تحلیل عوامل موثر بر فرونشست در دشت سبزوار

مخاطره فرونشست زمین به صورت خزنده از حدود پنج دهه قبل در حال شدت گرفتن است و همه دشت‌ها و مناطق شهری ایران را تحت تاثیر قرار داده است. دشت سبزوار ازجمله مناطقی است که با پدیده فرونشست مواجه است. در این پژوهش ضمن پهنه‌بندی فرونشست، عوامل پایین رفتن سطح آب‌های زیرزمینی، پراکنش قنات ‌ها، چاه ‌ها و گسل‌ها بررسی‌شد. به منظور مطالعه میزان فرونشست از شش تصویر slc باند c ماهواره انویست سنجنده asar در بازه زمانی سال‌های 2004 تا 2008 استفاده شد. همچنین سطح تراز آب‌های زیرزمینی طی دوره‌ی1392-1352 با داده‌های مکانی چاه‌ها و قنات ‌های منطقه اندازه ‌گیری شد. از تصاویر راداری؛ اینترفروگرام تفاضلی، کوهرنسی، کاهش اثرات توپوگرافی، فیلترینگ و درنهایت میزان جابه‌جایی استخراج شد. برای برآورد میزان افت آب‌های زیرزمینی و فرونشست حاصل از آن، سطح آب چاه‌های پیزومتری درون‌یابی شد. برای بررسی رابطه بین پراکنش فضایی فرونشست و موقعیت چاه ‌ها ، قنات‌ ها و گسل‌ها نیز از روش‌‌های همپوشانی استفاده شد. نتایج نشان داد که بیشترین فرونشست در شمال شرق منطقه رخ ‌داده و جهت فرونشست جنوب‌غربی و شمال‌شرقی است. شهرهای داخل  دشت سبزار  ازجمله شهر سبزوار با میانگین نشست حدود 10سانتی‌متر در یک دوره 5ساله مواجه است. روند گسل‌ها عمود برجهت فرونشست است که بیانگر نقش آن‌ها در جابه‌جایی عمودی رو به پایین است. نتایج تحلیل آمار درازمدت سطح پیزومتری نشان داده که درپهنه هموار دشت سبزوار، پایین رفتن سطح آب‌های زیرزمینی به سمت شرق رو به افزایش است؛بنابراین عوامل مهم در رخداد فرونشست در این منطقه در مرحله اول کاهش سطح آب‌های زیرزمینی است. آمار درازمدت سطح پیزومتری نیز تاکیدی بر این رخداد است. گسل‌ها به‌دلیل عمود بودن سطح فرونشست با جهت آن‌ها می‌ توانند نقش تشدیدکنندگی داشته باشند؛ به‌عبارت‌دیگر هرچقدر سطح آب‌های زیرزمینی در این منطقه کاهش یابد با یک ضریب افزایشی، با تشدید پدیده فرونشست مواجه خواهیم بود.

An analysis of factors affecting subsidence in Sabzevar plain

1. Introduction Sabzevar plain is one of the areas facing subsidence phenomenon in Iran due to a sharp decline of groundwater table, development of residential areas over aqueducts or tectonics processes. The present study investigates the impact of these cases. Sabzevar County is located in a northwestern plain in Khorasan on the hillside of Jogatai Mountains. Rapid agricultural development and increased water demand in recent decades have resulted in annual groundwater harvesting of about 400 million cubic meters and an annual deficit of about 30 million cubic meters in water reservoirs. The groundwater table in this plain annually experience an average decline of one meter. Despite increased precipitation in the last two years, only a 10 mm increased precipitation was recorded in Sabzevar station and the area still faces drought according to comparative analysis of rainfall.   2. Methodology Data used in the present study include 6 Cband singlelook complex (SLC) images received from the ASAR sensor of Envisat. These images were captured during June, May, October, and December 2004 – 2008. Moreover, data including the groundwater table and the depth of water in local wells of Sabzevar County were collected from Khorasan Razavi Water Management Organization for the statistical period of 2003 – 2008 and 1974 – 2014. Data collected from local water wells and aqueducts were used to investigate subsidence. Following the geometric recording of the images, related interferograms were prepared. In order to calculate ground displacement, other effects were removed from the interferograms, and the effect of topography was corrected using the STRM digital elevation model (DEM) with a spatial resolution of 90 m to further improve the results. An adaptive filter was applied on the images to reduce the level of noise. In the phase correction stage, DEM produced through interferometry was used to correct the images and separate the deformation signal resulting in a differential interferogram. In order to estimate the groundwater decrease and study the resulting subsidence, the depth and groundwater level of 88 piezometers in Sabzevar were interpolated using the IDW method. Overlap methods were also used to investigate the relationship between the spatial distribution of subsidence occurrence and the location of wells, aqueducts, and faults.   3. Results Results indicates that the deformation of the area is the consequence of the high rate of subsidence in this short period of time. The maximum level of subsidence has occurred in the northeastern parts of the study area with a southwestnortheast direction starting from the hillside of Mish Mountain and moving with an increasing trend towards the hillside of Joghatay Mountain. Sabzevar and other cities of the county face an average subsidence rate of 10 cm per year. Images of displacement in the study area were obtained through interferometry and based on their overlap with subsidence. These images were then used for spatial analysis of aqueducts, wells, faults to study their impacts on subsidence. Results indicates that the subsidence rate has changed from 1 cm/year in 2007 to 14.6 cm/year in 2008. Active faults were also located in the western part of the study area across formations such as conglomerate, sandstone, red marl, and gypsumbearing marls. Faults were generally developed perpendicular to the direction of subsidence indicating their role in downward displacement. Interpolation was performed for the 1974 – 2014 period to study long term consequences of this finding. Findings indicates that the decline in groundwater level has deteriorated moving from Sabzevar plain toward the surrounding areas.   4. Discussion and conclusion The study area was located on the hillside of Joghatay Mountain. Agricultural activities have developed in the area resulting in increased annual demand for water. Despite recent precipitations, the area still suffers from drought, decline in groundwater level, and subsidence. Results of a threeyear interferometry selected from the period for which appropriate images were available have proved the occurrence of subsidence in the study area. A comparison between this image and the piezometric level in similar statistical years indicated the significant impact of groundwater level decline on subsidence. A comparison between the distribution pattern of faults, wells, and aqueducts and the subsidence area showed that a large number of wells were associated with subsidence, and the dominant faults were perpendicular to the surface of subsidence areas (Figure 1). Therefore, groundwater decline was the most important factor contributing to subsidence in this region, and long term piezometric level also have confirmed this effect. Faults perpendicular to the surface of subsidence areas might also intensify this phenomenon. In other words, further decline of groundwater table in the region will result in a higher rate of subsidence.