Near field effect on horizontal equal-hazard spectrum of Tabriz city in north-west of Iran

The near field ground motions have a high amplitude pulse like at the beginning of the seismogram which are significantly influenced by the rupture mechanism and direction of rupture propagation. this type of ground motion cause higher demands for engineering structures and its response spectrum is dramatically different than far field spectra. tabriz is one of the ancient cities in azerbaijan province with many industrial factories, financial centers and historical monuments in north-west of iran. in this region, north tabriz fault which has a well known history of intense seismic activity is passing through in close distance of urban area. in this regard investigation of near field ground motion effect on current practice seismic design spectrum in this region is necessary. hence, probabilistic seismic hazard analysis is carried out using appropriate attenuation relationship to consider near field effect. the peak ground acceleration (pga) and several spectral accelerations (sa) over bedrock are estimated for different return periods and maps of iso acceleration contour lines are provided to indicate the earthquake hazard in different points of tabriz city. afterward, the generated horizontal equal-hazard spectrums considering near field effect are compared with different spectrums developed based on simple pulse model for near field motion. both types spectrum are used for verifying current practice seismic design spectrum of iranian code (2005) and international building code (ibc 2000). the results reveal the longperiod structures which are seismically designed based on current practice seismic codes are in high risk of being damaged by attenuation relationships during near fault ground motion.