadores da sismicidade observada em sobre o complexo de Monchique. Apesar de não se ter calculado mecanismos focais neste trabalho, a análise de formas de onda mostra que existem 100 pares de sismos com coeficiente de correlação maior que 90%, para a onda P, e 15 pares de sismos com coeficiente de correlação maior que 90%, para a onda S. Este facto é uma boa indicação de que o padrão de radiação desses eventos é semelhante e, que, consequentemente, os seus mecanismos focais são idênticos. Não foi possível retirar uma conclusão concreta e objetiva da identificação de famílias de sismos. Contudo, em algumas estações identificaram-se clusters o que sugere que, não existe um único tipo de o mecanismo focal de sismos na região do complexo ígneo de Monchique ou que a instrumentação usada não é adequada (ex: rede sísmica pouco densa).
When we discuss the seismic activity of the Iberian Peninsula, namely continental Portugal, the intraplate context plays a lesser role relatively to the convergent regime between the Eurasian and African lithospheric plates. However, the earthquake cluster identified in Monchique region in previous studies is related with intraplate seismicity. The Monchique complex is an igneous intrusion related to several other oceanic and continental intrusions formed during the upper Cretaceous alkaline magmatic activity (70-100 Ma) The uprise and installation of magma in the crust causes changes in the physical and chemical properties of the nesting rocks, making the lithosphere more favourable to formation of fractures and, consequently, to occurrence of earthquakes. In this work, we study the Monchique region to characterize, in the best way, its seismicity and link the observed seismic activity to the geotectonic emplacement. We also sought to understand the mechanisms that triggers earthquakes in this region. To this end, we relocated the events occurred in the region of Monchique between 2007 and 2015 using the NonLinLoc program. We also performed an analysis of the waveforms of the events closest to the Monchique massif to verify if the radiation pattern of these events and, consequently, the type of focal mechanism is similar. Since the calculated b-value was 0.99 ≈1 , we assume that the Monchique lithosphere resembles a lithosphere of a typical tectonically active zone. The seismicity observed in Monchique was considered as profound because most of the hypocentres are located between 5 and 20 km. The relocation allowed to recognize two alignments oriented NNE-SSW and E-W, already observed in previous studies, which the last one appears to be subdivided in two segments (ENE-WSW and WNW-ESE) separating the north and southern section of the massif. These alignments reflect roughly the rheological boundaries observed at the surface and there is a possibility that they can define existing active faults. Another hypothesis would be that the earthquakes would be provoked in small fractures indicating the presence of a zone of fracture (shear zone). According to the hydrology of this region, it is possible that the percolation of fluids and/or the rheological contrasts are some of the main factors that induce the seismicity observed in the Monchique complex. Although we didn’t calculate focal mechanisms in this work, waveform analysis shows that there are 100 pairs of earthquakes with a correlation coefficient greater than 90% for the P wave, and 15 pairs of earthquakes with a correlation coefficient greater than 90 % for S wave. This is a good indication that the radiation pattern of these events is similar and that, consequently, their focal mechanisms are identical. It was not possible to draw a concrete and objective conclusion from the identification of families of earthquakes. However, in some seismic stations we identified clusters which suggests that there isn’t a unique type of the earthquake focal mechanism in the Monchique igneous complex region or that the instrumentation used was not the best (eg. the seismic network is not dense enough).
When we discuss the seismic activity of the Iberian Peninsula, namely continental Portugal, the intraplate context plays a lesser role relatively to the convergent regime between the Eurasian and African lithospheric plates. However, the earthquake cluster identified in Monchique region in previous studies is related with intraplate seismicity. The Monchique complex is an igneous intrusion related to several other oceanic and continental intrusions formed during the upper Cretaceous alkaline magmatic activity (70-100 Ma) The uprise and installation of magma in the crust causes changes in the physical and chemical properties of the nesting rocks, making the lithosphere more favourable to formation of fractures and, consequently, to occurrence of earthquakes. In this work, we study the Monchique region to characterize, in the best way, its seismicity and link the observed seismic activity to the geotectonic emplacement. We also sought to understand the mechanisms that triggers earthquakes in this region. To this end, we relocated the events occurred in the region of Monchique between 2007 and 2015 using the NonLinLoc program. We also performed an analysis of the waveforms of the events closest to the Monchique massif to verify if the radiation pattern of these events and, consequently, the type of focal mechanism is similar. Since the calculated b-value was 0.99 ≈1 , we assume that the Monchique lithosphere resembles a lithosphere of a typical tectonically active zone. The seismicity observed in Monchique was considered as profound because most of the hypocentres are located between 5 and 20 km. The relocation allowed to recognize two alignments oriented NNE-SSW and E-W, already observed in previous studies, which the last one appears to be subdivided in two segments (ENE-WSW and WNW-ESE) separating the north and southern section of the massif. These alignments reflect roughly the rheological boundaries observed at the surface and there is a possibility that they can define existing active faults. Another hypothesis would be that the earthquakes would be provoked in small fractures indicating the presence of a zone of fracture (shear zone). According to the hydrology of this region, it is possible that the percolation of fluids and/or the rheological contrasts are some of the main factors that induce the seismicity observed in the Monchique complex. Although we didn’t calculate focal mechanisms in this work, waveform analysis shows that there are 100 pairs of earthquakes with a correlation coefficient greater than 90% for the P wave, and 15 pairs of earthquakes with a correlation coefficient greater than 90 % for S wave. This is a good indication that the radiation pattern of these events is similar and that, consequently, their focal mechanisms are identical. It was not possible to draw a concrete and objective conclusion from the identification of families of earthquakes. However, in some seismic stations we identified clusters which suggests that there isn’t a unique type of the earthquake focal mechanism in the Monchique igneous complex region or that the instrumentation used was not the best (eg. the seismic network is not dense enough).
Descrição
Tese de mestrado em Ciências Geofísicas, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, em 2018
Palavras-chave
Localização Hipocentral NonLinLoc Correlação cruzada Monchique Teses de mestrado - 2018