Inelastic Response Spectrum for Foreshock - Mainshock - Aftershock Sequences

Document Type : Research Article


1 Associate Professor, Structural Engineering Research Center, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran

2 M.Sc., University of Kurdistan, Sanandaj, Iran


The effect of aftershocks on structures are not usually considered in seismic design codes. In addition to mainshock events, aftershocks can cause major damage to structures, especially to mainshock-damaged structures. Analysis of the characteristics of the mainshock, foreshocks, and aftershocks reveal differences in the ground motion parameters. Structures may undergo a variety of seismic waves with different characteristics that can increase the chance of seismic amplification. The present study examined the effects of aftershock as well as foreshocks events on the response of single degree-of-freedom (SDOF) systems with nonlinear behavior. This allowed inclusion of possible differences during calculation of the response spectrum for cases having foreshock and aftershock effects and those excluding these effects. To this end, 38 mainshocks from different seismic regions with moment magnitudes (Mw) greater than 3.5 were used. More than 168 time acceleration histories from mainshock, aftershock, and foreshock events were applied to evaluate the effects of aftershocks and foreshocks on the response spectrum. The parameters of post-yield stiffness ratio (hardening and softening), ductility factor, period, and site classification were taken into account during 121,000 nonlinear analyses on 60 SDOF models. The results show that the aftershocks as well as foreshocks have a significant effect on the response spectrum, increasing the structural response. Consequently, the effect of aftershocks must be considered in the development of design spectra in seismic codes and guidelines.


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