A multi-scale risk assessment for tephra fallout and airborne concentration from multiple Icelandic volcanoes – Part 1: Hazard assessment

In order to assist the elaboration of proactive measures for the management of future volcanic eruptions in Iceland, we developed a new scenario-based approach to assess the hazard associated with tephra dispersal and sedimentation at various scales and for multiple sources The target volcanoes are Hekla, Katla, Eyjafjallajökull and Askja, se- lected either for their high probabilities of eruption and/or their high potential impact. By coupling tephrostratigraphic studies, probabilistic techniques and modelling, we devel- oped comprehensive eruption scenarios for both short- and long-lasting eruptions and compiled hazard maps for tephra ground deposition at a national scale and air concentration at a European scale using the TEPHRA2 and FALL3D models, respectively. New algorithms for the identification of realis- tic sets of eruptive source parameters are investigated, which assist the generation of probability density functions of erup- tion source parameters for the selected scenarios. Aggrega- tion processes were accounted for using various empirical models. Outcomes, i.e. probabilities conditioned to the oc- currence of an eruption, help the assessment and compar- ison of hazard levels at different scales. For example, at a national scale Askja has a 5–10% probability of blanketing the easternmost half of the country with a tephra accumula- tion of at least 1 kgm−2. At a continental scale, Katla has a 5–10% probability of producing ash clouds with concen- trations of 2mgm−3 over the UK, Scandinavia and north- ern Europe with a mean arrival time of 48–72 h and a mean persistence time of 6–18 h. In a companion paper, Scaini et al. (2014) present a vulnerability assessment for Iceland to ground deposition of tephra and for the European air traf- fic to airborne ash which, combined with the outcomes of the present paper, constitute one of the first comprehensive multi-scale risk assessment associated with tephra dispersal and sedimentation