Modeling of crustal deformation and fault development in the Ou Backbone range and the midNiigata region: Toward an understanding of the mechanisms of large inland earthquakes

Abstract

Modeling of crustal deformation and fault development in the Ou Backbone range and the midNiigata region: Toward an understanding of the mechanisms of large inland earthquakes : AbstractTo understand the generation processes of large earthquakes inland of convergent margins, it is necessary to investigate the tectonic loading processes caused by interactions of plates along plate boundaries, and internal inelastic deformation and fault processes within island arc crust and uppermost mantle. Internal deformation and faulting processes are controlled by non-uniform rheological structures in the island arc crust and the uppermost mantle. This paper reviews studies using numerical models to describe deformation and faulting processes in the crust in the Ou Backbone range (OBR) and the mid-Niigata region, by accounting for the non-uniform rheological structures in the island arc crust. Models of fault processes in these areas must consider non-uniform distributions of viscosity and frictional strength. Beneath the OBR, crustal rocks are thought to have been more weakened than those in surrounding areas because of the presence of aqueous fluids and high temperatures associated with volcanic activity. A theoretical study demonstrates that shortening deformation due to nonlinear viscous flow occurs in the hightemperature section of the lower crust, resulting in shear faulting in the upper crust. On the other hand, it is generally considered that many inland earthquakes occur along pre-existing normal faults formed in a pre-existing rift system within a back-arc region during the opening of the Japan Sea ( -Ma); these faults were reactivated as reverse faults during the shortening deformation that has occurred since . Ma. The numerical models confirm that ancient rift structures and weak zones in the basement caused the development of the complex fault configurations associated with the and earthquakes in the mid-Niigata region. The development of models for the generation processes of large inland earthquakes requires an understanding of the realistic rheological structures which depend on the tectonic settings of the model region.