Figure 3

Three representative examples of experimental datasets. Three representative examples of experimental datasets showing, on the left axes, the evolution (with time) of local strains (purple, red, green, and orange) parallel to the fault, and, on the right axes, mean shear stress (black), during an unstable slip event. The strain versus time curves are offset vertically from each other for clarity. Note that the strain values reported correspond to the ε _tt component of the strain tensor, where t is the direction tangential to the fault, and that positive and negative strains correspond to extension and compression parallel to the pre-cut surface, respectively. The colors associated with local strains correspond to the colors assigned to the strain gauges shown in Figure 1. The blue regions shown in A, B, and C demarcate the locations on these plots that correspond to the close-ups shown in D, E, and F, respectively. The arrows in D to F indicate the time of peak compression recorded by each strain gauge, reflecting the passage of a rupture front through that region of the sample assembly. (A and D) Olivine gouge (P _c  = 140 MPa). During this experimental run, the shear stress dropped within approximately 50 μs, with fast rupture propagation taking place at speeds of a few kilometers per second. (B and E) Liz/ctl gouge (P _c  = 170 MPa). During this experimental run, a slow stress drop associated with multiple slow rupture propagation events persisted for approximately 20 ms, after which a fast drop in stress (as shown in A and D) occurred at seismic speeds. (C and F) Liz/ctl gouge (P _c  = 60 MPa). During this run, a slow stress drop persisted for approximately 1 s until a residual stress level was achieved. Note that no high-frequency elastic radiation was observed after the unstable slip event.