  The Altels hanging glacier broke off on September 11, 1895. The ice volume of
this catastrophic rupture was estimated at $\rm 4.10^6$ cubic meters and is the
largest ever observed ice fall event in the Alps. The causes of this collapse
are however not entirely clear. Based on previous studies, we reanalyzed this
break-off event, with the help of a new numerical model, initially developed by
Faillettaz and others (2010) for gravity-driven instabilities. The simulations
indicate that a break-off event is only possible when the basal friction at the
bedrock is reduced in a restricted area, possibly induced by the storage of
infiltrated water within the glacier. Moreover, our simulations reveal a
two-step behavior: (i) A first quiescent phase, without visible changes, with a
duration depending on the rate of basal changes; (ii) An active phase with a
rapid increase of basal motion over a few days. The general lesson obtained
from the comparison between the simulations and the available evidence is that
visible signs of the destabilization process of a hanging glacier, resulting
from a progressive warming of the ice/bed interface towards a temperate regime,
will appear just a few days prior to the collapse.
