6:15 PM - 7:30 PM

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

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

*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

*d*, but it is often the case that those determined parameters depend on

_{c}*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 ∂

*f*/∂ln(

_{ss}*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 ∂

*f*/∂ln(

_{ss}*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 σ

*, 50 MPa pore water pressure, various temperatures*

_{e}*T*from 20

^{o}C to 200

^{o}C, and

*V*from 0.3 to 100 μm/s. They found that with increasing

*V*, the rate-dependency ∂

*f*/∂ln(

_{ss}*V*) increases from negative to positive at

*T*= 20

^{o}C, decreases from positive to negative at

*T*= 100

^{o}C and 150

^{o}C, and decreases more remarkably but stays positive in the studied range of

*V*at

*T*= 200

^{o}C. In order to account for these complex rate-dependencies, we modified the logarithmic RSF to a quadratic form:

*f*=

*f*

_{0}+

*F*

_{1}

*L*+

_{V}*F*

_{2}

*L*

_{V}^{2}+

*G*

_{1}

*L*+

_{W}*G*

_{2}

*L*

_{W}^{2}

where

*L*= ln(

_{V}*V*/

*V*

_{0}) and

*L*= ln(

_{W}*d*/

_{c}*V*

_{0}θ),

*f*

_{0}is a reference friction coefficient at a reference slip rate

*V*

_{0},

*F*

_{1},

*F*

_{2},

*G*

_{1}, and

*G*

_{2}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

*d*:

_{c}*d*θ/

*dt*= 1 ?

*V*θ/

*d*.

_{c}Note that at a steady-state,

*L*=

_{V}*L*and

_{W}*f*=

_{ss}*f*

_{0}+ (

*F*

_{1}+

*G*

_{1})

*L*+ (

_{V}*F*

_{2}+

*G*

_{2})

*L*

_{V}^{2}.

This is a generalization of the aging law, the original version corresponding to

*F*

_{1}=

*a*,

*F*

_{2}= 0,

*G*

_{1}= -

*b*, and

*G*

_{2}= 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 σ

*, and a rotation axis to mimic intersection of the fault plane and the surface. Distributions of*

_{e}*T*and σ

*are determined to be consistent with the heat-flow measurement and modeling by Gao and Wang [2014].*

_{e}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 ∂

*f*/∂ln(

_{ss}*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 ∂

*f*/∂ln(

_{ss}*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.