| On 30 December 2002 the coast of the volcanic island of
Stromboli, in the Tyrrhenian sea, Italy, was attacked by two tsunamis
generated by landslides that took place on the north-west flank of the
volcano. The landslides and the tsunamis represented the most impressive and
threatening episodes of a strong effusive eruption, started on 28 December
from a new vent which opened close to the north-east crater of the
volcano. In spite of the intensified monitoring carried out in response to
the eruption, the landslides and the ensuing tsunamis were not foreseen, and
the available instrumental data are insufficient to allow a precise
reconstruction of the sequence of the events. The seismic network recorded
two main landslides along the steep slope of Sciara del Fuoco, with onset
around 13:15 and 13:23 local time (GMT+1). The tsunamis were the direct
consequence of the mass movements. Three main post-event surveys helped make
assessment on the wave impact on the coast.
In this paper the attention is focussed on the accounts of the
eye-witnesses, that help us clarify and understand what happened. People in
the source area (Sciara del Fuoco) reported a small-volume subaerial slide
taking place first, then a sharp cut forming in the sea water down to the
sea floor (about 10–20 m deep) and propagating almost parallel to the
coastline, be concomitantly associated with a sea retreat and a subsequent
sea advance. It is suggested here that the cut was the effect of a large
submarine landslide that detached from very close to the coast and produced
the 13:15 signal in the recorded seismograms. The second, mostly subaerial,
slump was observed to slide down 7–8 min later and to excite a train of
waves some distance offshore. Not all the witnesses realised that two
distinct tsunamis occurred. The tsunami period was probably in the order of
100 s, but shorter period crests were seen to travel on the top of the
long-period waves by several persons. The duration of each tsunami was
appreciated to be around 5–7 min. It is difficult to ascertain which
tsunami was the largest, since there is no full agreement among the
observers, but certainly by accounts both were characterised by large
destructive waves. |