  Frictional weakening by vibrations was first invoked in the 70's to explain
unusual fault slips and earthquakes, low viscosity during the collapse of
impact craters or the extraordinary mobility of sturzstroms, peculiar rock
avalanches which travels large horizontal distances. This mechanism was further
invoked to explain the remote triggering of earthquakes or abnormally large
landslides or pyroclastic flows runout. Recent experimental and theoretical
work pointed out the velocity of vibration as the key parameter which governs
frictional weakening in sheared granular media. Here we show that the grains
mobility is not mandatory, and that the vibration velocity governs both
granular and solid frictional weakening. The velocity threshold controlling the
transition from stick-slip motion to continuous sliding is of the same order of
magnitude, namely a hundred microns per second. It is linked to the roughness
distribution of the asperities at the contact surface.
