Urban Seismic Microzonation Studies at Small Scales: Sisli/Istanbul (Turkey)

Abstract

The main purpose of this study is to provide a high-resolution application of urban seismic microzonation. A probabilistic seismic hazard analysis was undertaken in the study area using Poisson probabilistic approaches. The resulting hazard is characterized by a 15% probability that a given level of acceleration will be exceeded in a 50-year period. Using deterministic seismic hazard analysis, the magnitudes were estimated by a quadruple rupture model (with three different fault lengths, 108, 119 and 174 km) of the North Anatolian Fault Zone in the Marmara Region. Using both analyses (deterministic and probabilistic), the magnitude of the design earthquake was taken as 7.6. From this design earthquake, accelerations were estimated for several distances (from 25 to 50 km) using several attenuation relationships. In the second phase of the study, soil amplification factors and site characteristic periods were determined and estimated by seismic measurements and Standard Penetration Test data for the Sisli area of central Istanbul. Microtremor and earthquake data were evaluated to compare the characteristic site period data obtained. According to Eurocode soil classifications, the study area mainly consists of B type soils. A small part of the study area has C and A type soils. Characteristic site periods obtained by seismic methods agree with strong (earthquake) and weak (ambient noise) motion spectral values in some parts of the study area. Finally, a seismic microzonation map was prepared for integrated use, incorporating all geophysical and geotechnical data.