Earth Planets Space, Vol. 56 (No. 12), pp. 1087-1093, 2004
Bunichiro Shibazaki1, Norio Shigematsu2, and Hidemi Tanaka3
1International Institute of Seismology and Earthquake Engineering, Building Research Institute, 1 Tatehara, Tsukuba 305-0802, Japan
2Geological Survey of Japan, AIST, AIST Tsukuba Central 7, 1-1 Higashi, Tsukuba 305-8567, Japan
3Department of Earth and Planetary Sciences, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
(Received July 12, 2004; Revised November 25, 2004; Accepted November 30, 2004)
Important issues with regard to the generation processes of large inland earthquakes include how the stress concentrates and how nucleation starts in the deeper part of the seismogenic zone prior to the mainshock. We propose a model of earthquake generation processes that uses a constitutive law combining friction and flow processes. Using this law, we can represent fault behavior in which frictional slip coexists with flow processes at the frictional-viscous transition zone. We consider a limitted region where viscous deformation is high along the frictional-viscous transition zone, and investigate the role of this region in the nucleation process. During the interseismic period, slip velocity due to flow is much larger than frictional slip velocity in the region of low viscosity in the deeper part of the seismogenic zone. Large slip due to flow in this region is thought to cause stress to concentrate in the surrounding regions, and nucleation starts just above the low-viscosity region. Our numerical simulations indicate that the location of the nucleation process is determined by the nonuniform distribution of the depth of the frictional-viscous transition zone.
Key words: Nucleation process, a constitutive law combining friction and flow, frictional-viscous transition, low-viscosity region.