Analysis of volcanic threat from Nisyros Island, Greece, with implications for aviation and population exposure

Abstract. Nisyros island in the South Aegean volcanic arc, Greece, is a Quaternary composite volcano with a 3.8 km wide caldera that in 1996 entered a volcano-seismic crisis, which heralded the islands' return to a state of unrest. The caldera has been the locus of at least thirteen phreatic eruptions in historical times, the most recent in 1888, and the system is still presently affected by considerable hydrothermal activity. Although the recent unrest waned off without eruption, there are still open questions relating to the current threat of volcanic activity from the island. Here, we perform a detailed and systematic assessment of the volcanic threat of Nisyros using a threat analysis protocol established as part of the USGS National Volcano Early Warning System (NVEWS). The evaluation involves a methodical assessment of fifteen hazard and exposure factors, and is based on a score system, whereby the higher the score, the higher the threat is. Uncertainty in assessment criteria are expressed by allowing for a conservative and an extreme score for each factor. We draw our analysis from published data as well as from results of our research on Nisyros over the past years. Our analysis yields a conservative threat score of 163 and an extreme score of 262. The most adverse exposure factors include significant scores relating to aviation and population exposure to volcanic hazards from Nisyros. When looked at in comparison to US volcanoes both scores place Nisyros in the "Very High Threat (VHT)" category, grouping it with volcanoes such as Redoubt, Mount Ranier and Crater Lake. We identify a short-fall in recommended surveillance efforts for VHT volcanoes given existing monitoring capabilities on the island. We discuss potential pitfalls of applying the NVEWS scheme to Nisyros and suggest potential adaptation of analysis scheme to match industrial and societal conditions in Europe. At the same time, our findings indicate that that volcanic threat posed by Nisyros volcano may currently be underestimated.