Evaluation of spectral accelerations for Isfahan region and comparison of the results with the spectral acceleration of Iranian Building Code (Standard No. 2800) and IBC

The spectral acceleration، SA، is used as the most commonly tool for the building response analysis. In many international or national engineering standards/codes such asInternational Building Code (IBC) and Iranian Building Code (standard 2800)، the buildings are desiged on the basis of spectral acceleration method. This method is based on orthogonal functions and has received much attention because of its simplicity. The target of this study was to estimate the spectral accelerations of ground motions due to earthquakes and compare them with the results of the standard No. 2800 and IBC in Isfahan and the adjoining regions (the area between 49. 5-54° E and 31-34. 1° N). The spectral accelerations، which are the input data for building design، are determined via spectral attenuation relationships by multiplying the response factor given in Iranian Building Code (standard 2800) to the Peak Ground Acceleration (PGA) determined from attenuation relationships. To this end، first، the seismicity parameters in the interest region for each seismotectonic province were calculated using a unified، homogenized and a complete catalog in the method proposed by Kijko and Sellevoll (1992)، in which one can consider magnitude uncertainty and completeness of data in calculations. Geological maps with scales of 1:100000 and 1:250000 were used to provide the fault map of this region. We determined several probable faults in the interest region to help us to introduce the potential seismic sources more precisely. Based on the fault maps، the potential seismic sources of the interest region were determined (twelve and six potential seismic sources for Zagros and Central-East Iran provinces، respectively). Then، the maps of the maximum and the spectral acceleration seismic zonation were prepared using a modified probabilistic approach (Shi et al.، 1992) as well as the spectral attenuation relationships of Campbell and Bozorgnia (2003) and also Ambraseys et al. (2005). In the modified probabilistic approach، the concept of spatial distribution function is introduced. By calculating the spatial distribution function، the contribution to annual mean occurrence rate of seismotectonic province is made for each potential seismic source. In other words، a spatial distribution function characterizes the seismicity differences among potential seismic sources. The spectral acceleration seismic zonation maps are produced for Peak Ground Acceleration (PGA) and periods of 0. 2، 0. 4، 0. 6 and 1 seconds using the EZ-FriskTM computer program. By multiplying the spectral response given in the standard 2800 to the zonation map of PGA، zoning maps for other periods were obtained based on the spectral acceleration and compared with the results of the direct estimation method. Macrozonation probabilistic seismic hazard maps of the interest region for 10% and 63% probability of exceedance were produced for each attenuation relations. It was shown that the maximum horizontal acceleration in Isfahan city for a 10% exceedance in 50 years، via Campbell and Bozorgnia (2003) and Ambraseys et al. (2005) relations was equal to 0. 1g and 0. 19g، respectively. As the spectral acceleration curve for the Isfahan city showed، the largest horizontal acceleration using Campbell and Bozorgnia (2003) relation obtained as 0. 35g in the period 0. 1 sec، while with the use of the Ambraseys et al. 2005) relations، it was obtained as 0. 61g at period of 0. 11 sec. It means that the frequency content of acceleration derived from both attenuation relationships was the same. In addition، a comparison between the spectral acceleration obtained via attenuation relationships and the one derived from the IBC and standard 2800 of Iran in different periods، exclusively for the city of Isfahan showed that up to 0. 1 sec، the spectral acceleration of standard 2800 of Iran and the values obtained from Ambraseys et al. (2005) relations were substantially close to each other. However، the spectral accelerations derived from Campbell and Bozorgnia (2003) relations have a lower value than those obtained based on standard 2800 and Ambraseys et al. (2005) relations. Considering these results as well as the less involvement of Iranian event data in Campbell and Bozorgnia (2003) attenuation relations than Ambraseys et al. (2005)، the second one was more reliable at the interest region of this study. The spectral acceleration obtained from standard No. 2800 of Iran for the periods larger than 0. 1 sec showed some values more than those obtained from other methods. According to the comparison made in the present study، this discrepancy may be due to a lack of sufficient accuracy of the relations proposed in standard 2800 of Iran. All calculations in the present study have been conducted for soil type I (according to the standard 2800).