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Aseismic deformation transients in subduction zone and the role of fault dilatancy -- Numerical simulation in the framework of rate and state friction Yajing Liu Allan M. Rubin (Princeton) James R. Rice (Harvard) September 25, 2008, SEIZE workshop
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Frictional strength is a function of sliding velocity (rate V) and the memory of asperity contacts (state θ). Observed in lab velocity jump tests (fixed normal stress) for a variety of natural and synthetic materials. Rate and state friction (1) [Dieterich, 1978, 1981; Ruina, 1983 Dieterich & Kilgore, 1996]
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Rate and state friction (2) Rate-dependence a: No change in contact population, but contacts resist more because they are sheared faster. State-dependence b: No change in contact shear rate, but old (strong) contacts are destroyed and replaced with new (weak) ones. L = slip to renew asperity contact population (~ 10s m).
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Geometry and model setup [Liu and Rice., 2005] Stability transition ~ 60 km downdip Lithostatic stress Pore pressure p
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simulated slow slip events Several features comparable to observations: Below locked zone, around friction stability transition. Typical slip rate is 10 to 100 times of V pl (~ 10 -9 m/s). Along-strike propagation speed is only 2-3 km/yr – increases as effective normal stress decreases (here 100 MPa is used).
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Several lines of evidence suggest that pore pressure is high in the ETS source regions: Dehydration conditions would be met, around ~350 o C and above, for shallow-dipping subduction zones (Cascadia, SW Japan, S. Mexico), which exhibit short-period transients [Peacock et al., 2002; Wada et al., 2008]. Hypocenters of non-volcanic tremors in Cascadia sections mostly correspond to positive “unclamping” effective stress changes (<0.01MPa) on hypothesized vertical planes (fissures), due to transient slips [Kao et al., 2005; 2007; Liu and Rice, 2007]. Triggered tremors in Shikoku, Japan and Cascadia by passing surface waves from the 2004 Sumatra, and 2002 Denali earthquakes, respectively, and resonance-like response to tidal forcing, all suggest that “ETS” phenomena are sensitive to small stress changes, and indicate near- lithostatic fluid pressure in those source regions [e.g., Miyazaki and Mori, 2006; Rubinstein et al., 2007; Shelly et al., 2008]. Elastodynamic rupture: [Ida, 1973; Shibazaki and Shimamoto, 2007; Ampuero and Rubin, 2008]
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Analysis of response at high pore pressure p Realistic situation: effective normal stress is high (finite) in the seismogenic zone, but much lower from stability transition and further downdip. Simplified situation: most of the seismogenic zone is completely locked ( infinitely high) with width W extending up-dip of the stability transition and whole down-dip region at a much lower, uniform, due to dehydration.
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Most of velocity-weakening zone locked
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Finite effective normal stress in the seismogenic zone Features similar to observations can be produced: Interseismic period filled with aseismic transients. Average recurrence interval of ~ 2 yr. Slip rate 2-4 times of V pl Cumulative slip of ~ 1-2 cm
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Dilatancy of fault gouge 1 m/s 10 m/s 1 m/s [Segall and Rice, 1995] [Marone et al., 1990]
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In a subduction fault model, depth-variable friction parameters (gabbro), normal stresses Gabbro is a better proxy for oceanic crust. Stability transition at around 510 o C. a b < 0.01 up to ~600 o C. Particularly, we use the data under supercritical water conditions. [He et al., 2007] Pore pressure depth distribution is constrained by seismological observations where available, and by thermal and petrological models of northern Cascadia and SW Japan subduction zones. [Peacock et al., 2002; Hacker et al., 2003; Wada et al., 2008; Kodaira et al., 2004; Shelly et al., 2006]
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Friction parameters (a, a b, L) and effective normal stress depth distributions
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Without dilatancy, short- period spontaneous aseismic transients occur when W/h* is within a limited range.
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With dilatancy, short-period spontaneous aseismic transients can occur theoretically for unlimited W/h*. indicator of drainage dilatancy strengthening v.s. frictional weakening
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Implications for seismogenic zone limits and depths of slow slip events ? [Dragert, Wang & James, 2001]
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No dilatancy With dilatancy
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Summary Short-period aseismic deformation transients emerge spontaneously when interstitial fluids are present and pore pressure is near- lithostatic within certain depth range (limited W/h*). At low effective normal stress, fault stabilization by induced suction during dilatancy due to increased shear rates becomes important. Aseismic transients can appear for much larger W/h* (using lab values of L). Both slip and recurrence interval (approximately) linearly increase with W/h*. Maximum slip rate decreases as E increases toward 1.0. “Coseismic” rupture can also be stabilized, with reduced rupture propagation speed and spatial extent. Fault can be frictionally unstable (a-b<0) but undergo no seismic slip. Implications for the relative depths of thrust earthquakes and slow slip events?
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Need more constraints on model parameters Fault gouge dilatancy coefficient : Marone et al. [1990]: granite, 150MPa, Samuelson et al. [2007, 2008]: Fine grain angular quartz, saturated. 0.8 to 30 MPa Westerly granite gouge (dry): 5 to 30 MPa Clay-rich ODP gouge (dry): 5 to 30 MPa Dependence of on effective normal stress and temperature. Hydraulic diffusivity: assumed nearly “undrained” in current earthquake simulations. a more complete analysis is necessary to examine effects of permeability, viscosity, and characteristic diffusion length d. Rate and state friction parameters Significant differences in granite (dry and wet) and gabbro friction properties. [Blanpied et al., 1995, 1998; He et al., 2006, 2007] Dilatancy may also affect friction parameters a, b, and L. [Samuelson et al., 2008]
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