Seismogenic width control on the dynamics and scaling of laboratory elongated ruptures

After an initial phase of circular expansion, very large earthquakes primarily grow horizontally, with their vertical extent limited by the seismogenic width of the Earth’s crust. This geometric evolution is accompanied by a transition in rupture dynamics from crack-like to pulse-like propagation. Such events are commonly referred to as elongated ruptures.While classical models (f.e., Linear Elastic Fracture Mechanics (Freund, 1998)) successfully describe small to moderate earthquakes, they fail to capture the dynamics of large events. Recent theoretical and numerical work by Weng and Ampuero (2019) introduced a physical framework for elongated ruptures, which, although supported by numerical validation and natural observations, has yet to be experimentally validated.To address this, we conducted 2D rupture experiments in a biaxial direct shear apparatus under unbounded and bounded conditions. The unbounded case corresponds to a uniform velocity-weakening interface, while the bounded case consists of an elongated velocity-weakening region adjacent to a wide velocity-strengthening zone, mimicking a seismogenic layer whose width is bounded by deep aseismic regions. This experimental model successfully reproduces confined elongated ruptures and reveals distinct propagation styles: crack-like ruptures under unbounded conditions and pulse-like ruptures under bounded conditions. This transition is also reflected in the temporal evolution of seismic moment: during the initial phase of propagation, seismic moment scales cubically with rupture duration, while after saturation of the seismogenic width, it transitions to a linear scaling, as expected for pulse-like ruptures.Together, these observations highlight the role of the seismogenic layer in controlling rupture style and provide experimental support for the proposed theory of elongated ruptures.