سن يابي لرزه خيزي گسل اهواز و جابه‌جايي مسير رودخانه كارون با استفاده از ترمولومينسانس و مطالعات ژيوتكنيك و شناسايي گسل هاي نوشناخته در پروژه قطار شهري اهواز

Age Dating of Ahwaz Fault Activities and Karoun River Displacement by Thermal Luminescence and Geotechnical Studies and Recording of New-Found Faults During Performance of Ahwaz Urban Railway Projec

شهر اهواز، مركز استان خوزستان در جنوب باختر ايران قرار گرفته است. عناصر اصلي زمين شناسي در اين شهر عبارتند از: گسل اهواز، طاقديس اهواز، سازندآغاجاري و رسوبات آبرفتي و بادرفتي جوان. ساخت پروژه قطار شهري اهواز در سال 1385 شروع شد. گسل اهواز از ميانه شهر عبور كرده و آن را از ديد ژرفاي سنگ بستر به دو بخش شمال- شمال خاوري (فراديواره) و جنوب- جنوب باختري (فروديواره) جدا كرده است. ژرفاي سنگ بستر (سازند آغاجاري) در بخش شمال- شمال خاور به‌طور ميانگين 8 تا 12 متر و در بخش جنوب-جنوب باختر به دليل افت زمين‌ساختي قابل توجه در دسترس حفاري هاي ژيوتكنيك قرار ندارد (شكل 2). رودخانه كارون در دشت خوزستان و شمال اهواز شكل پيچان رودي دارد ولي در مركز شهر در محدوده تلاقي با گسل اهواز شكل خطي پيدا مي كند (شكل 1). به‌منظور بررسي تاثير گسل اهواز بر اين تغيير شكل، ستون چينه شناسي رسوبات كارون به‌طور عمودي و افقي بررسي شده و نتايج نشان از آن دارد كه رژيم رسوب گذاري كارون به‌طور عمودي و همچنين افقي (از مناطق باختري به‌سوي بستر كنوني) در چند هزار سال اخير وارون شده است (شكل 3). به منظور بررسي فرضيه تاثير فعاليت گسل اهواز بر اين پديده، سن رسوبات بستر كارون در ژرفاي مختلف با روش ترمولومينسانس اندازه گيري شد تا ضمن بررسي اين نظريه، زمان يكي از فعاليت هاي اصلي اين گسل معلوم شود. افزون بر اين در طي عمليات اجرايي، دو گسل نوشناخته در ايستگاه هاي ميدان فرودگاه و مصلا به همراه يك گسل احتمالي در محل ايستگاه مركز فرهنگي در منطقه شمال- شمال خاور گسل اهواز شناسايي شده و مختصات محل، شيب، امتداد و جابه‌جايي ظاهري آنها اندازه گيري شد.

Ahwaz city, the center of the Khuzestan province, is located in southwest of Iran. The four main geological phenomena of this area are Ahwaz Fault, Ahwaz Anticline, Aghajari Formation, and Quaternary Alluvium and Eluvium Deposits. The Ahwaz Urban Railway Project and its related comprehensive geological and geotechnical studies were started in 2006 and the first phase of the project consisted of 24 Km parallel twins tunnel & 24 stations. The project level is -17.5m beneath the ground surface, except for the crossing with the Karoun River. The twin tunnels cross under the Karoun River in lower elevation. The most important part of the project is crossing safely under the Karoun River. Therefore, this area, which is located in the southern vicinity of the Ahwaz Fault, was studied precisely by surface and under-ground methods. The Ahwaz Fault divides Ahwaz City into two sections. The north, north-east section (hanging wall), which depth of the bed rock is 8-12m and is covered by young quaternary sediments, and south, south-west section (foot wall), which its bed rock is settled down totally and cannot be reached by geotechnical drilling. The Karoun River has meander form in the Khouzestan Province (also north and south of Ahwaz City), but in center of the city (especially in crossing with the Ahwaz Fault) this form changes into straight shape, so it means that the Karoun River channel in center of Ahwaz is young and affected by the Ahwaz Fault. The logs of geotechnical drilling in river bed and river banks were studied vertically and horizontally in order to find out the stratigraphic changes of the Karoun River sediments. These studies show that stratigraphic sections of the Karoun River reveres from west to east horizontally and also vertically in recent thousands years in crossing area with the Ahwaz Fault. These evidences support the hypothesis of affection of the Ahwaz Fault activity on the Karoun River movement. Therefore, the age of river bed deposits was measured by thermal luminescence method in order to realize the age of one the main Ahwaz Fault activities, which change the river channel. The results of age dating show that the old river channel has changed to the recent one about 4000 years ago. In other words, the Karoun River channel changed after an earthquake struck this area. In addition, two new faults were discovered in the Foroodgah square and the Mosala stations during the excavation in north-northeast of the Ahwaz Fault. An inferred fault was discovered in the Cultural Center station as well.