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The VSI scale

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Guest post by Tallis

Humans have always had some fascination with rankings, some of the most popular videos and articles on the internet have dealt with comparisons and rankings. However, this doesn’t mean that it is just another useless fad, on the contrary, a well thought-out “Top 10 list”  can be very informative. For disasters, we have created scales, and we are all very familiar with these scales, the Fujita scale, Safir-Simpson scale, Moment magnitude, and the VEI scale. Just like us, these scales are imperfect, they don’t factor in all of the variables, most of which you would only know right when the disaster is taking place or afterwards but they get the job done. We don’t need to understand all of the variables to understand that when an EF5 is coming, that means get into a basement. However, volcanoes aren’t like most disasters, we don’t really see disasters that produce such varied and extreme consequences on so many levels. For example the Laki eruption, a VEI 5, killed more than 30,000 people directly, and the explosivity of that eruption had negligible effect on the actual population. The VEI scale is a great scale for scientific purposes, easy to understand and easy to apply but when it comes to the effects that is where it’s reliability breaks down somewhat. For instance a VEI 7 can cause less direct death and damage than a VEI 3.

My VSI scale will have it’s focus completely dedicated to the consequences of eruptions, and will serve as an excellent way of estimating the scenarios born from volcanoes. What does it mean? VSI stands for Volcanic Societal Impact, and it’s focus is dedicated solely to direct volcanic impacts, so climate consequences are not a factor. Climate effects of larger eruptions are chaotic and unpredictable. When a large eruption takes place, by the time it takes to get a general idea of the volcanic winter, the eruption could have already ended. The damage done by a on-going eruption does not include subsequent climate effects (although it could be factored in for historic eruptions).

For the VSI scale, lahars, ashfall, tsunamis, landslides, shockwaves, gas emissions, glacial flood outbursts, and pyroclastic flows can all be factored in with relative ease. The two primary factors are range and impact. Both are split into 4 categories, Short range: 0-30 km away from the volcano, Medium range: 30-70 km, Long range: 70-100 km, and Distal: 100+ km. The impact categories are, None or negligible, Minor, Moderate, Severe, and Catastrophic. If climate consequences were to be factored in, they would go under the Distal range.

The scale goes from 0-12, but how would we discern whether an eruption is a VSI 1 or 12? We think about what impacts the eruption would cause over any of the ranges listed above. We convert the impact categories to the numbers 0, 0.5, 1, 2, or 3; we then add all the numbers to get where the eruption lies on the scale. For instance, the recent eruption of Fagradalsfjall would be VSI 0 as it has negligible consequences on all ranges, so 0+0+0+0=0 and a truly Catastrophic eruption be 3+3+3+3=12.

Now this is where the problem shows, the definition of a moderate, severe or catastrophic impact would vary depending on who is using the scale, which would lead to some conflict on where to place certain eruptions, the Enhanced Fujita scale has similar issues. Another issue is that the scale doesn’t tell you what the primary hazards are, a VSI 8 over land and VSI 8 over water will have different hazards. Another issue is that this scale focuses on impacts and with some good preparation, the impacts of a massive eruption can be reduced substantially and as such the eruption would rank lower on the scale then a smaller eruption that took place without good prep. The largest eruption of the 20th century, the eruption of Novarupta ranks at 1.5! Needless to say, the population and infrastructure surrounding the volcano plays a crucial role in finding out what it’s potential ranking is.

Now that I’ve explained the scale, I will rank some past eruptions and I know the perfect place to start!

The 1883 eruption of Krakatoa

The town of Anjer, after the eruption

This eruption is one of the most infamous in all of history, and it was the first disaster to grab global attention, so naturally it’s the perfect place to begin. The eruption produced several explosions that were more energetic than most nuclear weapons and while the cause of the explosions are fascinating, it doesn’t matter for the ranking. Most of the deaths were caused by the four massive tsunamis produced by the eruption. Each explosion also caused massive pyroclastic flows that reached distances of up to 40 km in some areas. This eruption is one of the deadliest in Human history killing more than 36,000 people. In Anjer alone, 7600 local inhabitants and 14 Europeans died.

This volcano ranks pretty high for a low end VEI 6. The Short and Medium range impacts were all Catastrophic due to the tsunamis and pyroclastic flows, Due to the tsunamis, the volcano produced severe Long range impacts, and the ash fall and the tsunami would’ve produced moderate distal impacts. So this eruption would a VSI 9 (3+3+2+1=9)

The June 1991 eruption of Pinatubo

The most recent VEI 6 eruption, this eruption was one of the most important in the history of volcanology. It was the first VEI 6 and volcanic winter to be analyzed with modern technology, and this event greatly broadened our understanding of large volcanic eruptions. The response to this eruption was a triumph, as the preparations prevented 1000s of deaths. Still, this volcano killed more than 800 people due to falling ash. This eruption did substantial damage to the Philippine economy and it produced lahars for years but consequences for the country were nowhere near as bad as it could have been. As such his volcano ranks a bit lower on the scale. Short and Medium range impacts were severe, long range and impacts were moderate, distal impacts were minor. So Pinatubo would be a VSI 5.5

The April 1815 eruption of Tambora

This eruption was one of the largest and deadliest of the past 4.000 years, killing more than 75,000 people directly and causing global famine. All life surrounding the volcano was destroyed by the massive pyroclastic flows. Despite being 10s of km away from the coast, the pyroclastic flows still displaced enough water to cause a moderate sized tsunami that would kill over 7,000 people, the ashfall was so severe that it plunged several major islands into famine and the extensive rafts of pumice would hinder sea travel for years.

The short and medium range impacts were catastrophic, with severe consequences for Long and Distal ranges so this eruption would be a VSI 10

There are several big candidates for VSI 12 eruption, some you already know about but there are some that don’t get talked about but that’s an article for another day and in fact I would like that series to be a joint venture.

With this scale we can have a better gauge of the societal effects of major eruptions but it’s not perfect. If you have any suggestions on how to make the scale better, I would like to hear them in the comment section. No scale can be perfect, since the scale makers aren’t perfect. Perfection should be sought after but never expected.


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