Conventional inversion methods for in situ stress determination from fault slip data obtained from focal mechanism solutions, define the optimum stress as the maximum point of the object function. In this research, object functions are computed for a mesh of principal stress directions and stress ratios for fault slip data obtained from fault plane solution for aftershocks of Bam Earthquake in Iran, in 2003. Several stress state possibilities are found with close values of object function. To overcome this problem, a method is introduced in which the stress states with less than 5 percent difference with maximum value of the object function are selected and categorized according to their stress ratio. Significant stress states for each category are identified as high concentrated loci of principal stresses. The results obtained in this method are compared with results obtained from multiple inverse method which shows a close agreement.
Estimation of in-situ stress state is one of the major tasks in structural geology and rock engineering projects. Knowledge about stress state serves a vital role in certain cases such as large underground structures, deep drilling tasks and borehole stability in petroleum engineering. The process of estimating the stress state from focal mechanisms is called "stress inversion". Most of the stress inversion methods are based on Wallace-Bott hypothesis, which assumes that slip vector is parallel to the resolved shear stress on the fault surface and fault movements are expected to occur in the direction parallel to the maximum shear stress on this plane (Wallace 1951 and Bott 1959). The "Multiple Inverse Method", which is presented by Yamaji 2000, is a method based on separating stresses from heterogeneous fault slip data. This method is based on conventional inverse method and on re-sampling statistical techniques.