A hanging glacier at the east face of Weisshorn (Switzerland) broke off in
2005. We were able to monitor and measure surface motion and icequake activity
for 25 days up to three days prior to the break-off. The analysis of seismic
waves generated by the glacier during the rupture maturation process revealed
four types of precursory signals of the imminent catastrophic rupture: (i) an
increase in seismic activity within the glacier, (ii) a decrease in the waiting
time between two successive icequakes, (iii) a change in the size-frequency
distribution of icequake energy, and (iv) a modification in the structure of
the waiting time distributions between two successive icequakes. Morevover, it
was possible to demonstrate the existence of a correlation between the seismic
activity and the log-periodic oscillations of the surface velocities
superimposed on the global acceleration of the glacier during the rupture
maturation. Analysis of the seismic activity led us to the identification of
two regimes: a stable phase with diffuse damage, and an unstable and dangerous
phase characterized by a hierarchical cascade of rupture instabilities where
large icequakes are triggered.Comment: 16 pages, 7 figure