Once initiated, an earthquake rupture usually propagates along the fault plane at a speed somewhat less than that of the seismic shear wave velocity VS in the surrounding earth. On less frequent occasions, ruptures propagating at speeds exceeding VS, (but still less than the compressional velocity VP) have been observed from large strike slip earthquakes, such as the highly destructive 1906 San Franciscso earthquake.
As such it is important to understand the factors influencing these sub-shear and super-shear propagation models with the ultimate goal to better assess the conditions leading to the most destructive strike slip earthquakes. As this process still remains poorly understood, laboratory analog tests can provide insight into how such fractures grow during an event.
Rubino et al.  employ modern ultrahigh speed videography operating at up to 2 million frames per second to capture the propagation of ruptures whose fronts move as fast as 2.28 km/s. The ruptures propagate along pre-existing “fault” with a controlled surface roughness between two pieces of transparent acrylic glass. The assembly is loaded uniaxial to resolve normal and shear stresses onto the fault plane, the magnitudes of which depend on the orientation of the fault with respect to the loading. Ruptures are initiated from a small pulse produced from the electrical resistive failure of a wire embedded in the fault providing a time reference for controlling the video capture.
Digital image correlation applied to subsequent frame within a given video allowed the researchers to map particle velocity fields around the rupture both spatially and temporally. The results demonstrate differences in the near and far field wave propagation from the different rupture modes that agree well with theoretical predictions.
Citation: Rubino, V., Rosakis, A. J., & Lapusta, N. . Spatiotemporal properties of sub‐Rayleigh and supershear ruptures inferred from full‐field dynamic imaging of laboratory experiments. Journal of Geophysical Research: Solid Earth, 125, e2019JB018922. https://doi.org/10.1029/2019JB018922
—Douglas R. Schmitt, Editor, JGR: Solid Earth