The 3-D Spatial Distribution of Shear Strain Energy Changes Associated With the 2016 Kumamoto Earthquake Sequence, Southwest Japan

Shear strain energy is an essential physical quantity governing earthquake generation. To calculate the shear strain energy changes due to an earthquake, we need both coseismic stress changes and background crustal stress. The orientation of the background stress can be estimated from earthquake focal mechanisms, but its absolute level is still uncertain. Assuming the level of the background deviatoric stress to be frictional strength in the crust, we evaluated the three-dimensional distribution of shear strain energy changes associated with the 2016 Kumamoto earthquake sequence, southwest Japan. Its spatial patterns strongly depend on the background stress level. From the energy balance of shear faulting, we proposed that the volume integral of the shear strain energy changes could constrain the background deviatoric stress level. It should be >14 MPa at 10-km depth at the very least. We showed that approximately 75% of aftershocks occurred where the shear strain energy increased.