Tomorrow's Istanbul

The Mw 6.9 earthquake that took place offshore between the Greek island of Samos and Turkey’s İzmir province on 30 October 2020 didn’t come as a surprise (Figure 1 by F.S. Malcioğlu, BÜ, Animation 1 by Dr. M. Çağlar, BÜ). Due to the extensional tectonic regime of the Aegean and high deformation rates, earthquakes of similar size frequently occur in the Aegean Sea on fault segments close to the shores of Turkey, affecting the settlements on mainland Turkey and on the Greek Islands. However, the Samos-Sığacık earthquake was an outstanding and important event for several reasons, as this article demonstrates.

In this paper, we discuss the dynamic nature of risk through the lens of multi-hazard relationships and scenarios. Disaster risk is commonly expressed as (Risk = Hazard × Exposure × Vulnerability). This expression does not communicate the extent to which each term (and therefore risk and impact) can change over time, and any relationships between the four variables. To better convey and discuss multi-hazards and dynamic risk, in July and August 2020 we held two virtual workshops (40 and 35 participants) as part of the GCRF Tomorrow’s Cities Research Hub, which has as its focus four cities Istanbul, Kathmandu, Nairobi, and Quito, with a particular emphasis on the urban poor. During the two workshops, participants (including those from academia, NGOs, and the public sector) from each city generated multi-hazard scenarios that can be used to improve the understanding of dynamic risk and we highlighted three main examples of dynamic risk: (1) The hazard term can involve multiple hazards, with relationships between hazards, and the likelihood or magnitude of single natural hazards and multi-hazard scenarios varying over time. (2) Both the exposure and vulnerability components of the risk equation change over time, and can contribute to the triggering, amplification (or reduction) of multi-hazard events. (3) Progression through multi-hazard scenarios can influence or drive changes in both exposure and/or vulnerability terms. These three statements illustrate the dynamic nature of each component of the risk equation and the existence of relationships between each term. Furthermore, they demonstrate how understanding the multi-hazard landscape and potential multi-hazard scenarios can help to enrich understanding of dynamic risk. This understanding of multi-hazard scenarios can be used to consider potential interventions where risk is dynamic.

Istanbul is a major global urban centre. With city expansion expected to continue over the next few decades there is a real opportunity for urban growth that incorporates disaster risk reduction (DRR). But in order to develop DRR inclusive urban development strategies we need to understand the breadth of hazards that can affect the city and their potential interactions.

To create a single hazard overview for the city we searched through peer-reviewed literature, reports, government websites and international disaster databases for hazard occurrences. Of the 34 natural hazards in our global hazard table encompassing five major hazard groups (geophysical, shallow process, meteorological, hydrological, climatological and extraterrestrial), we found 27 of these had occurred or had the potential to occur in Istanbul. Notable absences were snow avalanches, glacial outburst floods and direct volcanic hazards. However, ash dispersal models show that ash from volcanic eruptions in the Mediterranean can affect the city.

Additionally, we present an interaction matrix for hazards relevant to the city that shows how one hazard may trigger or increase the probability of another. We adapted the global hazard interaction matrix of Gill and Malamud (2014) by removing hazards that were not relevant to Istanbul and supplementing it with specific examples that have occurred in the city. We found 85 such interactions that reveal the potential for interacting chains of natural hazards.

We discuss how multi-hazard scenarios, developed through expert stakeholder engagement and based on the hazard interaction matrix, are an effective way to explore and communicate the dynamic variability of exposure, vulnerability and therefore, multi-hazard risk.

Cevizlibağ Metrobus Overpass has a very important place in Istanbul transportation due to its high pedestrian density and its location on the main road with heavy vehicle traffic. Damage to the Cevizlibağ overpass in a possible earthquake may block the traffic of Istanbul and prevent the aid teams from reaching the earthquake victims. If it collapses in an earthquake while the density of people is high on it, it may cause many casualties. For this reason, this overpass should be designed in such a way that it will not be damaged even in the biggest possible earthquake. Also, there are 4 stairs on the overpass and a platform with turnstiles for Metrobus passes (Figure 2).

This study aims to determine the dynamic (Modal parameters such as modal frequencies and mode shapes) and static properties (Geometry and material properties) of the Cevizlibağ Overpass and to examine the earthquake performance. In this context, the building was analyzed under the earthquake levels defined in the 2018 Turkish Earthquake Code. As a result of these analyzes, the total shear force capacity of the structure and the amount of shear force demanded from the structure were calculated using different earthquake levels.