Site Specific Response Analysis and Microzonation for Urban Planning

Ansal, Atilla; Tönük, Gökçe; Kurtulus, Aslı

Site-specific assessment of design earthquake characteristics on the ground surface necessitates seismic hazard and site response analyses to be evaluated with respect to the exceedance levels of 2%, 10% and 50% for 50 years in order to support performance based design procedures. The analysis may be considered as composed of two consecutive stages that can be evaluated separately. The first stage is the estimation of design earthquake characteristics on the rock outcrop based on regional seismic hazard study for all exceedance levels. The second stage is the estimation of design earthquake characteristics on the ground surface by site response analysis based on the geotechnical and geological site conditions. The selected input acceleration time histories are scaled with respect to peak ground accelerations calculated from the earthquake hazard study for different exceedance levels, and are used as input for site response analysis. The peak ground acceleration, peak ground velocity and spectral accelerations corresponding to these exceedance levels are evaluated on the ground surface for microzonation with respect to round shaking intensity. Seismic microzonation involves generation of seismic hazard maps with respect to estimated ground motion characteristics on the ground surface. A grid system is implemented dividing the investigation area into cells according to the availability of geological, geophysical and geotechnical data. Site characterizations are performed based on available borings and other relevant information by defining representative soil profiles for each cell with shear wave velocities extending down to the engineering bedrock. 1D site response analyses are conducted to estimate site specific earthquake ground motion characteristics on the ground surface for each representative soil profile to estimate elastic response spectrum based on calculated acceleration time histories. Average of spectral accelerations between 0.1 and 1s periods of elastic acceleration response spectrum are calculated as one of the two parameters representing earthquake shaking intensity on the ground surface. Site specific peak spectral accelerations corresponding to 0.2s period are also calculated as the second microzonation parameter using the empirical amplification relationships proposed by Borcherdt (1994) based on equivalent shear wave velocities for the top 30m of the soil profiles. Superposition of these two parameters is assumed to represent overall effect of site conditions and is adopted as the criteria for the microzonation with respect to ground shaking intensity. The methodology is presented based on a detailed microzonation study with respect to earthquake ground shaking parameters conducted using an extensive site investigation study that was carried out on the European side of Istanbul as the first phase of the large-scale microzonation project for the Istanbul Metropolitan Municipality. The study was carried out for the Zeytinburnu district in Istanbul using part of these recently complied soil data and the regional probabilistic seismic hazard scenario proposed by Erdik et al. (2004). The microzonation maps are compared with the previously conducted Zeytinburnu microzonation studies (Ansal et al., 2005; 2007; Kılıç et al., 2006) where microzonation maps were produced with limited number of site investigations and site response analyses using more approximate microzonation procedures.