مروري بر مخاطرات ژيوتكنيكي و آسيبهاي سازه اي در زلزله هاي 21 مردادماه 1391 اهر- ورزقان

A Reconnaissance Report of August 11th 2012, North-West Iran Earthquake

در روز 21 مرداد ماه 1391 مصادف با 11 اوت 2012 در ساعات 16 و 53 دقيقه و 17 و 4 دقيقه دو زلزله به ترتيب با بزرگاي 4/6 و 3/6 استان آذربايجان شرقي را لرزاند. بلافاصله پس از وقوع زلزله گروهي از پژوهشگاه بين المللي زلزله شناسي و مهندسي زلزله براي بازديد از مناطق زلزله زده اعزام گرديد. اين مقاله بخشي از نتايج مشاهدات اين بازديد و ارزيابيهاي پس از آن است. بر اساس آثار مشاهده شده در منطقه زلزله زده، علاوه بر گسلش سطحي، مخاطرات ژيوتكنيك لرزه اي نيز شامل اثرات ساختگاه در تقويت امواج لرزه اي، زمين لغزش و سنگ ريزش به وقوع پيوسته است. بر اساس مشاهدات انجام يافته و گزارشهاي موجود، شهرستانهاي ورزقان، اهر و هرسين كانون بيشترين خسارات بوده است. گزارشهاي رسمي خسارات جاني را 306 كشته و بيش از 1500 زخمي اعلام نموده‌اند. مشاهدات آسيب در ساختمانها نشان مي‌دهد كه آسيبهاي وارده به اجزاي غيرسازه‌اي و تاسيسات خسارات مالي قابل توجهي را به همراه داشته است و موجب توقف خدمت رساني بسياري از تاسيسات گرديده است. همچنين بيشترين قرباني در اين زلزله در اثر تخريب ساختمانهاي روستايي خشتي و گلي يا آجري بدون رعايت ساده‌ترين ضوابط فني بوده است. بيمارستانهاي فعال در منطقه در زمان زلزله از خدمت-رساني خارج شده و بخشهاي غيرسازه‌اي مانند ديوارها و سقفهاي كاذب آسيب شديدي را متحمل شدند. در كنار مشاهده آسيبهاي موضعي، آسيبهاي سازه‌اي و جدي كه باعث توقف خدمت رساني در پلها و ابنيه حمل و نقل و سدهاي احداث شده در منطقه زلزله زده گردد، مشاهده نشد.

Abstract In August 11th 2012, at 16:53 and 17:04 two earthquakes with magnitudes of 6.3 and 6.4 occurred in East-Azerbaijan province, North-West Iran. The epicenter of the event as well as the main damaged area was close to Ahar, Varzaghan and Heris. The number of victims has been reported 306 people and more than 1500 people were announced as injured. Immediately after the events, IIEES dispatched a reconnaissance team to the area. This paper presents a preliminary report of geotechnical and structural damage according to the observations during this reconnaissance trip. The field observation revealed the existence of surface fault rupture with an E-W trending right-lateral strike-slip. Besides, some geotechnical instabilities such as landslides and site effects have been caused by the earthquake. According to the landslide instability, different earthquake-induced landslides have been occurred over the stricken area, with the domination of disrupted slides and falls in rock. Based on the geological map of the stricken area, the existence of the Pliocene lithology in some parts increases the sliding potential. Consequently, the recent earthquake reactivated some of the previous landslides which have occurred during the past decades. According to the topography and site effects, the highest damage states in the Varzeghan city were observed in buildings which are located next to the river. In the Ahar city, many of the damaged buildings were concentrated in the highest zones of the city. The Sattarkhan, Yamchi, Herris, Sabalan and Khodafrin embankment dams and Shahryar concrete dam were observed. Only the Sattarkhan dam suffered some minor damages and no observable damage reported in other dams. In this dam, some cracks in upstream-downstream direction were observed. The intake tower of the dam was also rotated slightly and a 6mm relative displacement was observed at the level of the deck of its access bridge. Moreover, some lightings of the access bridge to the intake tower were broken. The Sattarkhan Dam was operable after the earthquakes. During the inspection from the gallery, no evidence of abnormality including water discharge from the core and cutoff curtain were observed. The main damage occurred in adobe and non-engineered buildings mainly constructed in rural area. Considerable human loss was concentrated in the same area as a consequence of weak construction quality and heavy material. Some important buildings including hospitals and office buildings in the area were non-operational after the earthquake due to improper performance of non-structural systems and equipments. Four bridges were observed in the stricken area. Structural damage and failure of serviceability did not occur. The skewed bridge in Varzaghan experienced rotation of decks and pounding of decks to each other. This caused light damage to the bridge.