Tectonic Soil Vulnerability Mapping, Instanbul Site
Zulfikar, Can¹; Mine B., Demircioglu¹; Stramondo, Salvatore^2
1Bogazici University, KOERI, TURKEY; ²INGV, ITALY

Istanbul's long history of earthquake damage relates to the North Anatolian Fault that passes only a few tens of kilometres away beneath the Sea of Marmara. During the last 500 years, at least eight earthquakes with magnitudes greater than 7, have occurred close to Istanbul - causing high casualties and great damage. Recent studies show that the probability of an earthquake greater than 7 affecting Istanbul within the next 30 years now stands at 53%. Rapid population growth (10-fold in the last 50 years) has resulted in hastily constructed new building stock that often does not comply with required standards.

The European Space Agency's Terrafirma project has used radar satellite data, processed using the PSinSAR technique, to reveal where the geology and soils, that form the foundations of the city, are soft and more subject to earthquake shaking. This provides a key element in assessing vulnerability and risk across the city. Knowing this soft geology beneath cities is also important for those not subjected to major earthquakes.

Soil Vulnerability analysis
The PSInSAR (or PSI) coverage of the Metropolitan area of Istanbul was obtained for the period 1992-2010. 6 SAR frames were used with processing applied to two Envisat frames along the Ascending orbit, two Envisat frames along the Descending orbit, and two ERS frames along the Descending orbit.

Application and Validation
The basic approach towards the assessment of the spatial variation of geotechnical conditions of the foundation geology includes the determination of the soil classes to be used. For this purpose, the NEHRP (1997) soil classification was adopted. This classification has international acceptance in the earthquake engineering profession and facilitates the differentiation of ground motion (i.e. site response or site amplification) with respect to different site classes. The PSI results relating to soil vulnerability help in assessment of earthquake risk when analysed in combination with regional tectonics-geology, site classes, liquefaction potential, slope-stability and social exposure data of buildings, lifelines and infrastructures. The integration of PSI results and existing information help to determine areas at risk and appropriate resettlement areas. The PSI results combined with past earthquake damage data verify the consistency and utility of PSI data for further risk assessments. Considering Istanbul's current building stock, and the fluctuation between its day time and night time populations, these PSI results can help in developing mitigation measures and resettlement areas. In addition to the building stock in the city, the lifelines and infrastructure inventories can be also taken into account in order to assess their vulnerability according to the subsidence data provided by the PSI maps.

Interpretation
The PSI results on soil vulnerability in combination with regional tectonics-geology, site classes, liquefaction potential, slope-stability and social exposure data of buildings, lifelines and infrastructures, help to assess earthquake risk. The integration of PSI results and existing information helps to determine areas at risk and resettlement areas.