Constraint on the background stress in the source region of the 2016 Kumamoto earthquake sequence based on temporal changes in elastic strain energies and coseismic stress rotation

SUMMARY We investigated the deviatoric stress magnitude of the background stress fields before the 2016 Kumamoto earthquake sequence in Japan based on temporal changes in elastic strain energies caused by the mainshock and coseismic stress rotation. We modelled the six components of background stress fields from stress orientations together with the parameter of effective friction coefficients (μ’ = 0.3, 0.15, 0.1, 0.05 and 0.03) using the 3-D Mohr diagram. We computed the absolute stress fields immediately following the primary events (the largest foreshock and mainshock) of the earthquake sequence by combining coseismic stress change fields with background stress fields. The total amount of elastic strain energy released by the mainshock increased with the effective friction coefficient. Considering the energy balance in which some part of the released energy must be consumed as radiated energy, we understood that the model with μ’ = 0.03 was unrealistic. We also examined the dependence of coseismic stress rotation on the effective friction coefficient. Furthermore, we applied a stress inversion method to moment tensor data of earthquakes to directly estimate coseismic stress rotation. Comparing the theoretical and estimated coseismic stress rotations, we concluded that the models with μ’ = 0.3 and 0.15 were more consistent with the observations than those with μ’ < 0.1. In the reasonable models, the deviatoric stress magnitudes were 37–65 and 39−70 MPa at a depth of 10 km on the northern and southern source faults, respectively.