From our sample of 194 eruptions we find that about 80% of VALs fall into the categories `Missed’ (Category 1; 76 eruptions, 39% ) and `Too Late’ (Category 2; 76 eruptions, 39% ), about 14% as `Almost’ (28 eruptions), and only about 5% can be classified as `Timely’ (10 eruptions; 5% ) (Figure 2A). For the quantification of category `UwE’ we used the percentage of the number of `UwE’ from the total number of issued alerts (therefore excluding `Missed’). We found that overall 33% of alerts issued were `UwE’ (Figure 2B).
If we group the categories of `Timely’ and `Almost’ we find that only 19% of the alert levels were increased by any amount before the onset of eruption. We were surprised by this low percentage. Below we investigate what may be controlling the different categories by analyzing the results according to different parameters. First we look at the volcanic system: volcano type and eruption styles. Later we address the technical and societal factors that may be involved, including; the monitoring capacity, the effect of the number of eruptions within a system and the population at risk.
Volcano and eruption styles
We divided the eruptions according to the VEI (as assigned by the GVP) and found that in general there is an increase in the number of Category 5 (`Timely’) VALs as the eruption size increases (Figure 3). For VEIs ≤ 2 there is a high percentage of `Missed’ and `Too Late’ VALs (42% and 39% , respectively) and of the 154 eruptions, we see only 3% of the VALs in the `Timely’ category for these small eruptions. As the eruption size increases to VEI 3 the VAL’s in the `Missed’ category decrease (26% ), but those in the `Too Late’ increase (41% ). There is also an increase in the percentage of `Timely’ (12% ) and `Almost’ for eruptions of VEI’s < 2 to VEI 3. For VEI ≥4 eruptions the percentages in the `Missed’, `Too Late’ and `Timely’ categories are about the same (33% ), but the sample size (n = 6) is too small for statistical significance.
We also looked at the character of the volcano to determine if this would have any effect on the issuance of VALs. For this, we simplified our classification into: (1) Open-vent volcanoes that produce many frequent, small eruptions. They are often continuously degassing and tend to have more effusive eruption products. This continuous release of gas seems to make large eruptions less likely, unless the system seals itself (Newhall [2007]), (2) Closed-vent volcanoes have less frequent eruptions and are commonly dormant for decades, centuries, or longer. About 53% of the volcanoes in our database were classified as open-vent, and produced 76% of the eruptions, whereas the remaining 47% are closed-vent, and produced 24% of all eruptions. We found a higher percentage of `Timely’ and `Almost’ VALs (10% and 19% , respectively) in closed-vent (Figure 4) than in open-vent systems (3% and 12%, respectively). Open-vent systems also show a higher number of `Missed’ events (43% ) than closed-vent systems (22%). Finally the proportion of `UwE’ at open-vent volcanoes is 28% of the total number of alerts, but this proportion increases to 42% at closed-vent volcanoes.
Role of monitoring networks and equipment
We found that there is an increase of the `Timely’ and `Almost’ categories with the increase in the capacity of the monitoring network (Figure 5). Volcanoes with monitoring Levels 0 and 1 have a high number of `Missed’ and `Too Late’ VALs (88% and 80% , respectively for 13 total events). For volcanoes with Levels 2 and 3, we find that there is an increase in the `Timely’ and `Almost’ VALs from 14 (Level 2) to 43% (Level 3). The value of the `Almost’ category for monitoring Level 4 still increases to about 50% , although given that we only have 4 volcanoes in this category, it is not clear if the increase is statistically significant. At volcanoes with low monitoring levels, the `UwE’ alerts are a very high percentage of total alerts issued (72% for Level 0 monitoring) and then broadly decrease as the monitoring level increases (33% for Level 4 monitoring).
