Earthquakes are dynamic rupture events that initiate, propagate, and terminate on faults within the Earth's crust. Understanding rupture termination is essential for accurately estimating the maximum magnitude earthquake a region might experience. We study termination on sequences of M − 2.5 earthquakes that rupture a 3‐m granite laboratory sample. At this large scale, nucleation, propagation, and termination are either completely or partially confined within the sample–unique observations for experiments on rock. We compare measured termination locations to estimates from a fracture mechanics‐based model to quantify the fracture energy of the laboratory earthquakes, which compare well with estimates from small natural quakes. Our results provide a mathematical framework that links micrometer‐scale friction parameters to meter‐scale earthquake mechanics, shows that a 3‐m slab of granite can behave similar to a 200‐mm sheet of glassy polymer, and demonstrates how small events can prime a fault for larger, damaging ones.

The injection of fluids into the Earth-be it for CO 2 sequestration, enhanced geothermal systems, or oil and gas operations-is known to induce earthquakes (Ellsworth, 2013;Keranen et al., 2013;Raleigh et al., 1976). Minimizing induced seismicity requires an understanding of what causes a fault to begin to slip, the mechanisms driving the transition from aseismic to seismic slip (i.e., initiation of dynamic rupture), and how large the resulting seismic event will grow (i.e., how far dynamic rupture is sustained). These factors help inform the maximum event magnitude and potential for runaway ruptures. This study explores how background stress levels affect the initiation and termination of fluid-induced ruptures using a 3 m rock experiment.Fluid injection field experiments on the decameter scale highlight the important role of induced aseismic slip in the initiation of induced seismicity. Results from show that fluid injection primarily induced aseismic slip. They observed microseismicity as a by-product of aseismic slip rather than directly

In the quest to determine fault weakening processes that govern earthquake mechanics, it is common to infer the earthquake breakdown energy from seismological measurements. Breakdown energy is observed to scale with slip, which is often attributed to enhanced fault weakening with continued slip or at high slip rates, possibly caused by flash heating and thermal pressurization. However, seismologically inferred breakdown energy varies by more than six orders of magnitude and is frequently found to be negative-valued. This casts doubts about the common interpretation that breakdown energy is a proxy for the fracture energy, a material property which must be positive-valued and is generally observed to be relatively scale independent. Here, we present a dynamic model that demonstrates that breakdown energy scaling can occur despite constant fracture energy and does not require thermal pressurization or other enhanced weakening. Instead, earthquake breakdown energy scaling occurs simply due to scale-invariant stress drop overshoot, which may be affected more directly by the overall rupture mode – crack-like or pulse-like – rather than from a specific slip-weakening relationship.

Summary Earthquake ruptures are generally considered to be cracks that propagate as fracture or frictional slip on preexisting faults. Crack models have been used to describe the spatial distribution of fault offset and the associated static stress changes along a fault, and have implications for friction evolution and the underlying physics of rupture processes. However, field measurements that could help refine idealized crack models are rare. Here we describe large-scale laboratory earthquake experiments, where all rupture processes were contained within a 3-m long saw-cut granite fault, and we propose an analytical crack model that fits our measurements. Similar to natural earthquakes, laboratory measurements show coseismic slip that gradually tapers near the rupture tips. Measured stress changes show roughly constant stress drop in the center of the ruptured region, a maximum stress increase near the rupture tips, and a smooth transition in between, in a region we describe as the earthquake arrest zone. The proposed model generalizes the widely used elliptical crack model by adding gradually tapered slip at the ends of the rupture. Different from the cohesive zone described by fracture mechanics, we propose that the transition in stress changes and the corresponding linear taper observed in the earthquake arrest zone are the result of rupture termination conditions primarily controlled by the initial stress distribution. It is the heterogeneous initial stress distribution that controls the arrest of laboratory earthquakes, and the features of static stress changes. We also performed dynamic rupture simulations that confirm how arrest conditions can affect slip taper and static stress changes. If applicable to larger natural earthquakes, this distinction between an earthquake arrest zone (that depends on stress conditions) and a cohesive zone (that depends primarily on strength evolution) has important implications for how seismic observations of earthquake fracture energy should be interpreted.