Fig. 1: Distributions of gouge, shear stress, and foreshocks.

Distribution of gouge after the experiment a LH without PEG (LB12-012) and b MH with PEG (LB12-011). The distribution map was estimated from the image analysis of pictures taken just after each experiment (the original pictures are shown in Supplementary Fig. 3). Note that the distribution of gouge shown in (b) was dominated by the PEG generated from an earlier test (LB12-010). The color of grid indicates the gouge density and procedure for constructing these images can be found in “Methods”. Distribution of local shear stress estimated from the strain gauge array just before each stick-slip event for c LH without PEG and d MH with PEG. The color of lines indicates event ID. Hypocenters of foreshocks for e LH without PEG and f MH with PEG. The size of circles scales with the moment magnitude (M_w) of foreshocks. The color of circles indicates the normalized time-to-mainshock t* (see main text). g Frequency-size distribution of foreshocks for the two experiments. The estimated b value, moment magnitude of completeness, and corner moment magnitude, under the assumption that the detected foreshocks obey a TGR distribution, are listed in Supplementary Table 2. Red lines show the best-fit curves to a TGR distribution.