توسعه یک روش هندسی جدید به منظور برآورد نرخ لغزش گسل های زاگرس با استفاده از بردارهای سرعت شبکه ژئودینامیک

نوع مقاله: مقاله پژوهشی

نویسندگان

1 دانشیار، گروه ژئودزی، دانشکده فنی، دانشگاه تهران، تهران، ایران

2 دانشیار، دپارتمان اکتشافات معدنی، دانشکده مهندسی معدن، دانشکده فنی، دانشگاه تهران، تهران، ایران

3 دانش آموخته مهندسی نقشه برداری-ژئودزی، دانشکده مهندسی نقشه برداری و اطلاعات مکانی، دانشکده فنی، دانشگاه تهران، تهران، ایران

10.22131/sepehr.2019.36606

چکیده

این پژوهش روشی هندسی را برای محاسبه نرخ لغزش گسلهای زاگرس ارائه میدهد. در این روش هر گسل را به صورت مجموعه پیوستهای از قطعات گسلی در نظر میگیریم که موقعیت سطحی آنها معلوم است. اولین مسئله در انجام این محاسبات تعیین وضعیت صفحات گسلی موجود در لایه لرزهزای زاگرس است. این کار را با استفاده از موقعیت کانونی زمین لرزههای رخ داده در اطراف قطعه گسلی انجام میدهیم. با محاسبه ضرایب صفحات گسلی، میتوان آزیموت امتداد صفحات را نسبت به شمال نجومی به دست آورد. تنوع در مکانیزمهای حرکتی گسلهای منطقه، شکستگیها با راستاهای متفاوتی را بر روی سطح زمین ایجاد میکند که از اختلاف بین راستای گسل و راستای شکستگی سطحی و نوع حرکت گسل (شیبلغز و امتدادلغز) زاویه لغزش گسل را به دست میآوریم. در ادامه و به منظور محاسبه نرخ لغزش هر گسل، پوسته زاگرس را به صورت جسم یکپارچهای در نظر میگیریم که در اثر اعمال تنش به صورت یکنواخت تغییرشکل میدهد. به کمک این فرض بردارهای سرعت شبکه را بر روی سطح گسلها تصویر کرده و با استفاده از بردار جهت لغزش، نرخ لغزش گسل را محاسبه میکنیم. نرخ لغزش به دست آمده پارامتری است که برای هر قطعه گسلی به صورت جداگانه محاسبه میشود. با در نظر گرفتن اثر خطاهای سیستماتیک در موقعیت کانونی زمین لرزهها، نرخهای لغزش به دست آمده برای قطعات گسلی همواره دارای خطا هستند. لذا به منظور کاهش اثر خطا،برای هر گسل یک نرخ لغزش میانگین تعریف میکنیم. بردارهای سرعت مورد استفاده در این تحقیق، بردارهای سرعت ایستگاههای دائمی شبکه ژئودینامیک زاگرس هستند که توسط سازمان نقشهبرداری کشور تهیه شدهاند. موقعیتهای کانونی زمینلرزهها نیز توسط پژوهشگاه بینالمللی مهندسی زلزله و زلزلهشناسی منتشر میشوند.نتایج به دست آمده از این روش نشان میدهند مناطقی که گسلهای آنها نرخ لغزش بالایی دارند، از تراکم زمینلرزه بالایی نیز برخوردارند.

کلیدواژه‌ها


عنوان مقاله [English]

Developing a new geometric method to estimate the slip rate of Zagros faults using velocity vectors of geodynamic network

نویسندگان [English]

  • Mohammad Ali Sharifi 1
  • Abbas Bahroudi 2
  • Saleh Mafi 3
1 Research Institute of Geoinformation Technology (RIGT), School of Surveying and Geospatial Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran
2 Exploration Department, School of Mining Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran.
3 Graduated Student, School of Surveying and Geospatial Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran
چکیده [English]

Extended Abstract
Introduction
Attitude determination of the fault planes and slid movements occurring on these planes are among the topics of interest to geoscientists. Among the methods that have been introduced to determine the attitude of the fault planes so far, the use of geological tools for justifying the geometry of the faults with surface outcrops, and examining the changes of the stress field and the displacements appeared on the Earth’s surface can be mentioned. The slip rate is calculated using the displacement of the sedimentary rock layers relative to the displacement time and the simulation models.
Materials & Methods
In this research, a geometric method is presented to calculate the slip rate of Zagros faults. We consider each fault as a continuous set of fault fragments whose surface positions are known. Given that most of the Zagros faults are hidden, locating thefaults is carried out using the geologicalmap of Iran’s faults. The first issue in performing these calculationsisto determine the attitude of the fault planes in the Zagros seismogenic layer. The seismogenic layer is that part of the earth's crust whose deformation is elastic, and the major fractures caused by the earthquakes occur in this part. In order to determine the attitude of the fault’sfocal plane, we use the focal coordinates of the earthquakes occurringaround each fault segment. In performing these calculations, the focal locations of the earthquakes are transferred to the geodetic coordinate system and, the equation of the fault plane is calculated using the least squares method in the Cartesian coordinate system.
One can obtain the azimuth of the strike of the planes relative to the astronomical north by calculating the coefficients of the fault planes. To determine the azimuth, we first obtain the unit vector of the strike line by cross product of the geodetic z-axis (normal vector of the horizontal plane) and the normal vector of the fault plane. The fault plane azimuth will then be the angle between the strike line vector and the north vector.The north vector is the vector which is determined by connecting the point located on the center of each faultfragment to the intersection point of the horizontal plane and thez-axis. Variation in dynamic mechanisms of thefaults in the region creates fractures with different directions on the ground. We obtain theslip angle (rake) of the fault from the difference of the fault direction and the direction of thesurface fracture and the type of thefault (strike-slip, dip slip and oblique).By calculating the slip angle, we now calculate the unit vector of the slip direction from the rotation ofthe strike line vector as much asthe rake angle.
 
Results & Discussion
In order to calculate the slip rate of each fault, we consider Zagros crust as an integrated object, which deforms uniformly by imposing the stress. Based on this assumption, we project the velocity vectors of the Zagros geodynamic network on the fault planes and calculate the slip rates using the slip direction vectors. It should be noted that the velocity vectors of the geodynamic network have been defined in the navigation coordinate system. According to the definition of the fault plane equations, it is necessary to transfer the velocity vectors to the geodetic coordinate system. The resulting slip rate is a parameter which is calculated for each fault fragmentindividually.
Considering the effect of the systematic errors in the focalposition of theearthquakes, (including the error of the focal depth and the epicenter location), the slip ratesobtained for the fault fragmentsalways have systematic errors. Therefore, we define an average slip rate for each fault in order toreduce the error effect.
In this study, velocity vectors of seventeen permanent stations of the Zagros geodynamic network provided by the National Cartographic Center (NCC) are used. The focal positions of the earthquakes are also published by the International Institute of Earthquake Engineering and Seismology (IIEES).
 
Conclusion
The obtained results showed that the regions with high fault slip rate usually have dense earthquakes. In addition, the seismicity potential of any region can be found by comparing the slip rate of each fault and the density of its earthquakes in the region. According to the changes in the slip rate obtained in Zagros, faults in the western part of Zagros, especially in Ilam province, have low slip rates. However, the province is considered asone of the seismic areas of the state in terms of earthquake density.It means that most of the slip movements occurringon the faults of the western region have been accompanied by vibration.

کلیدواژه‌ها [English]

  • Slip rate
  • Fault plane
  • Focal coordinates of earthquakes
  • Velocity vector
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