Frequently asked questions about Etna
Can the next flank eruption be forecast or predicted?

In the long term, all that can be said is what has been said in the answer to the previous question. In the short term, an identification of an imminent eruption and its location will very likely be possible, based mostly on premonitory seismicity and ground deformation. An intrusion of magma into the flank of Etna will be detected by the seismic and geodetic network. In fact, nearly all flank eruptions since 1981 have been preceded by up to four days of intense seismicity that could be identified as probable precursor. In the case of the 1981 eruption, volcanologists of the Istituto Internazionale di Vulcanologia (IIV) were actually flying in a helicopter over the northern flank of Etna in the moment when the first eruptive fissures began to split open - right where the premonitory seismicity had occurred. The July-August 2001 eruption was preceded by four days of vigorous seismicity, ground fracturing and rapid inflation (uplift of the ground in specific areas), so that there was little doubt that an eruption was imminent. However, the next flank eruption, in October 2002, was preceded by only 2 hours (!) of such premonitory symptoms, and that eruption came as a surprise to many since volcanologists working on Etna had issued no assessments of the probability of a future eruption in the long term. Furthermore there is no means to predict the duration and volume of an imminent eruption - this is impossible even after an eruption has actually begun.

What is possible is to evaluate the probability of a new flank eruption in the near future by statistical means. This can be based on the recent eruptive behavior of the volcano but must also consider trends documented in the historical record of Etna's activity. The recent (let's take the period since 1971) past shows that flank eruptions are clustered, that is, they come in series with one flank eruption occurring every 1-2 years. A new series seems to have begun with the 2001 eruption, and the second eruption in this series occurred in 2002-2003. This is similar to the intervals between flank eruptions during 1971-1993, and it can be assumed that during the near future Etna will continue to produce flank eruptions at a similar rate. However, this relatively simple probabilistic approach must remain open to changes in the dynamics of magma production, accumulation and uprise, which seem to be under way since the 2001 eruption. Part of the 2001 and 2002-2003 magmas were fed from a reservoir that is distinct from the central conduit system, and which has not fed any eruption during the 20th century (but possibly some eruptions in the pre-1900 period were fed from the same or a similar reservoir). The behavior of this reservoir might have a crucial influence on the frequency, location and style of eruptions in the near future.

Another major problem volcanologists have to deal with (not only in the case of Etna) is that a volcano may show signs of unrest without producing an eruption thereafter. At Etna, in 1989 and late 1997-early 1998 there were clear indicators that magma was intruding to shallow depths under the flanks of the volcano, but in both cases no eruption occurred in the areas of the anticipated outbursts (southeastern flank in 1989, contemporaneously with an eruption on the northeastern flank; western flank in December 1997 and January 1998). However, the next eruption after the 1989 intrusive event (1991-1993) was accompanied by the formation of fractures adjacent to the 1989 fracture system.

In summary, at the current time (August 2003) there is no means to say with certainty when the next eruption will take place, where it will take place, and how long it will last. It is, however, likely that it will occur within less than 2 years from August 2003, with a good chance that it will affect generally the same areas which have been the sites of the 2001 and 2002-2003 eruptions. This eruption may or may not be preceded by summit activity. Why? Summit activity generally marks the recharging of the central conduit system of the volcano after its draining during a "normal" (lateral) flank eruption. This can occur if the magma does not find pathways leading directly from the central conduit system into the flanks of the volcano, that is, the volcano is stable. If, on the other hand, there are open fractures in the walls of the central conduit system, which lead into the flanks, then the magma might drain into these fractures without even arriving at the summit craters. Following the massive flank slip that shortly preceded and accompanied the beginning of the 2002-2003 eruption (and which has not ceased ever since then), the flanks of the volcano can be expected to be strongly fractured, as has also been noted by Patanè et al. (2003). In that case, intruding magma would not have to open new pathways and could rise to the surface rather passively, without even causing significant seismicity. This means that there might be very little direct and clear warning of the next eruption, and the message of the volcano must be read somewhere between the lines.

Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"