A multidisciplinary approach has allowed us to estimate the probability of the repetition of explosions more intense than Stromboli after one of them has occurred
Stromboli, the "lighthouse of the Mediterranean", is a volcano famous for its low-energy and persistent explosive activity, known as Strombolian activity. This feature has always been a strong attraction for visitors and volcanologists from all over the world.
However, occasionally - as recently occurred on 3 July and 28 August 2019 - more intense and sudden explosions occur which can represent a serious danger, the so-called "Strombolian paroxysms". Already described by the geologist Giuseppe Mercalli at the beginning of the last century, during these events more craters are simultaneously involved and higher volumes of pyroclastic materials are erupted.
The goal of the study “Major explosions and paroxysms at Stromboli (Italy): a new historical catalog and temporal models of occurrence with uncertainty quantification”, just published in the journal 'Scientific Reports' of Nature, it was possible to estimate the frequency of occurrence of Strombolian paroxysms and verify whether the volcano had its own "memory", i.e. whether it was possible to identify a statistical recurrence between one paroxysmal eruption and the next. He then tried to answer the questions "how likely are these more violent explosive phenomena?" and “How much more likely do they become after one of them has occurred, and for how long?”
To answer these questions, a team of researchers from the National Institute of Geophysics and Volcanology (INGV) and the University of Bristol (UK) has developed a new catalog which describes 180 violent explosive events of various scales that occurred on Stromboli from 1879 to 2020. In particular, 36 of the 180 explosive events surveyed are paroxysms, similar to those of summer 2019.
For this study, the researchers critically evaluated events described in scientific papers of the past and information reported in historical and narrative texts, determining, on an objective and homogeneous basis, the type and intensity of the explosive activity regardless of the emphasis of the stories .
“The new catalog we have developed”, explains Massimo Pompilio, principal investigator of INGV and co-author of the study, “it has made it possible to review the classification of numerous events through the critical analysis of historical sources. The analysis shows that the average annual rate of paroxysms over the last 140 years was 0.26 events/year, i.e. one event approximately every 4 years. This rate is close to that calculated in the last ten years, but much lower than that reached in the 40s, when these paroxysmal events were much more frequent. The volcano therefore alternates between periods of intense activity and periods of relative quiet." “The short time span of 56 days observed between the two paroxysms of summer 2019”, continues Massimo Pompilio, “It is therefore not a rare situation. Five times in the last 140 years there have been even shorter inter-event times. Conversely, there have been four periods without paroxysms lasting from 9 to 15 years, and an interval without paroxysms that even lasted for 44 years, from 1959 to 2003".
This information is also useful in a forecasting context, i.e. to estimate the probability of future occurrence of these phenomena.
Andrea Bevilacqua, INGV researcher and first author of the study explains: “When a phenomenon, such as a volcanic explosion, occurs at irregular intervals over time, what is studied is the distribution of the 'inter-event times', ie the times that elapsed in the past between one explosion and the next. In particular, the development of inter-event models allows us to calculate the probability of an explosion occurring as a function of the time elapsed since the last event of that type. An important piece of evidence that emerged from our research concerns the tendency of paroxysms to occur in clusters. Still on the basis of data from the last 140 years, we have estimated that there is a 50% probability that a paroxysm will occur within twelve months of the previous one and a 20% probability that it will follow in less than two months; on the other hand there is also a 10% probability that more than ten years will pass without further paroxysms occurring”.
A very similar "memory" of the volcano, albeit with different estimates of occurrence, emerges considering, together with the paroxysms, also the so-called "major explosions", explosions more frequent than the paroxysms but endowed with less energy and danger.
"This study has shown how, in terms of the occurrence of more violent than ordinary explosive phenomena, Stromboli is going through, in recent years, one of the most intense phases of activity in its recent history", concludes Augusto Neri, Director of the Volcanoes Department of INGV and co-author of the study. “The estimate of the 'memory' of Stromboli's most intense explosive activity will be able to make a significant contribution to quantifying the danger of these phenomena and, consequently, to reducing the associated risk. Furthermore, the data analysis suggests the existence of a physical process which to some extent influences the frequency of the volcano's explosions, making them not completely random eruptive events. Understanding the reasons and the physical mechanisms that determine this memory represents a further scientific challenge".
The published research has an essentially scientific value, without immediate implications regarding the aspects of civil protection at the moment.
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Stromboli, the "memory" of the volcano measured to estimate the probability of explosive eruptions
A multidisciplinary approach made it possible to estimate the probability of further intense explosions of Stromboli when one has just occurred
Stromboli, the “lighthouse of the Mediterranean”, is a volcano famous for its low-energy but persistent explosive eruptions, behavior known scientifically as Strombolian activity. This feature has always been a strong attraction for visitors and volcanologists from all over the world.
However, occasionally - as recently occurred on 3rd July and 28th August 2019 - more intense and sudden explosions occur, which can represent a serious danger, the so-called “Strombolian paroxysms”. These larger eruptions were described by the geologist Giuseppe Mercalli at the beginning of the last century and, during such events, several of Stromboli's craters are active simultaneously, with volumes of pyroclastic materials being erupted that are much greater than is “normal” for explosions from the volcano.
