Complex laboratory earthquake sequences show asperity interactions through creep fronts and illuminate the mechanics of delayed earthquake triggering

Abstract

Natural earthquakes occur in clusters or sequences that arise from complex triggering mechanisms, but direct measurement of the mechanisms responsible for complex temporal sequences and delayed triggering is rarely possible. A central question involved whether delayed triggering is due to slow slip and stress transfer or local weakening/fatigue processes such as stress corrosion. We investigate the origins of this complexity and its relationship to fault heterogeneity using a biaxial loading apparatus with an experimental fault that has two dominant seismic asperities. The fault is composed of a 5 mm layer of quartz powder, a velocity weakening material common to natural faults, sandwiched between 760 mm long polymer blocks that deform similar to the way 10 meters of rock would behave. Due to the higher local normal stress and the free surface boundary condition on the sample ends, the sample behaves like two asperities, one at each end, that can fail independently. As the quartz powder was continuously sheared, the friction properties changed, and we observed a transition from steady sliding to periodic repeating earthquakes that transitioned into aperiodic and complex sequences of fast and slow events. There is also reason to believe that friction properties evolved differently on the higher normal stress asperities and made them more unstable than the center part of the laboratory sample. Sequential ruptures on the two different asperities were linked via migrating slow slip which resembles creep fronts observed in numerical simulations and on tectonic faults. The propagation velocity of the creep fronts ranged from 0.1 to 10 m/s, which is broadly consistent with the velocity of slow slip fronts inferred from migrating tectonic tremor sources. 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.