Earthquake risk assessment for İstanbul

Abstract

Seismic risk analysis can be described as the exploration of seismic hazard and data reflecting the current feature of the site in order to reveal all the probabilities which will trace the level of impact. In general terms, risk is the combination of probability of occurrence and the consequence of a specified hazardous event. In the other words, the value of risk depends on the severity of hazard and the vulnerability of the elements which will be affected by the hazardous event. Vulnerability can be described as inherent characteristics of a system that create the potential for harm but are independent of the probabilistic risk of the occurrence (event risk) of any particular hazard or extreme event. Vulnerability in urban areas can be investigated according to environmental, physical, socio-demographic and economic structures of settlements. Istanbul faced several earthquakes such as it had experienced in 1509, 1799 and 1894. According to the recent scientific researches, in the next 30 year period, another major earthquake will likely occur and will cause vast damages in Istanbul .To examine the earthquake risk in Istanbul, 15 variables are used which represent hazard and urban exposure for 613 neighborhood of Istanbul Metropolitan Area. The methodology of the empirical part of this study is based on principle component analysis which provides to evaluate risk variables through the main factors. Firstly, in order to calculate the earthquake risk of the city, the information on earthquake hazard was compiled according to the previous study developed by Japan International Cooperation Agency and Istanbul Greater Municipality in 2002. The hazard data referred from the earthquake scenario with the MW=7.7 of JICA and IBB project. Moreover, the areas with a slope more than %30 were also included into the database. Since earthquakes can trigger secondary hazards such as urban fires and explosions, hazardous land uses were also mentioned. Land use pattern and building density have been used in order to reveal the general built-up structure of the city and to forecast the probable difficulties in search and rescue operations aftermath of the earthquake. In the demographic indicators, population density and vulnerable population rate according to the age groups have been presented as well. Furthermore, the number of students attending the neighborhood’s schools is defined as vulnerable population. Beside earthquakes give damages to built-up environment; they can affect economic structure in long term period in region and nation wide. Therefore, fiscal indicators are integrated to the database as they embody the likelihood of future economic losses by earthquakes. The potentials comprehend all kind of elements which are able to reduce the negative effects of earthquake. Especially, open spaces and health care facilities are vital to prevent the increase of further losses rising in the short term period aftermath of the earthquakes. Originally, 27 variables were collected, but after testing for multi-collinearity among the variables, 15 independent variables were used in the statistical analyses to calculate earthquake risk levels in Istanbul. These 15 independent variables form 5 main factors using the principal component analysis such as: (Factor 1) vulnerability, (Factor 2) density, (Factor 3) work places and hazardous land uses, (Factor 4) hazard and (Factor 5) potentials. The 5 main components sort according to their explanation percentage of the total variance. The total variance presents the earthquake risk level in this study. The vulnerability component which explains about 18,9% of the total variance, is also the most explanatory factor among the others. In the other words, while describing earthquake risk level in Istanbul, the major indicators are highly related with the exposure and the vulnerability levels of the city. As far as Factor 2 and Factor 3 are also associated with the built-up area of the city, so that the general conditions of urban pattern explain about 47,1% of earthquake risk. In the principal component analysis, the forth factor which refers earthquake hazards explains just 10,8% of the total variance of earthquake risk level.

Özet

Deprem risk analizi; sismik tehlike ve alana ilişkin verilerin bir arada incelenerek olasılıkların ortaya konulmasıdır. Deprem riskini etkileyen faktörler; deprem tehlikesi, arazi kullanımı, demografik yapı ve ekonomik yapı başlıkları altında incelenmektedir. Bu faktörlerin kendi grup ve alt gruplarındaki karşılaştırmaları ve değerlendirilmelerinin yanı sıra, birbirleriyle olan etkileşimleri de deprem risk haritalarının hazırlanmasında önem kazanmaktadır. Çalışma alanı olarak seçilen İstanbul, tarih boyunca sayısız deprem yaşamıştır. Bunların arasında 1509, 1766 ve 1894 depremleri kent genelinde ve çevre yerleşmelerde büyük yıkımlara neden olmuştur. Bu depremlerin oluşumuna neden olan Kuzey Anadolu Fay’ının Marmara içinden geçen kollarının önümüzdeki yıllar içinde yine kırılarak büyük bir deprem oluşturması beklenmektedir. Bu çalışmanın amacı, Türkiye’nin nüfus açısından en büyük şehri olan ve aynı zamanda Kuzey Anadolu Fay hattının batı kanadına yakınlığı nedeniyle deprem tehdidi altında bulunan İstanbul’un yapılaşmış, demografik ve ekonomik çevre değişkenleri ile mevcut potansiyelleri kapsamında deprem riski açısından değerlendirilmesidir. İstanbul’da depreme bağlı risk düzeylerinin hesaplanmasında kullanılan ana bileşenler analizi yöntemiyle belirlenen 5 temel faktörden arazi kullanımına ilişkin olan faktörlerin deprem tehlikesi değişkeninden daha yüksek değerler alması, risk değerlendirmede yerleşmelerin tehlikeye açıklık seviyelerinin ne kadar belirleyici olduğunun bir göstergesidir. Deprem tehlikesinin büyüklüğünün yanı sıra, İstanbul’un gelişmesindeki plansızlık deprem riskinin artmasında en büyük rolü oynamaktadır.