A study of the duration of strong ground motion using accelerometric data of subduction and normal faulting Mexican earthquakes is presented. Duration is obtained based on the time between 2.5 and 97.5 percent of the Arias intensity. An expression to predict this duration in terms of the magnitude, distance to the rupture area and site period is proposed. This expression is used together with the random vibration theory to predict response spectra. Three dimensional response spectra of seismic coefficient, structural period and number of inelastic cycles are obtained. Finally, the inelastic structural response of a concrete structure built over the lakebed zone in Mexico City is studied. A synthetic accelerogram in terms of strong motion duration and site period that yields the same inelastic response as the real accelerogram is proposed as a simplified tool to model inelastic behaviour in lakebed zone sites.
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