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Potential slab deformation and plunge prior to the Tohoku, Iquique and Maule earthquakes


Megathrust earthquakes rupture hundreds of kilometres of the shallow plate interface in subduction zones, typically at depths of less than 50 km. Intense foreshock activity preceded the 2011 Mw 9 Tohoku-oki (Japan) and 2014 Mw 8.2 Iquique (Chile) megathrust earthquakes. This pre-earthquake activity was thought to be generated1,2,3,4,5,6 by slow slip in the seismogenic zone before rupture, but where this slow slip originated and how it spread rapidly over long distances are unknown. Here we analyse seismic activity deep in the subduction zone before the Tohoku-oki and Iquique ruptures, as well as before the 2010 Mw 8.8 Maule earthquake in Chile. We find that, before each of these megathrust earthquakes, shallow seismicity occurred synchronously with bursts of seismic activity deep (100 km) in the subducting slab. The extensional mechanism of these deep shocks suggests that the slab was stretched at depth. We therefore propose that, before these megathrust quakes, the slab might have started to plunge into the mantle below part of the future rupture zone. We speculate that synchronization between deep and shallow seismicity may have marked the nucleation phase for these three giant earthquakes.

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Figure 1: Compared evolutions of foreshock activity and deep seismicity before Tohoku.
Figure 2: Plunge/deformation of the slab below northeastern Japan before Tohoku, imaged by the spatial distribution of deep activity increase between 13 January and 9 March 2011.
Figure 3: Evolution of deep and shallow activity before Iquique.
Figure 4: Correlation of shallow (≤40 km) and deep (≥80 km) seismicity occurring in a zone of 2° (222 km) radius around the Tohoku epicentre, in the two years preceding the earthquake.


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We thank the scientists, engineers, and technicians from the International Seismological Centre, the Japan Meteorological Agency and the Centro Sismologico Nacional of Chile who made the catalogues that are used in this study.

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M.B. and D.M. originated and developed the study. V.D. contributed to the methodology. M.C., H.P., R.M. and B.G. contributed to the data analysis and interpretation.

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Correspondence to Michel Bouchon.

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The authors declare no competing financial interests.

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Bouchon, M., Marsan, D., Durand, V. et al. Potential slab deformation and plunge prior to the Tohoku, Iquique and Maule earthquakes. Nature Geosci 9, 380–383 (2016).

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