Absolute Tectonic Stress Inversion in the 2016 M_w 7.0 Kumamoto Earthquake Source Region Using Topography and Rupture Models

Abstract

Absolute tectonic stress estimation is of fundamental importance to investigating regional earthquake hazards, while traditional algorithms only invert relative stress and its orientations. We use topography data and rupture models to invert the absolute tectonic stress in the source region of the 2016 Mw 7.0 Kumamoto earthquake. We adopt a basic assumption that the slip direction of each sub‐fault is in the shear direction of the total stress tensor, which is a combination of tectonic and topographic stresses. We use the Bayesian theorem to describe the probability density function of stress parameters and perform a grid search for the optimal values and uncertainties of stress parameters. Our synthetic test shows that the true parameters can be reliably estimated over a broad range of tectonic stress magnitudes. In the application to the Kumamoto earthquake, we find that the optimal tectonic stress is uniaxial tensional with a magnitude of 35 MPa and orientation of N6.5°W, which is at an angle of ∼47.5° to the fault strike. The average total shear stress on the fault plane is 16 MPa on the fault plane. With the inverted stress parameters, we evaluate the pore pressure based on the absolute stress field, the result of which is within a range of 0.3–0.45 of the lithostatic stress.