Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-SS Seismology

[S-SS29] Fault Rheology and Earthquake Dynamics

Sun. May 24, 2015 6:15 PM - 7:30 PM Convention Hall (2F)

Convener:*Wataru Tanikawa(Japan Agency for Marine-Earth Science and Technology, Kochi Instutute for Core Sample Research), Takeshi Iinuma(International Research Institute of Disaster Science, Tohoku University), Yuta Mitsui(Department of Geosciences, Graduate School of Science, Shizuoka University), Hideki Mukoyoshi(Department of Geoscience Interdisciplinary Graduate School of Science and Engineering, Shimane University)

6:15 PM - 7:30 PM

[SSS29-P12] Earthquake sequence simulations using measured frictional properties for JFAST core sample

*Hiroyuki NODA1, Michiyo SAWAI2, Bunichiro SHIBAZAKI3 (1.JAMSTEC, 2.Hiroshima University, 3.Building Research Institute)

Keywords:Earthquake cycle, Friction constitutive law, Numerical simulation, Friction experiment

Parameters in a rate- and state-dependent friction law (RSF) are often determined by velocity-step tests in which the slip rate V is stepped typically by a factor of 3 to 10. The test may yield a set of parameter values such as a, b, and dc, but it is often the case that those determined parameters depend on V if a logarithmically wide range of V is investigated. At this point, the originally assumed constitutive law is shown to be invalid, strictly speaking, and thus need to be modified. For example, the experiments by Dieterich [1978] show that the rate-dependency ∂fss/∂ln(V) increases as V increases, which can be explained by introduction of a cut-off time for healing [Okubo, 1989]. Such a proposal of a new constitutive law with a corresponding microphysical interpretation is a great advance in technology which enables us to implement a complex rate-dependency into earthquake sequence simulations, as well as in understanding of physics of rock friction and earthquake generation process. However, not all experimental data showing complex rate-dependency have been digested and implemented in a rate- and state-dependent framework. In this study, we propose a simple modification to the logarithmic RSF which enables implementation of rate-dependencies (∂f/∂ln(V) and ∂fss/∂ln(V)) that change with ln(V).

Sawai et al. [2014, AGU fall meeting] conducted a series of velocity-step tests with a core sample obtained in JFAST project at 50 MPa effective normal stress σe, 50 MPa pore water pressure, various temperatures T from 20 oC to 200 oC, and V from 0.3 to 100 μm/s. They found that with increasing V, the rate-dependency ∂fss/∂ln(V) increases from negative to positive at T = 20 oC, decreases from positive to negative at T = 100 oC and 150 oC, and decreases more remarkably but stays positive in the studied range of V at T = 200 oC. In order to account for these complex rate-dependencies, we modified the logarithmic RSF to a quadratic form:
f = f0 + F1 LV + F2 LV2 + G1 LW + G2 LW2
where LV = ln(V/V0) and LW = ln(dc/V0θ), f0 is a reference friction coefficient at a reference slip rate V0, F1, F2, G1, and G2 represent rate-dependencies which are assumed to be given by quadratic functions of ambient temperature T, and θ is the state variable representing recent slowness which evolves with a characteristic slip dc:
dθ/dt = 1 ? Vθ/dc.
Note that at a steady-state, LV = LW and
fss = f0 + (F1+G1)LV + (F2+G2)LV2.
This is a generalization of the aging law, the original version corresponding to F1 = a, F2 = 0, G1 = -b, and G2 = 0. We determined the rate-dependency functions by least-squares method from the experimental data by Sawai et al. [2014], and investigated the consequence by means of dynamic earthquake sequence simulations [e.g., Lapusta et al., 2003].

In preliminary simulations, we simulated earthquake sequences on a planer fault in 2-D (mode II) problems with depth-dependent T, depth-dependent σe, and a rotation axis to mimic intersection of the fault plane and the surface. Distributions of T and σe are determined to be consistent with the heat-flow measurement and modeling by Gao and Wang [2014].

Without additional complexity such as patch-like asperities and high-velocity weakening (e.g., thermal pressurization of pore fluid [Noda and Lapusta, 2013]), earthquakes are nucleated at about 30?50 km downdip from the trench where ∂fss/∂ln(V) is negative regardless of V, and rupture only the shallowest part of the plate interface. The nucleation is preceded by slow slip in the shallower part of the plate interface where ∂fss/∂ln(V) changes its sign with increases V and thus spontaneous acceleration to coseismic slip rate cannot occur. Effect of thermal pressurization and interaction of the system with embedded rate-weakening patches generating earthquakes shall be discussed in the presentation.