Local Site Effects on the Damage Distribution during the 2003 Bam Earthquake (Southeastern Iran) and Its Implementation in Preparation of Earthquake Design Spectra for the City of Bam

Abstract

This paper puts forward in-depth investigations on possible local site effects on damage distribution during the destructive 2003 Bam earthquake of southeastern Iran. In this investigation, detailed geotechnical characteristics of soil layers including subsurface soil profile, shear-wave velocity (V_S) profiles along with constitutive soil models were surveyed in different areas of the city. A series of one-dimensional dynamic site response analyses were carried out to evaluate possible local site effects during the earthquake and also to propose site-specific design spectra for the city of Bam. Based on field and aerial surveys after the earthquake, a damage distribution map was prepared and compared with local site effects obtained from the analyses. The calculated PGA values at ground surface in the eastern part of the city were higher than those in the western zone while the maximum PGA occurred in the central part of the city. It was found that the local site conditions affected the pattern of PGA distribution, primarily due to the existence of a thick sedimentary layer extended from the northwest towards the southeast of the city. The distribution of period of maximum amplification complies with the result of geotechnical and geophysical testing and it had the lowest value (less than 0.1 s) in the northeastern parts of the city near a bedrock outcrop. The observed damage distribution was also in close agreement with the computed strong ground motion parameters at the ground surface, clearly showing the impacts of local site conditions.