Absolute stress field inversion using focal mechanism rotation and co-seismic stress change: Application to 2011 M9 Tohoku, Japan, earthquake

The stress field is a key factor controlling the earthquake nucleation, rupture propagation, and arrest processes, which is essential for understanding the rupture process and evaluating the earthquake hazard. We utilize the focal mechanism rotation phenomenon that occurs after a large earthquake to determine the absolute stress field around faults by employing a non-linear inversion. We assume that there are numerous existing faults in the crust, and some of them with certain directions are allowed to rupture according to the Coulomb failing criterion. The co-seismic stress perturbance induced by the mainshock, calculated from co-seismic slip models, causes the focal mechanism rotation of aftershocks compared to earthquakes before the mainshock. Thus, we use the Bayesian method to invert the absolute stress tensor and friction coefficient before the mainshock for effectively explaining the focal mechanism rotation. Results of synthetic tests indicate that the true parameters can be tightly constrained by accurately fitting the P-axis distributions before and after the mainshock, especially when conjugate faults are absent, by incorporating the prior distribution of the P-axis and friction. Finally, we apply our inversion algorithm to the 2011 Tohoku earthquake. Based on the appearance of normal earthquakes at depths shallower than 10 km and comparisons of data fitting for different co-seismic slip models, we infer that the largest co-seismic slip occurred shallower than the hypocenter and extends upward to the seafloor. The optimal inversion results show an increase in deviatoric stress magnitude with depth, coupled with a rotation of the maximum compressional stress direction from horizontal to vertical. This suggests that deep creep-slip loading significantly influenced the stress field in the stick-slip zone. Moreover, we calculated the pore pressure from the isotropic stress magnitude, directly derived from our inversion algorithm. The average value in the source region of the mainshock is ~0.92. It's probably due to the existence of high-pressure fluid, the megathrust fault is relatively weak (~24MPa).