The transition from stick to slip at a dry frictional interface occurs
through the breaking of the junctions between the two contacting surfaces.
Typically, interactions between the junctions through the bulk lead to rupture
fronts propagating from weak and/or highly stressed regions, whose junctions
break first. Experiments find rupture fronts ranging from quasi-static fronts
with speeds proportional to external loading rates, via fronts much slower than
the Rayleigh wave speed, and fronts that propagate near the Rayleigh wave
speed, to fronts that travel faster than the shear wave speed. The mechanisms
behind and selection between these fronts are still imperfectly understood.
Here we perform simulations in an elastic 2D spring--block model where the
frictional interaction between each interfacial block and the substrate arises
from a set of junctions modeled explicitly. We find that a proportionality
between material slip speed and rupture front speed, previously reported for
slow fronts, actually holds across the full range of front speeds we observe.
We revisit a mechanism for slow slip in the model and demonstrate that fast
slip and fast fronts have a different, inertial origin. We highlight the long
transients in front speed even in homogeneous interfaces, and we study how both
the local shear to normal stress ratio and the local strength are involved in
the selection of front type and front speed. Lastly, we introduce an
experimentally accessible integrated measure of block slip history, the Gini
coefficient, and demonstrate that in the model it is a good predictor of the
history-dependent local static friction coefficient of the interface. These
results will contribute both to building a physically-based classification of
the various types of fronts and to identifying the important mechanisms
involved in the selection of their propagation speed.Comment: 29 pages, 21 figure
