During quiescent periods of time, faults experience slow slip episodes, similar to earthquakes but too slow to generate significant seismic waves and shake the ground. The largest episodes, known as slow slip events (SSEs) can be recorded by geodetic instruments (GPS). These events cause the rupture of "slow" micro-earthquakes known as low-frequency earthquakes (LFEs) or very low-frequency earthquakes (VLFEs). All these slow slip phenomena have moment (quantity defining the magnitude of an event) proportional to their duration (Ide et al., 2007) (the moment of regular earthquakes is proportional to the cube of their duration). We evidenced the existence of a new slow slip phenomenon, called secondary slip fronts (SSFs) filling the gap in magnitude and duration between VLFEs and SSEs (figure below) (Bletery et al., EPSL, 2017). Explaining the origin of the two distinct slip modes (slow slip, regular earthquakes) is one of the main challenge to progress in our understanding of fault mechanics and better anticipate earthquakes.