Tracking Tabletop Earthquake Nucleation

Where, when, and how does an earthquake start? Stress concentrations are thought to play an important role in nucleating earthquakes, but their roles are far from trivial. How stressed areas (known as asperities) interact to control the location and timing of quake nucleation is an inherently two-dimensional elasto-dynamic crack problem where the effects of heterogeneity are far from obvious. We experimentally probe the dynamics of nucleation using a tabletop model fault. We reduce the speed of the dynamic ruptures by using soft transparent elastomers and filling the interface with a thin layer of sand. Heterogeneous stress concentrations are imposed by compressing the interface with a rigid plate and adjustable beads to form localized asperities. The clear material allows us to directly image the propagating slip, which would not be possible in natural rock. We find that nucleation location and style is controlled by the stress concentrations. Weaker concentrations result in more migratory nucleation and more global slip events. Furthermore the location of these regions dictates their interactions; widely spaced concentrations are less likely to slip simultaneously, resulting in fewer global events and slower stress release in the case of a global event.