Calculating threat

Within the country profiles (Appendix B) individual volcanoes are ranked by risk; however, it would also be beneficial to understand the total volcanic threat borne by each country. We therefore develop two measures of volcanic threat2 to enable country ranking. The measures variously combine the number of volcanoes in the country, the size of the total population living within 30 km of volcanoes and the mean hazard score, which is calculated for each country from the relevant volcano hazard scores (VHI). We develop and use a ‘Pop30’ score, which calculates the number of persons, using Landscan 2011 (Bright et al., 2012) data, within a given country living within 30 km of one or more volcanoes with known or suspected Holocene activity. Note that 30 km is chosen as most fatal incidents that are caused directly by volcanic hazards fall within this distance of volcanoes [see Chapter 4]. VPI30, supplied by VOTW4.0 (Siebert et al., 2010) based on the analysis of Ewert & Harpel (2004) and Siebert et al. (2008), is specific to a volcano and thus cannot be used in place of Pop30 as this would double count persons living within 30 km of neighbouring volcanoes.

We first develop a simple measure of volcanic threat to life country by country based on the number of active volcanoes, an estimate of exposed population and the mean hazard index of the volcanoes. The sum of this measure (Measure 1) for all countries is itself a simple measure of total threat and so the distribution of threat between countries can be evaluated and they can be placed in rank order using a normalised version of Measure 1. However, this measure of threat distribution can be misleading because an individual country may vary considerably in the proportion of its population that is exposed to the volcanic threat. Volcanic threat is very much higher in relation to its economy and population in a small island nation with an active volcano than in larger countries even if they have many volcanoes. Nation states vary greatly in their populations from, for example, China with 1.3 billion people (<1% exposed) to St. Kitts and Nevis in the Caribbean with only 54,000 people (100% exposed). Thus we need a measure of threat that reflects its importance to each country.