Aeolian Islands, Italy (38.79 N, 15.21 E)
(This report is relative to the period 05.04 - 10.06.1995)
Last report (see Bullettin, v. 20, n.4) described the seismic activity at Stromboli before and after the big explosion of 5 March, which was preceded by a short lasting decrease of the tremor intensity, although accompanied by a considerable number of major shocks, and followed by its raise. We also noted how this pattern did not match the behaviour recorded after the paroxysmal phases of 1993, at least from the point of view of intensity alone. Similarities seem however to appear in the spectral domain of the tremor, although many more years of recording (and regular funding...) are necessary before talking about any statistically significant "precursor".
The analysis of the seismic data of April and May is particularly interesting for two reasons. On one side C. Blot made an eruption forecast for 20 April +/- 15 days, based on two deep shocks recorded beneath Stromboli island. On the other side local authorities declared an alert on Stromboli during almost the same period as a consequence of an anomalous decrease in the activity of the volcano.
As one can see from the graph of the seismic activity, the number of recorded events shows in fact two periods of decrease, the most significant being the one between 27 April and 6 May, with less than 100 recorded events per day on the average, a remarkably low number if compared with the values recorded during the first three months of the year (270 events per day on the average).
This is however not so anomalous if we have a look at the activity of end 1990 - begin 1991 (with our previous one-component instrumentation) or if we examine the evolution of the seismicity after the paroxysmal phases of 1993, finding a similarly low number of events in February, June, October and November. So we can hypothesize that this low activity may be in some way a "late effect" of the crisis which culminated in the big explosion of 5 March rather than a "precursor" of a new paroxysmal event, although this is of course too much a simplification of the complex dynamics of the volcano. In the possible explanation of this period of low activty one has also to take into account the fact that the previous part of the crisis was anomalous, especially for what concerns the lack of an abrupt decrease of the tremor intensity immediately after the explosion. A detailed spectral analysis of the period is in progress; preliminary results confirm that, as usual, the "state transitions" are a bit more evident in the spectral domain.
The second half of May was in any case characterized by a reprise of the activity, both in terms of the total number of recorded events and of the number of major ones (saturating the acquisition system with ground velocities exceeding 100 µm/s). Also the limitations imposed by local authorities to the access of the tourists to the volcano were reduced. The beginning of June shows a new slight decrease of the seismic activity, which is now characterized by a fairly constant number of events (100-150 per day) and very few major shocks. During all the period examined in this report, the tremor intensity showed a very slight decreasing trend in its usual oscillations.
Timely information about Stromboli seismic activity, accompanied by general description of the volcano, images, news etc. can now be found in the World Wide Web, starting your browser at http://udgtls.dgt.uniud.it/seismol/strombol.htm. In particular, one is now able to compare "almost-real-time" seismic data with the activity recorded since 1989 (1992 with current instrumentation). A number of links to other interesting sites, regarding both Stromboli and other volcanoes, can also be followed from this site.
To see the Figure, click here!
Fig. caption:
