Numerical simulation of seismic cycles at a subduction zone with a laboratory-derived friction law Naoyuki Kato (1), Kazuro Hirahara (2), and Mikio Iizuka (3) (1) Earthquake Research Institute, University of Tokyo (2) Graduate School of Environmental Sciences, Nagoya University (3) Research Organization for Information Science and Technology

Numerical simulation of seismic cycles at a subduction zone with a laboratory-derived friction law Outline 1. Composite rate- and state-dependent friction law 2. Forecast models for the Tokai earthquake 3. Mechanism of episodic slip events 4. Historical earthquake sequence along the Nankai trough - Future study -

Rate- and state-dependent friction law   =  * + a ln(V/V * ) + b ln(V *  /L)  ; friction coefficient V ; sliding velocity  ; state variable a, b, L,  *, V * ; constants d  /dt = 1 -  V/L ; slowness law d  /dt = - (  V/L) ln (  V/L) ; slip law d  /dt = exp(-V/V c ) - (  V/L) ln (  V/L) ; composite law V c ; constant Kato and Tullis (2001)

Simulation results of slide-hold-slide test Simulation results of velocity stepping test Stiffness independent higher healing rates for the slowness and composite laws explain laboratory data Symmetric responses to velocity increases and decreases for the slip and composite laws explain laboratory data Kato and Tullis (2001)

Kato and Tullis (2002) Simulation of stick-slip cycle of a spring-block system The recurrence interval and stress drop are the largest for the composite law.

Model for seismic cycle at the Tokai seismic gap along the Suruga trough, central Japan Back slip distribution estimated from GPS data by Sagiya (1999) 2-D model for the Tokai earthquake Kato and Hirasawa (1999) Seismic moment release of preseismic sliding Kato and Tullis (2002) seismic gap

Kuroki et al. (2002) 3-D model for the Tokai earthquake Strain change (for one day before EQ) Strain (1E-8)

Episodic strain event near the hypothesized source area of the Tokai earthquake detected by GPS Data from Geographical Survey Institute ( Hamamatsu

Saiki Misho Slow slip event beneath the Bungo channel, southwest Japan Hirose et al. (1999) Observed and model horizontal displacement Distribution of estimated fault slip over 10 months Moment magnitude = 6.6 Saiki Misho Velocity changes for 3 years in southwest Japan (a) Apr. 1, Apr. 1, 1997 (b) Apr. 1, Apr. 1, 1998 (c) Apr. 1, Apr. 1, 1999

Possible mechanism of episodic slip event on a plate interface For a spring-block model, the ratio of spring stiffness to the critical stiffness k c = (B-A)/L controls sliding mode. ( B-A = -d  ss /dlnV, L characteristic slip distance) k >> k c → aseismic k ～ k c → episodic k << k c → seismic For a finite fault in a uniform elastic medium, the critical fault dimension r c = cGL/(B-A) may be defined, and r/r c controls sliding mode. ( c ; constant ～ 1, G ; rigidity) r >> r c → seismic r ～ r c → episodic r << r c → aseismic k

Simulation of slip on a flat fault in an infinite uniform elastic medium A circular patch with negative A-B value is embedded in a uniform fault with positive A-B value. stable Possibly unstable The critical fault radius r c = 4.12 km. The radius r of the negative A-B patch is 3.0 km. r/r c = 0.73 Distribution of A-B (MPa)

Snapshot of distribution of slip velocity ln(V/V pl ) ( V pl ; the assumed plate velocity = 4 cm/yr) The negative A-B patch is more strongly locked.

Snapshot of distribution of slip velocity ln(V/V pl ) Time from the last slide = 5.7 years Episodic slip starts in the negative A-B patch.

Snapshot of distribution of slip velocity ln(V/V pl ) Time from the last slide = 13 hours Episodic slip propagates in the negative A-B patch, but the slip is not significantly accelerated.

Snapshot of distribution of slip velocity ln(V/V pl ) Time from the last slide = 32 days Slip is decelerated because the rupture front enters the positive A-B region.

Snapshot of distribution of slip velocity ln(V/V pl ) Time from the last slide = 150 days Very slow slip propagates in the positive A-B region, while healing starts in the negative A-B patch.

Snapshot of distribution of slip velocity ln(V/V pl ) Time from the last slide = 2.2 years The negative A-B patch is strongly locked and the next cycle starts.

r = 3 km, r c = 4.12 km A-B = 0.2 MPa at 1-6 A-B = -0.2 MPa at 7-8 Slip history during the entire cycle

r = 3 km, r c = 4.12 km A-B = 0.2 MPa at 1-6 A-B = -0.2 MPa at 7-8 Slip history at the aseismic slip event TIME (days)

Summary of simulation for episodic slip events Aseismic episodic slip event may be simulated for a negative A- B patch with r/r c = Episodic events with various time duration may be simulated by varying r/r c -value. The duration of the event decreases with an increase in r/r c. Episodic events may be simulated not only by nonuniform distribution of A-B but also by nonuniformity in the characteristic slip distance L or in the effective normal stress. If the source size and duration of an episodic event are obtained, the value of (B-A)/L may be estimated.

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In the Earth Simulator Project, we take into consideration heterogeneous viscoelastic structure, a rate- and state-dependent friction, and interactions of many segments of plate boundary and inland active faults. FEM; Iizuka, Poster Solving friction problem; Prabhakar, Poster

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