Propuesta de espectros de diseño para la ciudad de Tapachula, Chiapas

Abstract

El objetivo principal de este trabajo es establecer el peligro sísmico en la ciudad de Tapachula Chiapas y proponer espectros de diseño a partir de espectros de peligro uniforme (EPU), se incluye el estudio para algunas ciudades del estado de Chiapas. El análisis sísmico del peligro sísmico se realiza con el enfoque probabilista, el cual proporciona un marco de referencia en el que las incertidumbres pueden ser identificadas, cuantificadas y combinadas. Se emplea el programa Ez-frisk asociado al cálculo del peligro sísmico, obteniendo las curvas de peligro sísmico y espectros de peligro uniforme, empleando 3 diferentes distancias de influencia (500, 300 y 200 km.) con el objetivo de conocer la variabilidad de los resultados sísmicos al usar distancias amplias o pequeñas. Al emplear distancias diferentes, los valores obtenidos no tienen una diferencia notable una con respecto a otra. Usando este criterio, para los EPU que se calcularon para los diferentes municipios seleccionados se empleará una distancia de 300 km. Los periodos de retorno empleados fueron Tr de 475, 975 y 2,475 años, obteniendo EPU para diferentes estructuras (Grupo A, B, y especiales). La elección depende del nivel de peligro que se desea emplear, los objetivos de diseño y el nivel de desempeño sísmico para la estructura en estudio. Se comparan los resultados obtenidos, contra espectros de diseño de las normativas vigentes, así como la comparativa de la aceleración (PGA) de sismos real con las obtenidas de los resultados. Finalmente se obtiene el espectro de diseño, empleando la metodología descrita por Chopra ,1995. Empleando los valores obtenidos de este estudio, se considera 5 % de amortiguamiento para sistemas elásticos, estos espectros no tienen amplificaciones dinámicas se consideran en condiciones en roca.

The main purpose of this research is to define seismic design spectra for Tapachula and other cities within Chiapas, Mexico. Chiapas is a state at the southern part of Mexico, with a great seismic activity, it has been hit by M > 8.0 earthquakes in the past. The region where Chiapas is located is surrounded by different active seismic sources. The main seismic activity that affects Chiapas comes from inter and intraplate activity from this tectonic region, according to the historic events that have affected this region. Cocos, Caribbean and North American are the main tectonic plates that can produce earthquakes with medium to large magnitudes, according to: Zúñiga et al., 1999, Belén et al., 2008 and Ordaz et al., 2008. The attenuation relationships that were used considered the seismic activity of this region. All the models that were used characterize the seismic hazard with PGA, Spectral values, or hazard curves that were produce with UHS, PGA and Sa(T). Different attenuation relationships were used for the seismic sources. Probabilistic Seismic Hazard Analysis (PSHA) aims to quantify the uncertainties and combine them to produce an explicit description of the distribution of future shaking that may occur at a site, in this research a PSHA method was used using the EZ-Frisk software. Three different radii were used to capture the variability within the seismic outcomes (200, 300 and 500 km). No differences were significant for the outcomes within this research by using those radii. And therefore 300 km radii were used for all the different studied sites. However, the return periods that were considered were: Tr= 475, 975 and 2,475 years. UHS were obtained for A, B and special structures according to the Mexican Seismic Provisions. PGA values from earthquakes, as well as design spectra from the Mexican Provisions were compared to those values proposed in this research. And we concluded that the values proposed in this research are accurate. The design spectra proposed considered a 5% damping for an elastic system that is supposed to be founded on firm soil.