[J05-5] Crustal dynamics: Multidisciplinary approach to seismogenesis V

[J05-5-05] Importance of fault rheology around brittle-plastic transition in long-term slip rate of major faults

^○Hiroyuki Noda (Kyoto University, Kyoto, Japan)

A ductile shear zone underlies a seismogenic part of a major fault, and its rheology may significantly affect the long-term slip rate. I have conducted numerical simulations of earthquake sequences on a major fault penetrating a crustal plate under a constant far-field stress tau_pl, using a rate- and state-dependent friction-to-flow fault constitutive law. In this law, shear resistance is approximately given by a rate- and state-dependent friction law in a shallow brittle part of the fault, and by power-law creep in a deep fully plastic part. The rate-dependency of the shear resistance takes the maximum value in a transitional regime between them. Note that the peak in the rate-dependency does not necessarily correspond with peak shear resistance. If we assume excess pore pressure at depth which limits the effective normal stress at a certain value, then a Christmas-tree-like strength profile does not exist, but a remarkable peak in the rate-dependency still appears in the transitional regime. In present simulations, the fault hosts repeating earthquakes in the brittle part, and slips by a long-term speed V_pl on average which depends on tau_pl. The relation between tau_pl and V_pl is very well explained by a power law which is similar to what unstable steady-state solutions with uniform slip rates V_ss follow. It should be noted that V_pl is larger than V_ss for the same tau_pl approximately by a factor of 2 due to heterogeneous distribution of shear stress. Since the relation between tau_pl and V_ss is given by spatial average of the rate-dependency, the transitional regime having the prominent peak in the rate-dependency most significantly contributes to the amount of stress perturbation required to change the long-term slip rate of the fault.