Strain Energy Transfer by Plastic Flow in San-In Shear Zone, Japan: Shear Strain Energy Change Due To Out-Of-Plane Inelastic Strain Distribution

Abstract

The present study investigated the shear strain energy changes in a shear zone associated with distributed deep deformation and strike-slip earthquakes. A model of a very long strike-slip fault system was developed in which an inelastic strain distribution is introduced as deformation source. We derived analytical solutions for displacement and stress to estimate the changes in the shear strain energy. Applying this model to the San-in shear zone, Japan, we reproduced the observed surface velocity distribution. Our findings indicate that the deep shear deformation decreases the strain energy in deep crust and increases it in the shallow seismogenic zone, suggesting energy transfer. We also found that the deep shear deformation roughly follows a simple flow law, with the inelastic strain rate aligned with the background stress field, indicating plastic flow beneath the seismogenic zone. However, the width of the deep plastic flow remains poorly constrained by surface deformation observations. The coseismic energy drop at the centroid of the earthquake fault associated with two large earthquakes (Mw 7.0 and 6.6) was inferred to correspond to an energy accumulation period of approximately 500–2,000 years.