A new approach for evaluating the strombolian type activity

Portsmouth, UK

Volcanoes represent one of the most important application fields for risk mitigation techniques (Mc
Guire, Kilburn, and Murray, Eds, 1995) due to the ingent damages and casuality. This paper deals with a
monitoring system that could be successful used for risk mitigation related to forecast paroxysmal
explosions of strombolian type. The typical activity of Stromboli volcano consists of intermittent mid explosions lasting a few seconds,
which take place at different vents and at variable intervals, the most common time interval being 10-20
minutes (Chouet, Hamisevicz, and McGetchin, 1974; Blackburn, Wilson, and Sparks, 1976). However the
routine activity can be interrupted by more violent paroxysmal explosions, that eject m-sized scoriaceous
bombs and lava blocks to a distance of several hundreds of meters from active vents. On average, one or
two paroxysmal explosions occurred per year over the past century, buth this statistic may be
underestimated in absence of continuous monitoring. For this reason from summer 1996 a remote
surveillance camera works on Stromboli recording continuously the volcanic activity. It is located on
Pizzo Sopra la Fossa, 100 metres above the crater terrace where are the active vents. Moreover, from September 2001 an on-line image analyzer system referred to as VAMOS (Volcanic
Activity MOnitoring System) operates detection and classification of explosive vents in real-time. Using
VAMOS it is possible to identify changes in the explosive activity trend that could precede a particular
eruptive event, like paroxysmal explosions, fire fountains, lava flows. Since the camera installation up to
present, several explosions occurred at the different craters and the parameterization in classes of intensity
for each explosion on the base of tephra dispersion and kinetic energy have been made. The analysis
include the counting of the explosions occurred at the different craters and the parameterisation in classes
of intensity for each explosion on the base of tephra dispersion and, in generic sense, of the kinetic energy
released. The plot of dissipated energy by each crater versus time seems to exhibit a cyclic behaviour with
max and min explosive activity ranging from a few days to a month. Often the craters show opposite
trends so when the activity decreases in a crater, increases in the other. Before every paroxismal
explosion recorded, the crater that produced the event decreased and then stopped its activity from a few
days to weeks before. The other crater tried to compensate increasing its activity and when it declined the
paroxysmal explosion occurred suddenly at the former site (Coltelli, and Cristaldi, 2003). The system has
automatically recorded and analysed the change in energetic trend that proceded the 20 October 2001
paroxysmal explosion that killed a woman and the strong explosive activity that preceded the onset of 28
December 2002 lava flow eruption.