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Decay of aftershock density with distance indicates triggering by dynamic stress


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.

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Figure 1: Combined aftershocks of M = 3–4 mainshocks.
Figure 2: Distance from the mainshock hypocentre versus aftershock linear density.
Figure 3: Aftershock density versus distance from the closest point on the fault planes of M = 5–6 mainshocks.
Figure 4: Distance versus earthquake linear density for a time-randomized catalogue.


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We thank R. Abercrombie, A. Felzer, N. Field, M. Gerstenberger, J. Gomberg, S. Gross, J. Hardebeck, M. Harrington, A. Helmstetter, S. Hough, L. Jones, Y. Kagan, H. Kanamori, I. Main, S. Prejean, P. Shearer, R. Stein, J. Vidale, K. Richards-Dinger and A. Yong for comments. This work was supported in part by the National Science Foundation.

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Correspondence to K. R. Felzer.

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Supplementary Figures

This file contains Supplementary Figures 1–9. The figures include information on data behaviour beyond the distances and times shown in the main paper, aftershocks in Japan and Northern California, comparison of the static stress change and rate and state friction aftershock model with data, a demonstration of aftershock location independence from mainshock magnitude, and discussion of data measurement technique. (PDF 895 kb)

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Felzer, K., Brodsky, E. Decay of aftershock density with distance indicates triggering by dynamic stress. Nature 441, 735–738 (2006).

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