An 8 month slow slip event triggers progressive nucleation of the 2014 Chile megathrust

The mechanisms leading to large earthquakes are poorly understood and documented. Here, we characterize the long-term precursory phase of the April 1st 2014 Mw8.1 North Chile megathrust. We show that a group of coastal GPS stations accelerated westward eight months before the mainshock, corresponding to a Mw6.5 slow slip event on the subduction interface, 80% of which was aseismic. Concurrent interface foreshocks underwent a diminution of their radiation at high frequency, as shown by the temporal evolution of Fourier spectra and residuals with respect to ground motions predicted by recent subduction models. Such ground-motions change suggests that, in response to the slow sliding of the subduction interface, seismic ruptures are progressively becoming smoother and/or slower. The gradual propagation of seismic ruptures beyond seismic asperities into surrounding metastable areas could explain these observations, and might be the precursory mechanism eventually leading to the mainshock.