1. 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

2.  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]

3. 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).

4. Geometry and model setup [Liu and Rice., 2005] Stability transition ~ 60 km downdip Lithostatic stress  Pore pressure p

5. 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).

6. 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]

7. 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.

8. Most of velocity-weakening zone locked

9. 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

10. Dilatancy of fault gouge 1  m/s 10  m/s 1  m/s [Segall and Rice, 1995] [Marone et al., 1990]

11. 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]

12. Friction parameters (a, a  b, L) and effective normal stress depth distributions

13. Without dilatancy, short- period spontaneous aseismic transients occur when W/h* is within a limited range.

14. With dilatancy, short-period spontaneous aseismic transients can occur theoretically for unlimited W/h*. indicator of drainage dilatancy strengthening v.s. frictional weakening

15. Implications for seismogenic zone limits and depths of slow slip events ? [Dragert, Wang & James, 2001]

16. No dilatancy With dilatancy

17. 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?

18. 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]