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[SCG50-12] An inversion method of fault-slip analysis to deteremine stress tensor and friction coefficient
Keywords:fault-slip analysis, stress tensor inversion, friction coefficient, fault instability
Stress tensor inversion techniques are widely used to infer crustal stress state from fault-slip data and seismic focal mechanisms. A majority of the techniques is based on the assumption that a fault slips along shear stress direction on the fault planes (Wallace-Bott hypothesis). Such inversion techniques try to minimize the misfit angles between observed slip directions and calculated shear stress directions.
Since the orientations of fault planes themselves are expected to reflect mechanical condition of faulting, Sato (2016) proposed a method to calculate friction coefficient of observed faults by maximizing the fault instability (Vavrycuk, 2014). This method has a defect that it does not consider the precision of the optimal stress tensor separately determined by the preceding stress tensor inversion. In order to solve this problem, this study proposes a new method to simultaneously infer stress condition and friction coefficient from a set of fault-slip data. The present method combines the objective functions of inversion techniques to minimize the misfit angles and to maximize the fault instability.
Some artificial fault-slip data were analyzed to understantd the performance of the present method. As the result of the numerical experiments, the folowing two advantages were found. Firstly, the present method enhances the detectability of stresses when fault planes are concentrated in the orientations of high fault instability, although it was difficult to accurately determine the friction coefficients from heterogeneous (caused by multiple stresses) fault-slip data. Secondly, the method is moderately robust to the change in stress state after formation of fault planes.
The new method was applied to some examples of natural outcrop-scale faults. The first example is from the Pleistocene Oita Group, southwest Japan, which filled the Beppu-Shimabara graben. A NNE-SSW trending tensional stress and the friction coefficient of 0.58. The second example is from the Awa Group in the Boso Peninsula, central Japan, which filled a forearc basin of the Sagami Trough. The analysis showed NE-SW and NW-SE trending tensional stresses clearly. Additionally, a N-S trending compressional stress was detected, which has been difficult to detect with Wallace-Boot hypothesis-based stress tensor inversion techniques.
References
Sato, K., 2016, Journal of Structural Geology, 89, 44-53.
Vavrycuk, V., Bouchaala, F. and Fischer, T., 2013, Tectonophysics, 590, 189-195.
Since the orientations of fault planes themselves are expected to reflect mechanical condition of faulting, Sato (2016) proposed a method to calculate friction coefficient of observed faults by maximizing the fault instability (Vavrycuk, 2014). This method has a defect that it does not consider the precision of the optimal stress tensor separately determined by the preceding stress tensor inversion. In order to solve this problem, this study proposes a new method to simultaneously infer stress condition and friction coefficient from a set of fault-slip data. The present method combines the objective functions of inversion techniques to minimize the misfit angles and to maximize the fault instability.
Some artificial fault-slip data were analyzed to understantd the performance of the present method. As the result of the numerical experiments, the folowing two advantages were found. Firstly, the present method enhances the detectability of stresses when fault planes are concentrated in the orientations of high fault instability, although it was difficult to accurately determine the friction coefficients from heterogeneous (caused by multiple stresses) fault-slip data. Secondly, the method is moderately robust to the change in stress state after formation of fault planes.
The new method was applied to some examples of natural outcrop-scale faults. The first example is from the Pleistocene Oita Group, southwest Japan, which filled the Beppu-Shimabara graben. A NNE-SSW trending tensional stress and the friction coefficient of 0.58. The second example is from the Awa Group in the Boso Peninsula, central Japan, which filled a forearc basin of the Sagami Trough. The analysis showed NE-SW and NW-SE trending tensional stresses clearly. Additionally, a N-S trending compressional stress was detected, which has been difficult to detect with Wallace-Boot hypothesis-based stress tensor inversion techniques.
References
Sato, K., 2016, Journal of Structural Geology, 89, 44-53.
Vavrycuk, V., Bouchaala, F. and Fischer, T., 2013, Tectonophysics, 590, 189-195.