Considerations on the 21st Anniversary of the Earthquake of August 17, 1999
The scale of the damage and casualties, and the extent of the physical, social and economic consequences of the 17th August 1999 Kocaeli Earthquake, which happened right to the east of Istanbul, strongly demonstrated the importance of a paradigm shift from emergency response to hazard risk mitigation as part of established politics in urban development plans. In the 21 years following the 1999 earthquakes, initiatives and projects have been conducted to mitigate risks in the urban context, of infrastructure, and those of public and private properties. Research and implementation projects were initiated regarding the earthquake hazard and and risk; new laws and regulations were enacted for the earthquake risk management; and new structuring mechanisms were introduced to governmental institutions and organizations. Since 1999, in Turkey, there have been significant and destructive earthquakes whose magnitudes were more than 6.0. After each earthquake, the public questioned the safety of the cities and looked for some tangible examples of risk mitigation. Governmental and local institutions focused more on the emergency response and implementation plans; while designing and introducing new or improved initiatives towards the final aim of improving the earthquake risk management capabilities after each earthquake. We should accept the fact that the effects of the Silivri Earthquake that happened on 26/09/2019 were more than anticipated. It has been pleasing to see the governmental institutions refocusing on Istanbul’s earthquake problem and the new initiatives and studies in planning to tackle it.
Due to the complicated structure of the North Anatolian Fault (NAF) in the Marmara region, all settlements there have historically suffered from destructive earthquakes. Large magnitude earthquakes produced by the northern segments of the North Anatolian Fault (NAF) in the Marmara Sea affecting wider regions.
Looking at the earthquakes generated by NAF in the last 300 years, we see a sequence of four major earthquakes from East to West: One in 1719, similar to the 1999 Izmit Earthquake, another in 1754 in the Marmara Sea and lastly two earthquakes in May and August 1766. More recently , the 1894 earthquake caused considerable damage in Istanbul and in the Eastern part of the Marmara Sea, while in 1912 another major earthquake hit the settlements along the Tekirdağ-Şarköy-Mürefte line. The lack of a destructive earthquake in the central Marmara Sea in the 19th and 20th centuries is considered as a seismic gap. Among the significant earthquakes produced in the southern Marmara region are the two 1855 Bursa Earthquakes, the 1953 Yenice-Gönen and 1964 Manyas earthquakes. In addition to these earthquakes with magnitudes larger than 7.0, among the earthquakes in the magnitude range of 6.0-6.5 were the 1935 Marmara Islands, 1943 Hendek-Adapazarı, 1963 Çınarcık and 1983 Biga earthquakes. The recurrence time of the significant earthquakes on the main segments of the NAF is linked to the amount of stress accumulated on these segments. Thus, we can say that the possibility of earthquake occurrence on those segments with no recent large magnitude events is increasing steadily.
This kind of time dependent computations suggest a 10-15% increase in the earthquake hazard of cities such as Istanbul, Tekirdağ, Yalova and Bursa, as compared to traditional time-independent probabilistic estimates. On the other hand, as it is the case in several regions of Turkey, many settlements in the Marmara region are under the threat of smaller magnitude events which might occur at closer distances and become destructive due to their proximity. Moreover, the density of population and buildings in the region increases the destructivity of these earthquakes. For this reason, it is important to evaluate the earthquake risks, i.e. the damage, casualties, and economic losses that might be expected in all the major cities of the region.
Recent risk estimations conducted for the metropolitan area of Istanbul indicated that an earthquake with the magnitude of 7.5 in the Marmara Sea would cause heavy and very heavy damages to approximately 50,000 buildings and cost 15,000 lives related to structural damage. It should be noted that these estimations are valid for median ground motions caused by an earthquake of such magnitude. As shown in the study, the extent of damage and the number of casualties could considerably change depending on the source and rupture characteristics of the earthquake, resulting in different levels and distributions of ground motion.
It should be noted that the reported figures are only for the Istanbul metropolitan area. Large earthquakes in the Marmara Sea will inevitably affect neighbouring cities such as Yalova, Kocaeli and Tekirdağ in addition to Istanbul, thus increasing the total number and extent of damage. For this reason, studies on the risk estimation in significant cities in the entire Marmara region by taking different scenario earthquakes on the primary and secondary segments and parts of the NAF should be conducted. In order to find a reliable baseline for these kinds of studies, the inventories related to buildings, population and infrastructure in these cities should be carefully kept and curated. Such inventories need regular updates. Furthermore, qualitative and quantitative consistency among inventories kept and used by different institutions in terms of earthquake risk related data should be ensured. Studies on damage estimation should be reviewed every ten years and it should be monitored if there is any increase or decrease in the earthquake risk as result of urban renovation and transformation, increase in the number of buildings in compliance with the regulations or any other related reasons. Thus, the hazard risk management mechanisms should be guided more efficiently. In addition to these, it should be remembered that the Marmara region hosts many industrial installations. It is important to assess the earthquake risk in the industry in order to prevent the short, middle and long term economic loss and the environmental disasters caused by the leak of inflammable, explosive and poisonous substances. Finally, some risk assessment studies to calculate the potential damage in electricity, gas, waste-water and transportation networks should be conducted in all cities of the Marmara region.