Effect of eruption frequency on the issuance of VALs
To test whether an increased knowledge of the system (e.g. a given volcano) increases the reliability of the issuance of VALs, we looked for an increase in the number of `Timely’ and `Almost’ VAL categories with an increase in the number of eruptions at a given volcano. In our sample, 34 volcanoes had more than one eruption over the time period and this filter still includes 161 eruptions. We found a large spread in data and no relationship between the number of eruptions per volcano and the percentage of `Timely’ and `Almost’ VAL categories (Figure 6). However, some pattern emerges if we consider frequency of eruption and level of monitoring. Volcanoes with frequent eruptions (mostly, open vent volcanoes) with the highest percentages of appropriate alert levels are those with monitoring levels 2 and 3 (Figure 7). The lowest percentages of appropriate VALs are for volcanoes that have fewer eruptions (mostly, closed-vent volcanoes), and a moderately high level of monitoring (level 2 or higher).
Effect of population on VALs
The purpose of VALs is primarily to inform local populations (and, in some cases, air traffic) of the potential for volcanic hazard. Local authorities also use them to inform decision making for evacuations. We checked to see if the size of population living around a given volcano affects the issuance of VALs. We used the population figures from the Smithsonian GVP, which were obtained from the LandScan 2007 data (Oak Ridge National Laboratory [2008]). We considered the population size within 30 km of the volcano, and normalized the data by the number of volcanoes within each population bin size (Figure 8A), to account for variable number of volcanoes per bin. We found that there are more `Timely’ alerts in the bin with the highest population, but in general there are similar proportions (within a few percentage points) for the `Timely’ and `Almost’ categories for all population sizes (Figure 8A), meaning that there is no significant correlation between appropriate VALs and population size. The proportion of `UwE’ varies between about 24 and 48% for all population sizes, without a clear change with population. As an additional measure to test the influence of the population on the decision to change VALs, we compared data from two countries with very different demographics. Country A has a large population living within 30 km of volcanoes, whereas Country B has much lower numbers of people (on average less than a 100) living close to its volcanic centers.
For Country A we found that for volcanoes with populations of less than 1 million people about 21% of the VALs fall into the `Timely’ or `Almost’ (Figure 8B), but this increases to 45% for volcanoes with more than 1 million people, so there is an improvement with population size. The highest proportion of `Missed’ events occurs within the population range of 1,000 – 10,000. The `UwE’ show a somewhat bi-modal distribution for Country A with peaks for the lowest (1,000 – 10,000) and the highest (>1,000,000) populations. In contrast, for Country B with sparsely populated volcanoes (less than 2,500 people living within a 30 km radius of the volcanoes) we see that even in the lowest populations (less than 1000) there are higher proportions `Timely’ and `Almost’ VALs (Figure 8C) compared to the highly populated Country A. But there is also an increase in appropriate VAL’s with population size. There is no dominant trend in the `UwE’ alerts that can be attributed to differences in population. The general decrease in `UwE’ alerts for Country B is more likely a factor of the small sample size, rather than a true trend.
Has the issuance of VALs improved over time?
Since 1990 there has been an increase in the number of countries and institutes that have adopted a form of VAL, from one in 1990 to 60 in 2013 (Figure 9). To be able to compare the results from different years we have normalized the data by the number of volcanoes. We find that there is quite a large variety from year to year and not a linear increase of any of the categories with time (Figure 10). Many `Missed’ and `Too Late’ VALs occurred prior to 2000 while from 2004 onwards most years have higher `Timely’ and `Almost’ categories (in general > 20% ). To better test whether the VALs are improving with time, we performed a moving average analysis of the `Timely’ and `Almost’ categories (Figure 10) which shows a complex pattern but a general improvement after 2003.
We also compared the number of alerts and number of eruptions over time normalized by the number of volcanoes and some trends can be identified (Figure 11). We find that from 1996 to 2004 the normalized number of eruptions is higher than the number of alerts, which reflects that many eruptions occurred without any alert being issued. However, starting from 2005 the ratio of numbers of alerts to eruptions is similar or higher than one, so that on average at least one or more alert was issued per eruption, something that is also reflected in the decrease in `Missed’ events over this time period (Figure 10). Thus, in general we find that with time there is an increasing number of VALs issued per volcano and per eruption that could reflect an increase in the monitoring network or change in the scientific decision making process as discussed below.