Creep fronts and complexity in laboratory earthquake sequences illuminate delayed earthquake triggering

Laboratory earthquake experiments reproduce delayed earthquake triggering, similar to aftershocks, as a result of propagating slow slip fronts. The speed of the fronts can be highly sensitive to fault stress levels left behind by previous earthquakes. Earthquakes occur in clusters or sequences that arise from complex triggering mechanisms, but direct measurement of the slow subsurface slip responsible for delayed triggering is rarely possible. We investigate the origins of complexity and its relationship to heterogeneity using an experimental fault with two dominant seismic asperities. The fault is composed of quartz powder, a material common to natural faults, sandwiched between 760 mm long polymer blocks that deform the way 10 meters of rock would behave. We observe periodic repeating earthquakes that transition into aperiodic and complex sequences of fast and slow events. Neighboring earthquakes communicate via migrating slow slip, which resembles creep fronts observed in numerical simulations and on tectonic faults. Utilizing both local stress measurements and numerical simulations, we observe that the speed and strength of creep fronts are highly sensitive to fault stress levels left behind by previous earthquakes, and may serve as on-fault stress meters.

Automated summary

Laboratory earthquake experiments have revealed that delayed earthquake triggering, similar to aftershocks, can be reproduced through propagating slow slip fronts. The speed of these fronts is highly sensitive to the fault stress levels left behind by previous earthquakes. The experiments also showed periodic repeating earthquakes transitioning into complex sequences of fast and slow events.