Mechanical behavior and the degree of localization in large displacement faulting experiments N. M. Beeler and T. E. Tullis, Brown University, Providence, RI [Chester et al.., 1993] 1 mm 2 to 4 km depth,  =d/w=508 (Chester et al., 1993) (2) (4) 1 mm

1. Description of experiments and conditions - intially bare surfaces - gouges 2. Granite: Textures and mechanical behavior 3. Implications for seismic faulting 4. General conclusions Deformation and mechanical behavior large displacement faulting experiments Power et al., 1989; Yund et al., 1990 Beeler et al., 1996 Blanpied and Beeler, 1998 {

Conditions in experiments: dry, room temperature

rate dependence of strength (e.g., Dieterich 1978;1979) dilatancy (e.g., Morrow and Byerlee, 1989; Marone et al., 1990) Measurements velocity weakening velocity strengthening

Fault configurations

Initially bare surfaces

d=376mm  =4800, ~ 60% amorphous material [Yund et al., 1990],  c =0.62MPa

natural faulting (from Power et al., 1989) oops, too much strain!  =1000  =10

Simulated gouge

(from Beeler et al., 1996) d=10 mm  =13

d=65 mm  =75 (from Blanpied and Beeler, 1998)

d=194 mm  =255

d=407 mm  =530 (from Blanpied and Beeler, 1998)

(Charles and Hillig, 1962) Subcritical fracture growth (Atkinson, 1987) (also see Scholz, 1968; Das and Scholz, 1981)

Characteristics of seismic failure in the laboratory (from Beeler and Lockner, 2002) Simple friction or fracture based relation for seismic failure Dieterich, 1992 Lockner, 1998 Marcellini, 1995 Shaw, 1993 Atkinson, 1984 Das and Scholz, 1981 Knopoff, 1969 Scholz, 1968 Griggs, 1940

Predicted duration of earthquake nucleation = 10MPa/75 yr a =  n =18MPa/km Depth km t n = days (Dieterich, 1994) Implications of delayed failure: Earthquake occurrence insensitive to periodic stress of periodless than the delay time Lab-based prediction: earthquake occurrence should not correlate with the passage of seismic waves and daily earthtides

Summary of laboratory observations: Deformation in laboratory faulting experiments is accommodated by rigid rotation, sliding between particles and grain fracture. Grain-size reduction from fracture accompanies shear localization Shear localization is required for seismic faulting Seismic failure is delayed, defining a duration of nucleation Conclusions: Fault deformation during and prior to earthquake occurrence involves grain- scale fracturing The duration of nucleation for earthquakes is long (> 1 week) Duration is likely due to short term non-linear viscous effect from sub-critical fracture growth