تعيين ميدان جابه‌جايي ناشي از فعاليت هم‌لرزه گسل شيب‌لغز و امتدادلغز با استفاده از روش المان محدود

Finite element modeling of deformation field induced by dip and strike slip fault coseismic activity

ميدان جابه‌جايي حاصل از فعاليت هم‌لرزه گسل‌هاي شيب‌لغز (نرمال و معكوس) و امتداد‌لغز (چپ‌گرد ) با استفاده از آناليز عددي المان محدود در يك محيط همسانگرد همگن كشسان خطي برآورد شده است. با كمك روش عددي المان محدود مي‌توان پيچيدگي‌‌هاي محيط واقعي را براي توليد تابع‌‌‌‌هاي گرين دقيق‌تر و به‌كارگيري در حل مسيله‌‌ معكوس به‌منظور استخراج نرخ لغزش گسل‌ها، مدل‌سازي كرد. به همين منظور با به‌‌كارگيري المان‌‌‌هاي تماسي در سطح گسل ميدان جابه‌جايي حاصل از گسل‌‌هاي عمقي و امتدادلغز با مقدار جابه‌جايي واحد محاسبه شد. گسل شيب‌لغز، سطحي در نظر گرفته شده و عرض آن 20 كيلومتر است كه براي مدل‌سازي محيط نيم‌‌فضا طول و عمق محيط به‌ترتيب 1000 و 500 كيلومتر لحاظ شده است و در مقابل ابعاد گسل بزرگ است. طول و عرض گسل امتداد‌لغز به‌ترتيب300 و 20 كيلومتر و ابعاد محيط نيم فضا 1000، 300 و 120 كيلومتر است. گسل شيب‌لغز در محيط دوبُعدي و گسل امتدادلغز در محيط سه‌بُعدي مدل‌سازي شده است. ميدان جابه‌جايي حاصل از مدل عددي با مدل تحليلي اوكادا مقايسه شده است. مقايسه نشان مي‌‌دهد كه انطباق قابل‌‌قبولي بين ميدان جابه‌جايي افقي و قايم حاصل از مدل عددي و تحليلي وجود دارد.

Many earthquakes occur in Iran every year and some of these earthquakes cause loss of life and property. Consequently earthquake is one of challenging topics not only in Iran but also in other active tectonic regions in the world. Investigating the mechanism of earthquake as a natural disaster is the first and important step. To study earthquakes, different information such as geometry and behavior of active faults, as well as the mechanical properties of the earth’s upper most layers, are required. Geometric and rheology properties of earth’s layers as well as details of the contemporary strain, temperature and stress have increased significantly over the past decade. Furthermore thanks to availability of Global Navigation Systems (GNSS), like GPS, modern space geodesy data processing and new tools like PS-InSAR that provide unforeseen spatial coverage of precise observations of the Earth’s surface deformations. The only processing approach that composes all geometrical and physical complexities is Finite Element Modeling (FEM). The first step in using FEM is to examine its capabilities. In this paper deformation field of a dip slip (normal and reverse) and a strike slip fault (left lateral) in linear homogenous isotropic elastic medium by means of 2D and 3D Finite Element Method (FEM) has been investigated. By means of FEM, the complexity of mechanism of a fault related disaster for determination of precise Green operator and solution of a reverse problem for extraction of fault slip rate can be modeled. As a sample, we apply contact elements and develop a frictionless fault surface and then deformation field of dip and strike slip faults for one meter of slip for each side of fault surface. Fault top lines for dip and strike slip faults are assumed to be on the ground. The dimensions of semi infinite medium for dip and strike slip faults are respectively 1000*500 and 1000*3000*120 km and these dimensions relative to fault’s dimensions are large. FEM deformation field are compared to Okada analytical model (an analytical model). The comparison shows that there is a good agreement between FEM and the analytical model. Our procedure can be summarized as follows: 1- 2D geometrical modeling of 90, 70 and 25 degree dip slip faults and 3D vertical strike slip fault. 2- Meshing of medium and assign material properties (linear homogenous isotropic elastic). 3- Apply boundary conditions by horizontal and vertical displacement vectors. 4- Determination of horizontal and vertical displacement vectors on the ground by means of FEM. 5- Comparison of analytical (Okada model) and FEM results and computation of Root Mean Square (RMS) as an efficiency test of results.