Faulting processes controlled by the nonuniform thermal structure of the crust and uppermost mantle beneath the northeastern Japanese island arc

摘要

We performed a finite element analysis, taking into account nonlinear viscoelasticity and plasticity, to construct a two-dimensional model of the deformation and faulting processes that occur in the deeper parts of the seismogenic zone beneath northeastern Japan. Our finite element code was developed on the basis of the GeoFEM parallel finite element code using plug-ins to adopt several nonlinear functions. An extensive onshore-offshore wide-angle seismic expedition conducted across northeastern Japan in 1997 revealed a detailed image of the deep crust and uppermost mantle structures. In modeling the deformation and faulting beneath the Ou Backbone Range and Dewa Hill, northeastern Japan, we consider a realistic crust and uppermost mantle structure revealed by the 1997 seismic expedition and the existence of the two hot regions revealed by geothermal observations in a compressional tectonic setting. The numerical results reveal that shortening deformation due to nonlinear viscous flow occurs in the hot region in the lower crust and the uppermost mantle, resulting in shear faulting in the upper crust. Our model reproduces three reverse fault zones that are consistent with those observed. We investigate the stress accumulation process over a period approximately corresponding to the earthquake recurrence interval by prohibiting plastic faulting and found that the stress is loaded along the fault zone at magnitudes much lower than the absolute stress level. The results indicate that the loading processes of large inland earthquakes in northeastern Japan are determined by the nonuniform thermal structure in the deeper crust and uppermost mantle.