افراز آهنگ لغزش بین گسل‌ها‌‌ی فعال بخش جنوبی البرز مرکزی با وارد کردن برهمکنش مکانیکی بین گسل‌ها‌‌

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

1 1دانشگاه تبریز، تبریز، ایران

2 دانشگاه صنعتی خواجه نصیرالدین طوسی، تهران، ایران

چکیده

برای یک گسل ایده‌آل در محیط کشسان، توزیع لغزش حول یک مقدار مرکزی بیشینه، متقارن و بیضی‌شکل است، ولی در طبیعت این توزیع نه به شکل بیضی و نه متقارن است. توزیع آهنگ لغزش در صفحه گسل به برهمکنش پارامترهای متعددی از جمله هندسه خود گسل و گسل‌ها‌‌ی مجاور، شرایط مرزی روی گسل‌ها‌‌ و مناطق دور و معادله رفتاری محیط اطراف بستگی دارد. از طرفی بررسی این توزیع، نقش مهمی در بررسی انتقال لرزه‌خیزی از یک گسل به گسل‌ها‌‌ی دیگر دارد. باتوجه به واقع شدن کلان‌شهر تهران در بخش جنوبی البرز مرکزی، بررسی برهمکنش مکانیکی بین گسل‌ها‌‌ی فعال شمال تهران، اهمیت ویژه‌ای در تحلیل خطر لرزه‌ای در پایتخت دارد.
در این تحقیق با در نظر گرفتن یک نیم‌فضای کشسان همگن و همسان برای منطقه مورد بررسی اطلاعات هندسی گسل و پارامترهای رئولوژیکی منطقه از منابع متفاوت انتخاب و ثابت فرض شد. در ادامه شرایط مرزی تنش با استفاده از مشاهدات GPS محاسبه شد که عامل ایجادکننده لغزش در گسل است. بر این اساس جهت بیشینه کوتاه‌شدگی در منطقه مورد بررسی دارای امتداد N36.5E است که دامنه آن بزرگ‌تر از جهت بیشینه کششی است. سپس گسل‌ها‌‌ در راستای عمود برهم قفل و در راستای مماسی به‌صورت آزاد رها شدند. با اِعمال شرایط مرزی به سامانه گسلی، آهنگ لغزش بین گسل‌ها‌‌ افراز شد. برای افراز از روش المان‌های مرزی استفاده شد. مدل نابرجایی تحلیلی اکادا (1985) نیز درحکم حل اساسی انتخاب شد.
    نتایج حاصل، نشانگر خروج آهنگ لغزش توزیع شده روی گسل‌ها‌‌ از حالت متقارن و بیضی‌شکل است و از وجود برهمکنش بین گسل‌ها‌‌ تحت تأثیر تنش منطقه‌ای و هندسه گسل‌ها‌‌ی مجاور حکایت می‌کند. همچنین اکثر گسل‌ها‌‌ دارای رفتار چپ‌گرد و معکوس‌اند و آهنگ لغزش‌های چپ‌گرد عموماً بیشتر از آهنگ لغزش‌های معکوس است. علت بیشتر بودن آهنگ لغزش چپ‌گرد نسبت به آهنگ لغزش معکوس در مورد اکثر گسل‌ها‌‌ ناشی از دو عامل برهمکنش مکانیکی بین گسل‌ها‌‌ و تمایل محورهای اصلی جهت بیشینه کوتاه‌شدگی و کشیدگی تانسور آهنگ تنش منطقه‌ای نسبت به امتداد گسل‌ها‌‌ است، به‌نحوی‌که تصویر مؤلفه‌های اصلی تنش در امتداد اکثر گسل‌ها‌‌ بیشتر از تصویر مؤلفه‌های متناظر در امتداد عمود بر آنها است. از طرفی در مدل‌سازی صورت گرفته شیب گسل‌ها‌‌ ثابت در نظر گرفته شده است، درحالی‌که به‌ویژه در مورد گسل‌ها‌‌ی معکوس، شیب گسل با عمق تغییر می‌کند
 
 

کلیدواژه‌ها


عنوان مقاله [English]

Slip rate partitioning among the southern central Alborz active faults by considering mechanical interactions among faults

نویسندگان [English]

  • Asghar Rastbood 1
  • Behzad Voosoghi 2
  • Haniye Tabatabaei 2
چکیده [English]

For an idealized fault, slip distributions are symmetrical about a central slip maximum and follow an elliptical distribution in an elastic material. However, slip distributions in nature are neither symmetric nor elliptical. The distribution of slip along a fault depends on its geometry and that of neighboring structures, the remote boundary conditions and boundary conditions along the fault(s), and the constitutive behavior of the surrounding host rock (Bürgmann et al., 1994). In fact, an interaction among the mentioned parameters determines the manner of the slip distribution on the fault(s).
On the other hand, fault slip distributions play an important role in earthquake studies. Because faults are loaded at very slow rates in continental interiors, interactions among them and the resulting slip distribution can give rise to earthquakes on other faults after a long period of quiescence and seismicity can migrate from one fault to the other (Landgraf et al., 2009).
The Alborz Mountains accommodate about one-third of the Arabian-Eurasian convergence (e.g., Priestley et al., 1994; Berberian and Yeats, 1999; Jackson et al., 2002). The Mosha-Fasham Fault, the Northern Tehran Thrust and the Taleghan Fault are active faults of the North of Tehran in Southern Central Alborz. It is necessary to analyze the seismic hazard in this area by considering the mechanical interaction among faults. 
In this research, a slip partitioning is done among the faults in the North of Tehran. First, an elastic and homogeneous half-space was considered for the study area. Then the geometric data of faults are gathered from geological and geophysical references including the fault length, width, dip, upper and lower locking depths. For Lame coefficients, we used average global values. Both mentioned geometrical and physical data were kept fixed in the modeling process.
Then, a displacement gradient tensor that best fitted the study area is calculated using GPS data by least squares method. The strain-rate tensor and finally stress rate tensor were then estimated using the generalized Hook’s law. It is necessary to note that the orientation of the regional stress field (N36.5E) was kept fixed in modeling for all of the study area. The stress rate tensor acts as a boundary condition in the model. As another boundary condition, the faults were locked in a normal direction but they were allowed to slip freely in strike and dip directions under the influence of stress boundary conditions.
Our problem involved a medium containing faults. Each fault had two surfaces or boundaries, one effectively coinciding with the other. A boundary element method called "the displacement discontinuity method" can cope with this problem. It is based on the analytical solution (Green function) to the problem of a constant discontinuity in a displacement over a finite line segment in a plane of a half-space elastic solid. Okada (1985) analytical solutions were used as Green functions for modeling.
Regarding the strike and dip changes of the selected active faults, fault surfaces were divided into different segments in strike and dip directions with constant strike and dip. In this way, we had 22 fault segments in total. Then the fault segment surfaces were divided into 1×1 km elements. Finally, we had 8248 free slipping elements in strike and dip directions as input for modeling.
In most cases, the results showed that the partitioned slips did not have an elliptical shape. Also, they were not symmetric around a central maximum.
The modeling results showed that most of the faults in the study area were left-lateral strike slip and reverse dip slip faults. Also, the left-lateral strike slip rate magnitudes were often greater than the reverse dip slip ones. This was due to the obliquity of the horizontal principal stress axis relative to the fault strike, so that the along strike of the fault component of the principal stress was greater than the fault normal component.
 
 

کلیدواژه‌ها [English]

  • Mechanical interaction
  • slip rate partitioning
  • Boundary element method
  • Displacement discontinuity
  • GPS
  • stress rate tensor

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