SEISMIC HAZARD AND RISK ASSESSMENT OF TURKEY

Turkey, being located in a highly seismically active region, has experienced several devastating earthquakes throughout its history. The recent Mw 7.4 1999 Kocaeli and Mw 7.2 2011 Van earthquakes caused a considerable amount of economical losses and physical damage, along with great numbers of casualties. This is due to not only to the high seismicity of the region but also due to the high physical vulnerability of the built environment. Therefore, developing an effective seismic hazard assessment along with a comprehensive seismic risk evaluation becomes an important matter for a sustainable development of the country. As both of these components involve uncertainties, a probabilistic scheme to calculate the seismic losses needs to be employed. This paper provides some important outcomes of a probabilistic seismic risk analysis for Turkey. The results of this comprehensive assessment are crucial to understand how the risk is distributed across Turkey and thus how this risk can be effectively managed. INTRODUCTION The focus on seismic risk calculation has become an important topic in recent years in order to take action to decrease the human and economic losses due to earthquakes. The estimated risk profile then can be used to strengthen the current buildings, to propose design methods for new buildings and to establish an effective disaster management. Three components are required for seismic risk assessment: seismic hazard, exposure and physical vulnerability. Considering the seismic hazard, probabilistic seismic hazard assessment (PSHA) is conducted in order to account for both epistemic and aleatory uncertainties in the source models and ground motion prediction equations (GMPEs). The required seismic sources for the PSHA used herein were obtained through the European FP7 Project SHARE (Seismic Hazard Harmonization in Europe, and were used together with a set of GMPEs applicable to Turkey. In addition to the comprehensive seismic hazard analysis, an exposure data, which should sufficiently reflect the structural characteristics, value and spatial distribution of assets, is required. The 2000 Building Census Survey carried out by Turkish Statistical Institute (TUIK) contains information on buildings at the province-level. According to this source, the Turkish building stock is mainly composed of lowto mid-rise reinforced concrete (RC) infilled frames and unreinforced masonry (URM) structures. 1 PhD Candidate, REM Programme, UME School, IUSS Pavia, Italy, [email protected] 2 Dr, Department of Civil Engineering, University of Aveiro, Portugal, [email protected] 3 Dr, EUCENTRE, Italy, [email protected] S.Özcebe, V.Silva, H.Crowley For what concerns the physical vulnerability, fragility functions, providing the probability of exceeding different damage states conditional on ground shaking intensity, derived by Erberik (2008) for the most common building typologies in Turkey were combined with a damage-to-loss model modified from DEE-KOERI (2003) and Bal et al. (2007) to obtain a set of vulnerability functions in terms of the loss ratio (i.e. ratio of cost of repair to cost of replacement). The seismic risk calculation for Turkey was carried out using the OpenQuake-engine (Silva et al., 2013), the open-source software for seismic risk and hazard calculations developed by the Global Earthquake Model (GEM) initiative. Risk metrics in terms of the economic losses due to structural damage at different return periods have been investigated. These results were disaggregated according to the different building typologies, in order to assess which construction types are contributing the most to the aggregated loss. Although the economic losses are higher for the RC building stock, the ratio between the structure-specific average annual loss (AAL) and the respective economic value is estimated to be greater for masonry typologies. THE METHODOLOGY The probabilistic seismic hazard assessment for Turkey has been carried out using source models, which were developed within the European FP7 Project SHARE. In the SHARE Hazard model, there are three seismic models to assess the occurrence of earthquake activity: a classic Area Source Model (see Figure 1 this is a model that combines activity rates based on fully parameterized faults imbedded in large background seismicity zones), the Fault-Source and Background (FSBG) Model (Figure 2), and a kernel-smoothed model that generates earthquake rate forecasts based on fault slip and smoothed seismicity (SEIFA). These models use different assumptions to estimate earthquake activity rates in the European region (more information can be found on the project website: http://www.share-eu.org/ and portal: http://www.efehr.org). Figure 1 SHARE Area (Active Crustal) Source Model v6.1 for Turkey S.Özcebe, V.Silva, H.Crowley Figure 2 SHARE Fault-Source and Background v6.1 for Turkey By using the OpenQuake hazard engine, the seismic source models together with GMPEs are combined within a classical PSHA framework using a logic tree approach to produce hazard curves at a range of fractiles. For this study, the GMPE proposed by Akkar and Bommer (2010) is used to calculate peak ground acceleration (PGA) and peak ground velocity (PGV). To account for site conditions in the hazard calculations, the shear wave velocity in the upper 30 meters of the soil (Vs30) is used to identify soil types in the region. For this purpose, as a simplified approach, Vs30 values are obtained by using the Global Vs30 Map Server developed by Wald and Allen (2007) and were included in the hazard calculations. The mean hazard map in terms of peak ground acceleration (PGA) for a probability of exceedance of 10% in 50 years is shown in Figure 3. It can be noted that the resulting hazard map is comparable with the current earthquake zoning map of Turkey (http://www.deprem.gov.tr/sarbis/Shared/haritaaciklama.aspx). Figure 3 Mean seismic hazard map in PGA (g) for a probability of exceedance of 10% in 50 years (475 years return period). S.Özcebe, V.Silva, H.Crowley The 2000 Building Census carried out by Turkish Statistical Institute (TUIK) contains data at the province level on several attributes of buildings such as structural system and material, number of stories, date of construction etc.