Modeling Deformation Processes of Northeastern Japan Island Arc Considering Rheological Structure Affected by Hot Mantle Fingers

Abstract:

The northeastern Japan island arc is one of the most favorable locations for studying rheological structures because many observational data are available, including seismological, geodetic, and geothermal data, and also because significant crustal deformation has been taking place since the 2011 Tohoku-oki earthquake. Dense geothermal observations were conducted using Hi-net boreholes (Matsumoto, 2007). By considering a thermal structure based on the dense geothermal observations, we model the stress state of the northeastern Japan island-arc crust using a finite element method with viscoelasticity and elastoplasticity. We consider realistic petrological structures of the upper crust, lower crust, and uppermost mantle to define flow properties. We apply an E–W contraction as a boundary condition. We reproduce several elongate low-stress regions as shown in the figure, where brittle-ductile transition zones are shallow, striking transverse to the arc with viscous deformation. These low-stress regions correspond to hot fingers (high-temperature regions in the mantle wedge, indicated by white lines in the figure). Regions with seismicity correspond to regions with high stress accumulation outside of the hot fingers and many intraplate earthquakes occur within these regions.

The viscous relaxation process after the 2011 Tohoku-oki earthquake could have been affected by the existence of low-viscosity regions caused by the hot fingers. A finite element model is developed to investigate the viscoelastic deformation processes after the earthquake, which consider the realistic crustal and mantle structures, viscoelasticity, and coseismic fault slip distribution (Iinuma et al., 2012). We also consider the realistic 3D heterogeneous viscosity distribution in the crust and the upper mantle beneath the northeastern Japan island arc. Our numerical results indicate that significant extensional viscous deformation occurrs in the low-viscosity regions in the hot fingers. This affects the crustal movements in the inland area significantly.