مدل سازي رياضي تنش موثر ناشي از مخزن سد كرخه و اثر آن در ايجاد گسيختگي روي گسل دالپري

Mathematical modeling on the Karkheh reservoir stresses and its application to the Dalpari fault

In this research, a new mathematical modeling on strength changes due to reservoir elastic stresses across the preexisting fault plane is introduced. The method has been applied to the Dalpari fault, which is one of the potential seismic sources in the vicinity of the Karkheh reservoir. In this method the distribution of total stress across the fault cannot E-mail: hodhodil355@gmail.com -Y\-AA- -W- ♦Y\-Y\TYRT^ ^ A if? ajji tlsii j ij+tj be determined because the initial stress is unknown; the pore pressure due to the reservoir is also not considered. The mathematical modeling method has been explained briefly in the following. The lake first is divided into small rectangles of sides a and b by two sets of orthogonal straight lines, one set conveniently east-west and the other north-south. The mean water depth h in each rectangle with area S is estimated, and the water pressure on the floor of the rectangle is replaced by a vertical force F = pgS h at the center of rectangle. It is clear that rather smaller rectangles lead to more precise modeling, hence, each rectangle with increasing h is divided into some parts. The water pressure of the lake is simulated by a set of point forces F which applied in the -X3 direction and acting on the rectangles. We define now a mathematical model of the single force F in the elastostatic fields using the delta function conception: The point force F is defined as: F = Fa5(r) The ith component of displacement at point p(Xpx2,x3) due to F in the jth direction, u^, is given by: uUF/87^(5ljrkk -r^) where j=3, for the water pressure of the lake, and r = ((xt)2 +(x2)2 +(x3)2)2 is the distance from origin to point P. The general three dimensional relationships between nine Cartesian strain component e- and three Cartesian displacements (u,,u2,u3) are given by: These nine terms constitute the infinitesimal strain tensor, a symmetric tensor with six independent quantities. The stress tensor is given by stress-strain relationships based on constitutive law called Hookeʹs law is given by: Qij ~ +2|I Gjj Using the conception of the stress tensor and well-known relationships in elastostatic theory the various stress parameters such as shear and normal stresses due to reservoir can be determined at the point P in a plane with normal n. In this way, we would be able to achieve the strength values due to the reservoir across the specified preexisting fault plane. The shear stress (xm) and strength (snr) due to the reservoir across the preexisting fault plane are, respectively, as follows. Tnr=Tn.COS(0) Snr ~ Tnr ~~ H^n 6 is angle measured in the preexisting fault plane between resolved shear stresses due to reservoir and ambient causes, and it is measured from the direction of the latter coefficient of friction along the fault plane, ^ is coefficient of friction across the preexisting fault plane. The earthquakes near new reservoirs is classified into the following three cases on the basis of positive, negative and zero values of snr; a reservoir based on this classification may stabilize some parts and destabilize other parts of the same nearby fault surface. Case I: induced or reservoir assisted natural earthquakes; snr > 0. This situation will arise when 0 < 0 < 90 and the reservoir stresses have a net destabilizing influence on