Imaging laboratory rupture nucleation at the source: A friction experiment using ultrafast ultrasound

At the field scale and in most laboratory studies the rupture nucleation mechanism of an earthquake, landslide or glacier stick-slip cannot be directly imaged. Near-field and source effects are thus difficult to observe. We use correlation of highspeed ultrasound images to track shear wave propagation at the rupture nucleation source and in its near-field. The particle velocity and accumulated displacement of the shear wave field emitted by the rupture are observed in-situ on a very dense grid. The grid consists of the ultrasonic imaging plane inside the frictional body and resolution is defined by the ultrasonic wavelenth (0.3 mm). The rupture process is generated by controlling a driving slab through a motor and a granular layer of sand or gravel constitutes the stick-slip behavior. The frictional body is a homemade Poly-Vinyl-Alcohol hydrogel. Although its properties are differing from those of classically investigated rocks, it constitutes a linear elastic material and reproduces rupture processes that are known from the field and rock physics. Through the elastic wave field we observe microslips which precede supershear rupture propagation along the frictional interface. We experimentally show that the source mechanism of a breaking asperity depends on the material contrast of the adjacent halfspaces. Neither a double-couple nor a single-force mechanism perfectly reproduce the experimental data of a rupturing asperity, while micro-slips are well reproduced by a singular shear point force.