Processos físicos para a simulação de movimentos sísmicos intensos para os Açores
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2013
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A ocorrência de um evento sísmico de relevante magnitude provoca um impacto devastador, a curto e a longo prazo na zona afetada, a nível social e económico. No sentido de minimizar os danos, é necessário o conhecimento da sismicidade global para que sejam implementadas as medidas regulamentares e financeiras que possibilitem reforçar a educação de populações e a prevenção, resposta e reparação de danos provocados por sismos. É importante a coordenação e a contribuição de todas as equipas multidisciplinares, de alguma forma relacionadas com a mitigação do risco sísmico.
No sentido de estabelecer os cenários sísmicos inerentes ao arquipélago dos Açores, são determinados parâmetros de caraterização dos movimentos intensos do solo da região. A caraterização da ação sísmica é assumidamente composta por três processos distintos mas complementares de libertação de energia na fonte sísmica, propagação da energia sísmica e alteração do conteúdo energético devido a efeitos locais.
As características da fonte sísmica e da propagação de energia no meio, têm origem nos registos em rocha da rede acelerográfica dos Açores e da rede sísmica de banda larga. A informação recolhida, sob a forma de acelerogramas, após processamento, origina os espetro de amplitude de Fourier em aceleração e em deslocamento.
Do espetro de amplitude de Fourier em aceleração, é determinado o fator de decaimento 𝑘, referente ao efeito de atenuação na crusta superior diretamente por baixo do local em estudo (válido para distâncias na ordem das centenas de metros até poucos quilómetros).
Do espetro de Fourier em deslocamento da fonte sísmica, são obtidos a frequência de esquina (𝑓𝑐), o momento sísmico (𝑀0) e a queda de tensão (𝜎), considerando a fonte sísmica como uma fonte pontual e homogénea, descrita pelo modelo ómega quadrado (Boore, 1983).
A determinação dos parâmetros da fonte, permitem a calibração do modelo estocástico de simulação dos movimentos intensos do solo, a utilizar para obter a descrição do movimento do solo num determinado local (amplitude, direção e caraterísticas espetrais), e a obtenção de uma base de dados sintéticos que complementará a base de dados reais e permitirá, entre outros estudos posteriores, a obtenção de leis de atenuação espetrais regionais, em desenvolvimentos futuros.
The occurrence of a seismic event of significant magnitude causes a devastating impact in the short and long term in the affected area, as in the social and economic level. In order to minimize the damage, it is necessary to know the global seismicity to create regulatory and financial measures that enable populations to enhance education and prevention for seismic situations and adequate response to the damage caused by earthquakes. Therefor, it is important the coordination and contribution of all multidisciplinary teams, somehow related to seismic risk mitigation in order to establish the seismic scenarios, in seismic regions, namely in the Azores archipelago, to prevent the strutural behavior of constructions. For the knowledge of the seismic scenarios it is necessary the determination of the strong ground motion of the region, which is assumed to be composed of three distinct but complementary processes, namely the release of the energy in the seismic source; the propagation of the energy and the change in energy content due to local effects. Considering the characteristics of the source and propagation, the study has its origins in the national acelerographic network and broadband seismic network. The information collected by the accelerographs is treatned and the acceleration and displacement Fourier spectrum is generated. From the Fourier amplitude spectrum of acceleration is determined the decay factor k, referring to quench the upper crust directly beneath the site under study (valid for distances on the order of hundreds of meters to a few kilometers). From the Fourier spectrum of the displacement of the seismic source are obtained the corner frequency (fc), the seismic moment (M0) and the stress drop (σ), considering the seismic source as punctual and homogenous, described by the square ómega model (Boore, 1983). The source parameters, allows the calibration of the stochastic simulation of intense movements of the ground, used for the description of ground motion at a given location (amplitude, direction and spectral characteristics), and is obtained a synthetic database to complement the actual database. The obtained parameters can also be used in future developments to determine regional spectral attenuation laws.
The occurrence of a seismic event of significant magnitude causes a devastating impact in the short and long term in the affected area, as in the social and economic level. In order to minimize the damage, it is necessary to know the global seismicity to create regulatory and financial measures that enable populations to enhance education and prevention for seismic situations and adequate response to the damage caused by earthquakes. Therefor, it is important the coordination and contribution of all multidisciplinary teams, somehow related to seismic risk mitigation in order to establish the seismic scenarios, in seismic regions, namely in the Azores archipelago, to prevent the strutural behavior of constructions. For the knowledge of the seismic scenarios it is necessary the determination of the strong ground motion of the region, which is assumed to be composed of three distinct but complementary processes, namely the release of the energy in the seismic source; the propagation of the energy and the change in energy content due to local effects. Considering the characteristics of the source and propagation, the study has its origins in the national acelerographic network and broadband seismic network. The information collected by the accelerographs is treatned and the acceleration and displacement Fourier spectrum is generated. From the Fourier amplitude spectrum of acceleration is determined the decay factor k, referring to quench the upper crust directly beneath the site under study (valid for distances on the order of hundreds of meters to a few kilometers). From the Fourier spectrum of the displacement of the seismic source are obtained the corner frequency (fc), the seismic moment (M0) and the stress drop (σ), considering the seismic source as punctual and homogenous, described by the square ómega model (Boore, 1983). The source parameters, allows the calibration of the stochastic simulation of intense movements of the ground, used for the description of ground motion at a given location (amplitude, direction and spectral characteristics), and is obtained a synthetic database to complement the actual database. The obtained parameters can also be used in future developments to determine regional spectral attenuation laws.
Descrição
Orientação: Alexandra Carvalho; co-orientação: Maria João Falcão
Palavras-chave
ENGENHARIA CIVIL, MESTRADO EM ENGENHARIA CIVIL, SISMOS, PROCESSOS ESTOCÁSTICOS, AÇORES, CIVIL ENGINEERING, EARTHQUAKES, STOCHASTIC PROCESSES, AZORES