The effects of restraining and releasing fault bends on the propagation of shear ruptures: direct experimental measurements

The geometry of long strike-slip faults often includes local deviations from the general fault orientation, commonly known as restraining and releasing bends. As earthquake ruptures propagate along the fault, these bends experience either an increase or decrease of stress, influencing the rupture dynamics. While it is well established that these geometric features can dramatically affect rupture parameters such as propagation velocity, style, and extent, the exact dynamics and mechanics of the rupture-bend interaction are not yet clear. Here, we present direct experimental observations of the interaction of shear ruptures with restraining and releasing bends of different angles, deviating from a planar interface by up to 30 degrees. We trigger dynamic ruptures that spontaneously propagate along matching interfaces between two loaded PMMA plates and image ruptures at the area of the bends with an ultra-high-speed camera, operating at a rate of million frames per second. By applying Digital Image Correlation on the imaged frames, we produce high-resolution, full-field maps of the evolution of displacements, particle velocities, and stresses as the ruptures propagate through the bends. While all ruptures reach the bends as self-healing slip pulses propagating at sub-Rayleigh velocities, different bend geometries have different effects on the ruptures. These include arresting the ruptures, transitioning them into supershear and crack-like ruptures, and triggering secondary ruptures and back-propagations. These results can illuminate the complex dynamics that evolve around restraining and releasing bends during earthquakes and can be used for calibrating earthquake models and refining earthquake hazard assessments.