Heterogeneity of Stress Field in NE Japan and Implications for Fault Strength and Earthquake Occurrence Mechanism

Abstract

 The state of stress in the Earth's crust is a key to understanding crustal dynamics. Recent seismological observations in NE Japan clearly show spatial variations of stress orientations in the crust; the stress field is basically consistent with relative plate motions as a first-order approximation, but it rotates significantly beneath mountain ranges or in and around the source regions of large earthquakes. Such observations suggest that the deviatoric stress field produced by relative plate motions is small enough to be disturbed by other stress sources such as topography and earthquake faulting. According to quantitative modeling studies, the deviatoric stress magnitude in the seismogenic zone in NE Japan should be in the order of tens of MPa, which is too small to overcome laboratory-derived fault strength. This suggests that fault weakening mechanisms, including the low frictional coefficient of the fault surface and overpressurized fluids, play an important role in the generation of earthquakes. Deviatoric stress magnitude in seismogeneic zones can be lower than in stable continents if the fault weakening mechanism is related to the subduction process. Given that deviatoric stress magnitude is generally small in the seismogeneic zones of island arcs, various stress sources can lead to heterogeneous states of stress. It is important to take account of stress heterogeneity when modeling crustal dynamics.