The Trans-Mexican Volcanic Belt (TMVB) is an active volcanic chain being deformed by an intra-arc extensional fault network. Although several crustal earthquakes with magnitude>7 have originated in the TMVB since the 16th century, the background seismicity of this geological structure is very low and the region is usually considered of low seismic hazard. In this study, we present an updated probabilistic seismic hazard model of the TMVB. The seismicity catalog used here includes forty-three historically and instrumentally recorded earthquakes, from 1858 to 2014; five of these are large earthquakes that occurred in the TMVB during the XIXth century. Due to the lack of a statically representative sample, we propose, in a qualitative manner, the seismicity catalog is complete for M≥4 since 1964 and for M≥6 since 1858. Moreover, we introduce three different earthquake frequency-magnitude relations. The first one is a conventional Gutenberg Richter fit of the distribution of the instrumentally recorded earthquakes data. The other two are non-conventional, semi-parametric approaches that integrate the historical and the instrumental data to determine seismicity rates in the region. Our preferred model (seismicity model B) fits separately the instrumental and the historical data and merge the two fits into one curve. A uniform seismic hazard (USH) of the TMVB for a return period of 500 years was calculated considering three major sources of earthquakes: 1) Subduction thrust-faulting events in the Middle American Trench (MAT); 2) Earthquakes within the subducted Cocos plate and, 3) Shallow crustal earthquakes in the TMVB. According to the seismicity model B, the average recurrence time of a M≥7 earthqua-ke on the TMVB is approximately 150 years. In contrast, the recurrence time estimated from the instrumental catalog is 12,000 years. The results of this seismicity model, which is based on historical and instrumental data, agrees also with the return periods of prehistoric earthquakes, estimated for short segments of the fault system in the TMVB in paleoseismological studies. When comparing the results of our preferred seismicity model, the PGA estimated using only the instrumental seismicity are 18 to 56% smaller than those predicted by the model using the historical catalog.
RESUMENEn este artículo se estudia el problema que representa la combinación de los efectos sísmicos en las estructuras causados por la acción simultánea de los dos componentes ortogonales horizontales del movimiento del suelo. Mediante el uso de la teoría de vibraciones aleatorias se desarrollan algunas expresiones analíticas que permiten combinar los efectos sísmicos causados por la acción de ambos componentes ortogonales horizontales del movimiento del suelo con el propósito de estimar la máxima respuesta elástica, bi-direccional de la estructura. La principal hipótesis que se hace en el desarrollo de estas expresiones, se refiere a la representación del espectro de amplitudes de Fourier de ambos componentes del movimiento del suelo mediante funciones tipo delta de Dirac. Por medio del análisis elástico bi-direccional, paso a paso de distintos modelos estructurales, se verifica la precisión del planteamiento de combinación propuesto. El procedimiento desarrollado es aplicable exclusivamente al caso de suelo blando y considera explícitamente el ángulo de incidencia del temblor y el tipo de respuesta que se analiza (ortogonal o colineal). ABSTRACTThis work studies the problem that represents the combination of the seismic effects on structures caused by the action of two horizontal orthogonal components of the ground motion. By using random vibrations some analytical expressions were developed to combine the two horizontal orthogonal seismic effects in order to estimate the elastic bi-directional peak response. The main hypothesis made in the development of these expressions was to represent the Fourier's spectrum for the two orthogonal components of the ground motion by a Dirac's delta. Through the fully bidirectional elastic "step by step" analysis of some structural models, it is verified the accuracy of the proposed method. The exposed procedure is exclusively applicable to soft soil. It considers explicitly the angle of earthquake incidence and the kind of response in terms of the direction of its components (orthogonal or collinear). INTRODUCCIÓNDurante la ocurrencia de un temblor, las estructuras se ven sometidas a un campo complejo del movimiento del suelo. Sin embargo, en la práctica es común analizarlas, únicamente, bajo la acción de dos componentes ortogonales horizontales del movimiento del suelo. En algunos casos (regiones cerca del epicentro del temblor, por ejemplo) se incluye un tercer componente ortogonal, el cual actúa en la dirección vertical.Artículo recibido el 14 de noviembre de 2007 y aprobado para su publicación el 20 de noviembre de 2008. Se aceptarán comentarios y/o discusiones hasta cinco meses después de su publicación
A vida tem sido muito generosa comigo. A Ortodontia me deu muito, não apenas cientificamente, mas também por me abençoar com a oportunidade de encontrar grandes colegas e fazer novos amigos. Peço permissão para apresentar um deles, uma estrela em nossa especialidade.
Pode-se dizer, com toda a segurança, que a Dra. Máyra Seixas é um daqueles raros talentos, difíceis de se encontrar em qualquer profissão. A Ortodontia foi o lugar que ela escolheu, especialmente, para disponibilizar o seu virtuosismo, figurando como uma clínica habilidosa, exímia escritora e professora carismática. Essa baiana de humor refinado, misturado com posicionamentos firmes, agrega atributos suficientes para se tornar uma pessoa querida por muitos e respeitada por todos. Faz parte de uma geração de profissionais que valorizam a Ortodontia “raiz”, na qual as novidades podem e devem ser bem-vindas, desde que apresentem evidências clínicas e científicas consistentes e vantajosas, que justifiquem possíveis mudanças.
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