Understanding dynamic friction through spontaneously evolving laboratory earthquakes

Abstract

Friction plays a key role in how ruptures unzip faults in the Earth’s crust and release waves that cause destructive shaking. Yet dynamic friction evolution is one of the biggest uncertainties in earthquake science. Here we report on novel measurements of evolving local friction during spontaneously developing mini-earthquakes in the laboratory, enabled by our ultrahigh speed full-field imaging technique. The technique captures the evolution of displacements, velocities and stresses of dynamic ruptures, whose rupture speed range from sub-Rayleigh to supershear. The observed friction has complex evolution, featuring initial velocity strengthening followed by substantial velocity weakening. Our measurements are consistent with rate-and-state friction formulations supplemented with flash heating but not with widely used slip-weakening friction laws. This study develops a new approach for measuring local evolution of dynamic friction and has important implications for understanding earthquake hazard since laws governing frictional resistance of faults are vital ingredients in physically-based predictive models of the earthquake source.

Emergence of Cracklike Behavior of Frictional Rupture: The Origin of Stress Drops

“…see Fig. 8 in [72]) and is strongly supported by recent experimental data we have extracted from recent experimental work [35,36], see Sect. IV.…”

“…The discussion above, both for the N -shaped and the no-minimum steady-state friction curves, seems to lead to the quite remarkable conclusion that based on basic physics considerations we expect no finite and welldefined steady-state stress drops to emerge at all in the context of friction rupture, and hence no crack-like behavior as well. This appears to be in sharp contrast to ample evidence indicating the existence of finite and welldefined steady-state stress drops in various frictional systems [1,29,30,32,33,35,63,64]. How can one reconcile the two apparently conflicting conclusions?…”

“…Following the discussion above, since in these calculations H → ∞, the finite stress drop persists indefinitely (while in finite size systems it persists for times ∼ O(H/c s ), cf. the experiments of [35], to be discussed later). The stress drop ∆τ observed in Fig.…”

“…Measurements of both τ res and v res behind rupture fronts for various τ d have been recently performed by two independent experimental groups using two different experimental systems and techniques [35,36]. The first focused on the frictional dynamics along the interface between two blocks of Homalite probed through a novel ultrahigh full-field imaging technique [35].…”

“…edge singularity, are expected to emerge. These assumptions have been adopted in an extremely broad range of theoretical and numerical studies [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], and their implications have been consistent with geophysical observations [26,28] and have been confirmed in some recent laboratory experiments [29][30][31][32][33][34][35][36].Yet, to the best of our knowledge, currently there is no basic understanding of how the effective crack-like behavior of frictional rupture emerges from fundamental physics and what its range of validity is. More specifically, there is a need to understand what the physical origin of a finite stress drop ∆τ > 0 is and under what conditions it is valid.…”