An initial volcanic hazard assessment of the Vestmannaeyjar Volcanic System: Impacts of lava flow and tephra deposit on Heimaey

Vedurstofa Islands

This work is the first step of a comprehensive long-term volcanic hazard assessment of the
Vestmannaeyjar volcanic system focusing on critical infrastructure on Heimaey and is suitable
for long-term planning during the quiescent time between eruptions. It can be further developed
after more geologic mapping and dating of deposits has been made to include more knowledge
of the eruptive history and the magmatic system and be refined after acceptable volcanic risk
in Iceland has been defined. Probabilistic modeling of lava flows and tephra fall from potential
future eruption scenarios is used to analyze how different eruption conditions produce hazards
of different sizes from various locations. The impacts of the hazards on the community are
quantified by considering their potential to destroy critical infrastructure and the resulting
potential economic damage in the absence of mitigation actions. Eruption frequency for the
Vestmannaeyjar volcanic system is low relative to the frequently active volcanoes of the
Eastern Volcanic Zone. In the case of an eruption within the whole system there is only a
3–8% likelihood that a vent will open on Heimaey. In other words, there is a 92–97% likelihood
that the next eruption within the volcanic system will not be on Heimaey. The most densely
populated parts of Heimaey in the north and around the harbor are the most vulnerable to
Moderate and Large lava flows originating on the island. Almost all infrastructure on the island
is vulnerable to lava inundation from a Large eruption originating anywhere on the island. Half
of the Heimaey residence roofs are at risk of collapsing due to tephra load within six days of
some Moderate sized eruptions only if an eruption occurs on or close to Heimaey; if the winds
promote transport over the island; and if the tephra is wet (as precipitation adds to the mass).
Mitigation actions have proved to be extremely beneficial and the experiences from the 1973
eruption should be relied on to help with future actions. Pre-eruption mitigation can occur in a
variety of ways including Civil Protection contingency planning, building up specialist
knowledge and within vulnerable communities and via land-use planning.