Modeling the San-in shear zone in southwest Japan: development of the immature shear zone along the volcanic front

Abstract Recent crustal deformation observations have revealed a right-lateral strain-concentration zone trending ENE–WSW in southwest Japan: the San-in shear zone (SSZ). Numerous large earthquakes—such as the 1943 Tottori earthquake (M JMA 7.3), the 2000 Western Tottori earthquake (M JMA 7.3), and the 2016 Central Tottori earthquake (M JMA 6.6)—have occurred in the SSZ. Therefore, understanding the mechanisms of the SSZ is essential in investigating the processes responsible for generating such large earthquakes. In the San-in region, the geothermal gradient is large along the volcanic front where Quaternary volcanoes exist. This study models the development of the SSZ using a finite element method with nonlinear viscoelasticity and Mohr–Coulomb plasticity, considering a heterogeneous thermal structure and strike-slip stress regime. Based on the observed geothermal gradient distribution, we assume that temperature increases linearly with depth. Our model generates a shear zone within the high geothermal gradient zone along the volcanic front. Geological investigations indicate that faults in the San-in region are immature, suggesting that the accumulated shear strain in the SSZ is small. In our model, when the cumulative shear strain is small, on the west side of the SSZ, small fault zones are generated in the oblique direction to the strike of the SSZ in the areas of the local high thermal gradient. As the simulated SSZ develops, fault zones become connected along it. Observations also show that fault zones that are oblique to the SSZ are dominant on the west side of the SSZ. Simulation results indicate that the SSZ is an immature shear zone that has developed along the volcanic front. Our model also reproduces the fault zone for the 1943 Tottori earthquake on the east side of the SSZ. We also investigate the role of localized thermal anomalies in fault formation and find that conjugate faults are formed around regions with a local high geothermal gradient. Many major earthquake faults in the San-in region are thought to be influenced by rheological structures caused by thermal anomalies. Graphical Abstract