Inversion of field data in fault tectonics to obtain the regional stress-II. Using conjugate fault sets within heterogeneous families for computing palaeostress axes

Abstract

SUMMARY In a family of faults that have been neither reactivated nor reoriented, each fault whose direction and sense of movement are known can be assigned Pθ and Tθ axes. If the rocks are isotropic and the orientation of each fault is controlled only by the orientations of the active principal stress axes and the apparent angle of internal friction φ that controls the angle 2θ between conjugate faults, Pθ and Tθ are parallel to the maximum and minimum principal stress axes, respectively. In most cases, however, this relationship does not hold accurately. Defining probability density regions around Pθ and Tθ that take into account variations of fault geometry and using the mathematical process of convolution, one can estimate the principal stress axes associated with any family of faults, whether the family be homogeneous or heterogeneous. The probability regions around Pθ and Tθ are in the form of Gaussian functions. The orientations of principal stress axes determined using different values for the constant C and reasonable values for φ (and therefore θ), are in good agreement with other numerical methods. The only major requirement of the method is the presence of conjugate fault systems within the fault slip data set studied.