In seismology, the rupture mechanisms of an earthquake, a glacier stick-slip and a landslide are not directly observed, but inferred from surface measurements. In contrast, laboratory experiments can illuminate near field effects. The near field reflects the rupture mechanism but is highly attenuated in the case of real-world surface data. We directly image the elastic wave-field of a nucleating rupture non-invasively in its near-field with ultrasound speckle correlation. Our imaging yields the particle velocity of the full shear wave field at the source location and inside the 3D frictional body. We experimentally show that a strong bimaterial contrast, as encountered in environmental seismology, yields a unidirectional or linear force mechanism for pre-rupture microslips and decelerating supershear ruptures. A weak contrast, characteristic for earthquakes, generates a double-couple source mechanism for sub-Rayleigh ruptures, sometimes preceded by slow deformation at the interface. This deformation is reproduced by the NF of a unidirectional force.

We observe the nucleation phase of in‐plane ruptures in the laboratory. We show that the nucleation is composed of two distinct phases, a quasi‐static and an acceleration stage, followed by dynamic propagation. We propose an empirical model which describes the rupture length evolution: The quasi‐static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time. The transition from quasi‐static to accelerating rupture is related to the critical nucleation length, which scales inversely with normal stress in accordance with theoretical predictions, and to a critical surfacic power, which may be an intrinsic property of the interface. Finally, we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions.

This study is devoted to the experimental investigation of the interaction of a propagating rupture with one or several mechanical heterogeneities. We developed a friction laboratory experiment where a soft elastic solid slides past a rigid flat plate. The system is coupled to an original medical imaging technique, ultrasound speckle interferometry, that allows observing the rupture dynamics as well as the emitted elastic shear wavefield into the solid body. We compare the dynamics of propagating rupture for a homogeneous flat interface and for three cases of heterogeneous sliding surfaces: (1) an interface with a single point‐like barrier made of a small rock pebble, (2) an interface with a single linear barrier that joins the edges of the faults in a direction perpendicular to slip, and (3) an interface with multiple barriers disposed on half of its surface area, creating a heterogeneous zone. We obtain experimental observations of dynamic effects that have been predicted by numerical dynamic rupture simulations and provide experimental observations of the following phenomena: a barrier can stop or delay the rupture propagation; a linear single barrier can change the rupture velocity, increasing or decreasing it; we observe transition from subshear to supershear propagation due to the linear barrier; a large heterogeneous area slows down the rupture propagation. We observe a strong variability of the rupture dynamics occurring for identical frictional conditions, which we impute to heterogeneity of the stress field due to both the loading conditions and memory of the stress field due to previous rupture events.

To get an insight into the processes underlying dynamic friction that plays an important role in seismic sources for example, we developed a sliding dynamic experiment coupled to elastography imaging. This experimental setup permits to observe simultaneously the frictional interface and the waves emitted in the bulk during slipping. We use soft solids made of hydro-organic gel of PVA, in contact with either glass or sandpaper. The huge interest of such soft solids is that ultrasonic waves allows to observe in real time the rupture nucleation and propagation, as well as shear waves themselves inside the medium. We investigate the friction in two different cases. In the case of friction on sand paper, links are formed between the gel and the sand paper by local pinning. The breaking of these links emits a characteristic wave pattern, and their occurrence is related to the local sliding velocity. In a very different way, when the gel slide on a glass surface, with an interlayer of sand grains, the slip occurs as successive rupture events, with a rupture front crossing the whole surface. We can study then the rupture velocity, and in the cases of ruptures faster than the shear wave velocity, we observe a Mach cone of shear waves.

Ultrafast ultrasonic speckle interferometry, an imaging technique derived from elastography, is used to follow the dynamic of the interface failure in a friction experiment. Experimental results that characterise two slipping regimes are presented: a slow slip regime associated with depinning events at the interface and a supershear rupture regime associated with the emission of Mach waves fronts. These results are discussed in the light of geophysical observations made at the scale of the Earth on the slip dynamics in active faults. Copyright (C) EPLA, 2011

We address the problem of modeling dynamic rupture on multiscale heterogeneous faults in 3D. Under the assumption of slip-weakening friction, we numerically construct effective friction laws that integrate the effects of small-scale heterogeneity during the rupture. This homogenization process is based on the description of the initial phase of the rupture by the dominant unstable spectral mode. Its dynamics is influenced by the geometry of the fault, the static friction heterogeneities and the friction law. We first define a periodic small-scale heterogeneous model, introducing heterogeneity in the distribution of the static friction coefficient. We then describe a method for constructing this effective friction law. Applying this new law homogeneously on the fault permits to reproduce the dynamic evolution of the heterogeneous fault. Furthermore, we show that the effective friction law can be used to replace small-scale heterogeneities in two-scale heterogeneous models, while preserving their effects. We study three kinds of two-scale models, with growing complexity: first periodic at both scales, then periodic only at small scale, and finally irregular at both scales. This homogenization method can be adapted to the case where the heterogeneity is introduced in the initial stress rather than in the static friction value. Finally, we show in a simple example that the effective friction law permits to reproduce the transition between subshear and supershear rupture propagation, originally produced by heterogeneities on the fault.