The aim of the study"Major explosions and paroxysms at Stromboli (Italy): a new historical catalog and temporal models of occurrence with uncertainty quantification”, just published in the journal 'Scientific Reports', was to estimate the frequency of occurrence of Strombolian paroxysms and to investigate if the volcano has its own “memory” as evidenced, in statistical terms, by a temporal recurrence relationship between one paroxysmal eruption and the next. The study then tackled the questions: “how likely are these more violent explosive phenomena?” and “how much more likely do they become after one of them has just happened, and for how long?”.
To answer these questions, a team of researchers from the National Institute of Geophysics and Volcanology (INGV) and the University of Bristol (UK) has developed a new catalog which describes 180 violent explosive events of varying scale that occurred at Stromboli from 1879 to 2020 In particular, 36 of these historic explosive events were of the much larger paroxysm type, similar to those of the summer of 2019.
For this study, the researchers critically evaluated events described in past scientific works and from information recorded in historical texts, determining, on an objective and homogeneous evidential basis, the type and intensity of the explosive activity --regardless of any narrative hyperbole in the written accounts.
“The new catalog that we have developed”, explains Massimo Pompilio, senior researcher at INGV and co-author of the study, “made it possible to review the classification of numerous events through the critical analysis of historical sources. From the analysis it emerges that the average annual rate of paroxysms of the last 140 years was 0.26 events / year, ie one event every 4 years or so. This rate is close to that calculated in the last ten years, but much lower than that achieved in the 40s of the last century, when these paroxysmal events were much more frequent. The volcano therefore alternates periods of intense activity and periods of relative quiet”. “The short span of 56 days observed between the two paroxysms of summer 2019”, continues Massimo Pompilius, “is therefore not a rare situation. Five times in the past 140 years there have been even shorter inter-event times. Conversely, there have been four periods without paroxysms lasting from 9 to 15 years, and an interval without paroxysms that lasted for 44 years, from 1959 to 2003”.
This information is also useful in a forecasting context, ie to estimate the probabilities of future occurrence of these volcanic phenomena.
Andrea Bevilacqua, INGV researcher and first author of the study explains: “When a phenomenon, such as a volcanic explosion, occurs at irregular intervals in time, what is studied is the distribution of the 'inter-event' times, ie the times elapsed in passed between one explosion and the next. In particular, the development of inter-event models allows us to calculate the probability of an explosion occurring as a function of the time elapsed since the last event of that type. An important evidence that emerged from our research concerns the tendency of paroxysms to occur in groups. Still on the basis of data from the last 140 years, we have estimated that there is a 50% probability that a paroxysm will occur within twelve months of the previous one and a 20% probability that it will follow it in less than two months; on the other hand, there is also a 10% probability that more than ten years will pass without any other paroxysms occurring".
A very similar “memory” property of the volcano, though with different rates of occurrence, emerges if, together with the paroxysms, the so-called “major explosions” are also considered. The latter explosions occur more frequently than paroxysms but with less energy and less danger.
“This study showed how, in terms of the occurrence of the most violent explosive phenomena of the ordinary, Stromboli is going through, in recent years, one of the most intense phases of its recent history”, concludes Augusto Neri, Director of the Volcanoes Department of the INGV and co-author of the study. “The estimation of the 'memory' of the most intense explosive activity of Stromboli will make a significant contribution to the quantification of the danger of these phenomena and, consequently, to the reduction of the associated risk. Furthermore, the analysis of the data suggests the existence of a physical process that to some extent influences the frequency of the volcano's explosions, making them not completely random eruptive events. Understanding the reasons and physical mechanisms that determine this memory represents a further scientific challenge”.
The published research has a scientific value with no immediate implications in terms of civil protection issues.
Figure 1 - In (ae) the photos of five different paroxysms of Stromboli, namely those of 1915, 1959, 2003 and 2019 (July 3 and August 28).
(Figure 1 - Photographs of five different paroxysms of Stromboli: those of 1915, 1959, 2003 and 2019 (3 July and 28 August).
Figure 2 - Cumulative count of the number of paroxysms from 1879 to 2020. Groups with inter-event times of less than one year are highlighted with a blue dotted curve. The years of six paroxysm sequences separated by waiting periods greater than 9 years are labeled. Also shown in the lower right corner are the shortest and longest periods between two consecutive paroxysms.
Figures 2 - Cumulative count of the number of paroxysms between 1879 and 2020. Blue dashed lines highlight groups with inter-event intervals of less than a year. The years of six sequences of paroxysms separated by intervals of more than 9 years are indicated. On the right side, the shortest and longest periods between paroxysms are reported.
Figure 3 - Example of probability of daily occurrence of paroxysms, as a function of the number of months that have passed since the last event of the same type. The colored band shows the uncertainty associated with considering two different statistical models. The horizontal lines indicate a daily occurrence probability of 0.1% (dotted line) and the average occurrence rates of the last 10 and 25 years in the hypothesis of no memory of the volcano, according to a Poissonian model (solid lines).
(Figure 3 - Example for the probability of daily occurrence of paroxysms, in function of the number of months elapsed since the last event of the same type. Colored band shows uncertainty related to the two different statistical models used. Horizontal lines indicated a 0.1% probability of daily occurrence (dotted line), and mean rates of occurrence in the past 10 and 25 years, in the hypothesis that the volcano has no memory, according to a Poisson model (continuous lines).