  The process of frictional rupture, i.e. the failure of frictional systems,
abounds in the technological and natural world around us, ranging from
squealing car brake pads to earthquakes along geological faults. A general
framework for understanding and interpreting frictional rupture commonly
involves an analogy to ordinary crack propagation, with far-reaching
implications for various disciplines, from engineering tribology to geophysics.
An important feature of the analogy to cracks is the existence of a reduction
in the stress-bearing capacity of the ruptured interface, i.e. of a drop from
the applied stress, realized far ahead of a propagating rupture, to the
residual stress left behind it. Yet, how and under what conditions such finite
and well-defined stress drops emerge from basic physics are not well
understood. Here we show that for rapid rupture a stress drop is directly
related to wave radiation from the frictional interface to the bodies
surrounding it and to long-range bulk elastodynamics, and not exclusively to
the physics of the contact interface. Furthermore, we show that the emergence
of a stress drop is a transient effect, affected by the wave travel time in
finite systems and by the decay of long-range elastic interactions. Finally, we
supplement our results for rapid rupture with predictions for slow rupture. All
of the theoretical predictions are supported by available experimental data and
by extensive computations. Our findings elucidate the origin of stress drops in
frictional rupture, i.e. they offer a comprehensive and fundamental
understanding of why, how and to what extent frictional rupture might be viewed
as an ordinary fracture process.
