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The long precursory phase of most large interplate earthquakes


Many earthquakes are preceded by foreshocks1,2. However, the mechanisms that generate foreshocks and the reason why they occur before some earthquakes and not others are unknown3,4,5,6,7,8. Here we use seismic catalogues from the best instrumented areas of the North Pacific to analyse the foreshock sequences preceding all earthquakes there between 1999 and 2011, of magnitude larger than 6.5 and at depths shallower than 50 km. The data set comprises 31 earthquakes at plate boundaries, and 31 in plate interiors. We find that there is a remarkable contrast between the foreshock sequences of interplate compared with intraplate earthquakes. Most large earthquakes at plate interfaces in the North Pacific were preceded by accelerating seismic activity in the months to days leading up to the mainshock. In contrast, foreshocks are much less frequent in intraplate settings. We suggest that at plate boundaries, the interface between the two plates begins to slowly slip before the interface ruptures in a large earthquake. This relatively long precursory phase could help mitigate earthquake risk at plate boundaries.

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Figure 1: Geographical distribution of the earthquakes.
Figure 2: Evolution of the cumulative numbers of events in the few days before 22 interplate earthquakes ( 70% of the data set).
Figure 3: Time evolution of the seismic moment released in the 50-km-radius zone surrounding the epicentre before interplate earthquakes.
Figure 4: Comparison of the characteristics of the different types of pre-earthquake sequence.


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We thank J. R. Grasso, M. Campillo, F. Renard, P. Y. Bard, M. P. Bouin, O. Coutant, P. Bernard, L. Géli, P. Henry, R. Archuleta, O. Lengliné and G. Poupinet for discussions.

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M.B. and V.D. analysed the data and investigated the time and space evolutions. D.M. developed and performed the statistical analysis, H.K. and J.S. investigated the characteristics of the precursory phase. All authors contributed to the conclusions presented in the manuscript.

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Correspondence to Michel Bouchon or Virginie Durand.

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The authors declare no competing financial interests.

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Bouchon, M., Durand, V., Marsan, D. et al. The long precursory phase of most large interplate earthquakes. Nature Geosci 6, 299–302 (2013).

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