HR: 08:30h
AN: S11E-03    [Abstracts]
TI: Transition to Pulse-Like Rupture, With and Without Inclusion of Evolving Temperature and Pore Pressure, When Accounting for Extreme Weakening at High Slip Rates
AU: * Noda, H
EM: nodahiroyuki@kugi.kyoto-u.ac.jp
AF: Dept. Geophys., Kyoto Univ., Kitashirakawa Oiwake cho, Sakyo ku, Kyoto, 6068502, Japan
AU: Dunham, E M
EM: edunham@fas.harvard.edu
AF: Dept. Earth Planet. Sci., Harvard Univ., 20 Oxford St., Cambridge, MA 02138, United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Dept. Earth Planet. Sci. and Sch. Engin. Appl. Sci., Harvard Univ., 29 Oxford St., Cambridge, MA 02138, United States
AB: We have conducted rupture propagation simulations incorporating the combined effects of thermal pressurization of pore fluid by distributed heating within a finite width shear zone, and flash heating of microscopic contacts. These are probably the primary weakening mechanisms at high coseismic slip rates. For flash heating, we use a rate- and state-dependent friction law in the slip law formulation, accounting for extreme velocity weakening above a weakening slip rate V_w ~ 0.1 m/s that depends on the background temperature, and a very short state evolution distance, L, of ~ 10 μm, which is comparable to the asperity length. We have also conducted a series of calculations with neglecting evolving change in macroscopic temperature, T, and pore pressure, p, and compared the results. Slip rate, V, at a point on a fault increases when a rupture front approaches, and decreases behind it. In the pulse-like solutions, V decreases below V_w, and the point is eventually locked. On the other hand, in the crack-like solutions, V increases again only if we allow evolving change in T and p. In the cases when we neglect changes in T and p, V continues to decrease behind the rupture front as long as we simulate. Here, a question emerges; is the solution crack-like because of the short calculation time? Zheng and Rice [1998] proposed an intuitive criterion between crack-like and pulse-like solutions as follows: If and only if the background shear stress, τ^b, is larger than a critical value, τ^pulse, there are roots of τ_ss(V) = τ^b - μ V/2 c^s, where τ_ss is steady-state strength, μ is shear modulus and c_s is shear speed. If T_ZR = - (μ/2c_s)/(dτ_ss/dV) at the largest root is near unity, the solution is pulse-like. Our calculations without T and p changes show that the pulse-like solution regime extends above τ^pulse, at least up to the point where T_ZR = 0.176, if a rupture is initiated by a perturbation in shear stress in a certain manner. The transition time to pulse-like solution increases with τ^b and diverges at a finite τ^b. If τ_ss depends only modestly on V at high slip rate (like for flash heating), and if L is short enough, an expanding singular crack solution may be a good approximation to a hypothetical crack-like solution. At the center, slip rate is V_∞ = F(V_r/c_s) (Δτ/μ) V_r, where Δτ is stress drop, V_r is rupture velocity and F(V_r/c_s) is near unity. Off the center, V changes from infinity at the tip to V_∞. Suppose V^pulse is a slip rate at which T_ZR = 1. When V becomes smaller than V^pulse, the shear stress deviates from τ_ss and V nears zero. Therefore, if V_∞ is below V^pulse, we will see the transition to a pulse-like solution. Although there is an uncertainty in estimating Δτ, this criterion seems to work well. With evolving change in T and p, the crack-like solutions (V increases again) are associated with the situation that shear stress τ > τ_ss. This indicates that increases in T and p lower τ_ss, which passes current value of τ before V reaches V^pulse. This is possibly how the crack-like regime extends to lower τ^b, even below the τ^pulse for the initial T and p values.
DE: 7209 Earthquake dynamics (1242)
DE: 7260 Theory
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
SC: Seismology [S]
MN: 2007 Fall Meeting