Stress inversion from the focal mechanism solution of Bam earthquake aftershocks (Iran, 2003)

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Stress determination can be performed by different inversion techniques using fault-slip data resulted from focal mechanism solutions obtained from earthquake shock records. Inversion methods, explicitly or implicitly, use the object function that evaluates the fitness of some assumed stress state to the fault-slip data. The optimal stress state is indicated by the extremum value of the function. Therefore, the process comes down to a peak detection problem. The authors used slip data corresponding to the aftershocks of the Bam earthquake that occurred in Iran in 2003. Several stress states are found in close neighbourhood to the maximum value of the object function. Therefore, the selection of the optimal solution becomes very difficult. To overcome this problem, all the stress states with less than 10% difference from the maximum value of the object function were selected. These states were then categorized according to their stress ratios. Grid points related to the maximum and minimum principal stresses were separately projected on a stereogram. Significant stress states were identified as highly concentrated grid points for each category. The results obtained by this method were compared with those obtained by applying the multiple inverse method. The stress results obtained also matched very closely with the pre-existing fault direction in the region.