افراز نرخ لغزش در گسل شمال تبریز با استفاده از مشاهدات دائم و دوره‌ای GPS

نوع مقاله : مقاله تحقیقی‌ (پژوهشی‌)

نویسنده

استادیار دانشکده عمران، دانشگاه تبریز، تبریز، ایران.

چکیده

برای گسل ایده­آل در محیط کشسان، توزیع لغزش حول یک مقدار مرکزی بیشینه متقارن است. در طبیعت، توزیع لغزش در صفحه یک گسل به برهم‌کنش پارامترهایی ازجمله هندسه خود گسل و گسل­های مجاور، شرایط مرزی روی گسل­ها و مناطق دور و معادله رفتاری محیط اطراف بستگی دارد. مطالعه این توزیع، نقش مهمی در بررسی انتقال لرزه­خیزی دارد. باتوجه به واقع شدن کلان‌شهر تبریز در مجاورت گسل شمال تبریز، مطالعه برهم‌کنش مکانیکی بین بخش­های مختلف این گسل از اهمیت ویژه‌ای جهت تحلیل خطر لرزه‌ای برخوردار است.
    در این تحقیق با درنظرگرفتن یک نیم­فضای کشسان همگن و همسان، اطلاعات هندسی گسل و پارامترهای رئولوژیکی منطقه از منابع مختلف انتخاب و ثابت فرض شد. شرایط مرزی کرنش با استفاده از مشاهدات دائم و دوره‌ای GPS شمال غرب کشور محاسبه و گسل در راستای عمود برهم قفل و در راستای مماسی به‌صورت آزاد رها شد. با اعمال شرایط مرزی در حالت­های مختلف، نرخ سالیانه لغزش روی گسل افراز شد. جهت افراز از روش اجزای مرزی استفاده و مدل تحلیلی اکادا نیز برای حل اساسی انتخاب شد.
    نتایج نشان­دهنده وابستگی نرخ لغزش توزیعی روی گسل به شرایط مرزی و مؤید وجود برهم‌کنش بین بخش­های مختلف این گسل است. همچنین نرخ لغزش افرازی، امتدادلغز راست‌گرد بودن گسل را در کلیه حالت­ها نشان می­دهد. نرخ لغزش توزیعی روی کل صفحه گسل تقریباً متقارن است و در حوالی کلان‌شهر تبریز به بیشینه مقدار خود یعنی 5/5 میلی­متر در سال می­رسد. نزدیک بودن نرخ لغزش افرازشده به مقادیر دیرینه­لرزه‌شناسی نشان­دهنده نزدیک به واقعیت بودن نتایج افراز با روش اجزای مرزی نسبت به نتایج دیگر روش­های تحلیلی و عددی است.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Slip rate partitioning in the North Tabriz fault using permanent and campaign GPS observables

نویسنده [English]

  • Asghar Rastbood
Assistant Professor, Civil Engineering Faculty, University of Tabriz, Tabriz, Iran
چکیده [English]

Fault slip distribution plays an important role in earthquake studies. Because faults are loaded at very slow rates in continental interiors, interaction among faults and 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.
    In this research, slip partitioning was done along the North Tabriz Fault (NTF). First, an elastic and homogeneous half space was considered for the study area. Then geometric data of NTF collected from geological and geophysical references including fault length, width, dip, and locking depth. For Lame coefficients, we used average global values. Both mentioned geometrical and physical data were kept fixed in the modeling process.
    Then, displacement gradient tensor that best fits the study area estimated using GPS data by least squares method. Next, strain rate tensor and finally stress rate tensor were estimated using generalized Hook’s law. Stress rate tensor acts as a boundary condition in the model. As other boundary conditions, the NTF was locked in normal direction but it was allowed to slip freely in strike and dip directions under the influence of boundary conditions.
    Our problem involves a medium containing NTF. Each fault section has two surfaces or boundaries, one effectively coinciding with the other. A boundary element method called displacement discontinuity can cope with this problem. It is based on the analytical solution (Green’s function) to the problem of a constant discontinuity in displacement over a finite line segment in a plane of a half space elastic solid. Analytical solution of Okada (1985) is used as Green’s function for modeling.
    Regarding the strike changes of NTF, the fault surface was divided by different segments in strike direction with constant strike and dip. As a result, we had eleven fault segments. Next, fault segment surfaces were divided into elements. Finally, we had free slipping elements in strike and dip directions as input for modeling. The results indicate the dependency of the distributed slip rate on the boundary conditions and confirm the existence of interaction among different parts of fault. Also, partitioned slip rate shows that NTF is right-lateral strike slip fault in all cases. Moreover, it is almost symmetric and reaches its maximum near the Tabriz metropolis. We show that the maximum slip rate in the fault plane is reduced by partitioning, which it will be definitely closer to reality. According to the meshing done for slip rate partitioning, we get a maximum slip rate of 5.5 mm/year in the northern part of Tabriz city.
The proximity of the partitioned slip rate to the paleo-seismic values indicates the closeness of the partitioning results to reality with the Boundary Elements Method compared to other analytical and numerical methods.

کلیدواژه‌ها [English]

  • slip rate partitioning
  • Mechanical interaction
  • Boundary Elements Method
  • GPS observation
  • North Tabriza Fault (NTF)
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