The weakened lower crust beneath the Nobi fault system, Japan: Implications for stress accumulation to the seismogenic zone

The 1891 M8 Nobi earthquake, which occurred along the Nobi fault system in central Japan, is the largest crustal earthquake in Japanese historical records. Here, we present a new estimate of the 3-D seismic velocity structures around the Nobi fault system using a large number of arrival time data obtained from both temporary and permanent seismic stations. The results show that the middle and lower crust in the northern part of the Nobi fault system has lower seismic velocities that are 4-9% lower than those of the surrounding area. This low-velocity crust most likely represents a zone containing 2-3 vol % of pore fluids that reduce the strength of the middle and lower crust. This inference suggests that deformation in this weakened crust, caused by a regional stress regime regulated by the eastward movement and collision of the Amurian plate with the North American plate, is dominated by anelastic processes. In addition, the seismogenic layer in the northern part of the fault system is ~ 5 km thinner than in the southern part, suggesting that stress is efficiently concentrated within the seismogenic layer in the northern part. This finding explains why the seismic rupture for the Nobi earthquake nucleated at the northern end of the fault system. Our results suggest that a weakened zone in the middle and lower crust is an important control on stress loading process within the seismogenic layer and thus the seismogenesis of crustal earthquakes.