17:15 〜 19:15
[SCG61-P08] 断層方位解析による応力・摩擦係数の変遷史解明
キーワード：小断層解析、応力逆解析、摩擦係数、滑り傾向係数
Crustal stress state and friction conditions are crucial parameters to understand fault activities. In order to extimate them, stress tensor inversion techniques are widely applied to geological 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). In this study, the fault instability is conbined with the Wallace-Bott-based technique to estimate both stress and friction conditions from a set of fault-slip data. The new method is planned to minimize the misfit angles between shear stress directions and observed slip directions and to maximize the fault instabilities.
Some artificial fault-slip data were analyzed to test the performance of the new method. As a result, 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 natural outcrop-scale faults found in the Pleistocene Oita Group and underlying Sekinan Group, southwest Japan, which filled the Beppu-Shimabara graben. A N-S trending tensional stress was obtained from both groups, and a NE-SW trending tensional stress was detected only from the Sekinan Group. This result suggests that the tensional principal stress axis was rotated from NE-SW to N-S at ca. 1 Ma. The frictional coefficients were estimated as 1.0 for N-S tension and 0.58 for NE-SW tension in the Sekinan Group and 0.4 for N-S tension in the Oita Group. These results imply that the friction coefficients of faults in relatively unconsolidated sediments are smaller than those in consolidated seiments probably because of diagenesis.
References
Sato, K., 2016, Journal of Structural Geology, 89, 44-53.
Vavrycuk, V., Bouchaala, F. and Fischer, T., 2013, Tectonophysics, 590, 189-195.
Some artificial fault-slip data were analyzed to test the performance of the new method. As a result, 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 natural outcrop-scale faults found in the Pleistocene Oita Group and underlying Sekinan Group, southwest Japan, which filled the Beppu-Shimabara graben. A N-S trending tensional stress was obtained from both groups, and a NE-SW trending tensional stress was detected only from the Sekinan Group. This result suggests that the tensional principal stress axis was rotated from NE-SW to N-S at ca. 1 Ma. The frictional coefficients were estimated as 1.0 for N-S tension and 0.58 for NE-SW tension in the Sekinan Group and 0.4 for N-S tension in the Oita Group. These results imply that the friction coefficients of faults in relatively unconsolidated sediments are smaller than those in consolidated seiments probably because of diagenesis.
References
Sato, K., 2016, Journal of Structural Geology, 89, 44-53.
Vavrycuk, V., Bouchaala, F. and Fischer, T., 2013, Tectonophysics, 590, 189-195.