On 23 October 2011 (13.41 LT) a disastrous earthquake with a magnitude of MW = 7.2 occurred in Van Province of eastern Turkey. That is the region where the Arabian Plate drifting towards north-northeast collides with the Eurasian Plate, while the other smaller plates in the region are moving apart as if pushed by the stronger Arabian. As a result, the seismic history of the region is quite rich usually resulting in severe losses. The death toll due to the main shock reached 604 people and injured 4,152, with at least 188 pulled out of the rubble of collapsed buildings. The most heavily damaged Ercis city was at a distance of 35-40 km to the north of the causative fault. The city of Van suffered much less damage, being to the south of the fault at a distance of 25-30 km. The damage was concentrated mainly in the old city centers of both cities. On the 9th of November 2011 (21.23 LT) a strike slip earthquake of MW = 5.7 occurred very close to the city center of Van, associated with a different fault. Due to this shock, an additional number of 40 people were killed and 260 injured, while the Bayram Hotel, in Van city, where the authors stayed from 25 to 29 October 2011, collapsed. The hotel building was visually inspected by the first author following a widely accepted methodology and it was considered to be earthquake safe. The available cross-checked information concerning the mechanism of the hotel's collapse is, at first glance, contradicted by the strong motion records presented in the paper. Unfortunately, during the main shock in both Van and Ercis cities, there were no strong motion recordings. In order to infer some basic characteristics of the ground motion due to the main shock, strong motion records from an aftershock, occurring in the same focal volume with that of the main shock, were used. Observations of the response of structures immediately after the main shock were carried out by the authors during their reconnaissance trip and led towards the same goal. The heaviest damage was observed to engineered reinforced concrete buildings. There are some cases of modern and even recently built, multistoried buildings with rather good reinforcement detailing that suffered extended damage or even collapse. On the contrary, nearby non-engineered, low-rise simple or traditional masonry houses, we¬athered the earthquake in the epicentral region almost without any damage. Most of the reinforced concrete structures are quite flexible, without shear walls, and used a flat-slab constructional system of rather small thickness compared to their spans. In spite of those characteristics justifying high flexibility, no noticeable horizontal motion or pounding was observed between adjacent buildings. For this reason it was thought helpful to present a critical evaluation of the published Turkish seismic building codes since 1940. A crucial subject directly related to the incurred damage and discussed by the authors is the widespread practice of building construction in the region that it is not according to the Turkish earthquake code requirements. It is shown that due to the main shock the numerous and heavy losses in Ercis city were due to the dominance of a severe vertical seismic component, while those in Van city due to resonance phenomena caused by the relatively weak horizontal ground motion. The losses due to the event of 9th November in Van city are attributed to the catalytic function of the vertical seismic component. The resulting collapse is usually quite abrupt and does not allow occupants time to egress safely.
Dr Efthymis Lekkas
Ομ. Καθηγητής
