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Decay of aftershock density with distance indicates triggering by dynamic stress

2006; Nature Portfolio; Volume: 441; Issue: 7094 Linguagem: Inglês

10.1038/nature04799

1476-4687

K. R. Felzer, E. E. Brodsky,

High-pressure geophysics and materials

Aftershocks, the most common type of earthquake, were thought to be triggered by static stresses induced by an earlier 'mainshock'. Recent work suggested that dynamic stresses, or shaking, may also be a factor, and a study based on analysis of the earthquake locations in the 1984–2002 Southern California catalogue confirms that view. Precise measurements of the decay of aftershock density with distance show that the probability of an aftershock is consistent with a maximum amplitude of seismic shaking at distances of 0.2 to 50 km from a mainshock. The majority of earthquakes are aftershocks1, yet aftershock physics is not well understood. Many studies suggest that static stress changes2,3 trigger aftershocks, but recent work suggests that shaking (dynamic stresses) may also play a role4,5. Here we measure the decay of aftershocks as a function of distance from magnitude 2–6 mainshocks in order to clarify the aftershock triggering process. We find that for short times after the mainshock, when low background seismicity rates allow for good aftershock detection, the decay is well fitted by a single inverse power law over distances of 0.2–50 km. The consistency of the trend indicates that the same triggering mechanism is working over the entire range. As static stress changes at the more distant aftershocks are negligible, this suggests that dynamic stresses may be triggering all of these aftershocks. We infer that the observed aftershock density is consistent with the probability of triggering aftershocks being nearly proportional to seismic wave amplitude. The data are not fitted well by models that combine static stress change with the evolution of frictionally locked faults3.