In this paper we have obtained the solution of the plane problem whi-ch has two semi-infinite cracks (they are situated on the same line) and is subjected to the stress along the boundaries at infinity whose principal-vector is a constant.We have analysed the character of this stress field:The boundaries of the tension-pressure stress regions and the cracks divide a plane in six parts.These distributions are essentially decided by the di-rection of the principal-vector of that exterior forces.
Since the linear solution and its stability are discussed,it is possible for us to analyse the extended form of the cracks and obtain stronger fr-acture condition than "coulomb's fracture criterion".The approximate ex-pression of the fracture length is obtained.
In all the development process of an earthquake we can assume that the exterior forces are continually strengthened along the same direction by the aid of the geological mechanics.Calculation,however,shows that the elastic deformation has made the direction somewhat deflective,which is related to the locked segment.Hence the developmental process can be di-vided into two phases as the principal-vector attains the particular direc-tion.In the first phase the cracks have no extension.Through calculating the additional stress field with timeparameter,we discuss the changes of the stressfield which are caused by the aforesaid deflection.In the second phase the cracks have extension or the pure shear dislocations at either end of the locked segment take place.We analyse the stress character in the phase with the help of the calculation of additional stress field which is caused by the dislocations.We discuss the relations of the earthquake sequences and the strong earthquake.
Finally,We discuss the plane problem with two semi-infinite cracks which are situated on different lines and obtain the chief results which are similar to what we have said before.