A tale of two cities: why ashfall depth may not be the key variable in determining impacts on

STEWART, CAROL; WILSON, THOMAS; JOHNNY WARDMAN; HEATHER BICKERTON; GUSTAVO VILLAROSA; OUTES, VALERIA

Cities on Volcanoes 7

Congreso; Cities on Volcanoes 7; 2012

Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI)

Volcanic ash is the most widely-distributed product of explosive volcanic eruptions. Ashfall thickness (depth) has been the most common parameter used for assessing the intensity of ashfall damage. However, while this approach has been appropriate for relatively coarse level assessment, it tends to overlook other ash characteristics (eg grain size, surface chemistry, density) which may also influence or even control the level of impacts sustained. Site-specific factors such as climate, topography and pre-existing infrastructure vulnerability are also important. This complex interplay of factors is explored by comparing the May 2010 eruption of Pacaya volcano, Guatemala, which deposited 2-3 cm coarse black basaltic ash on Guatemala City (population ~1.1 million), located ~30 km NNE of the vent; and the June 2011 eruption of Puyehue-Cordón Caulle, Chile, which deposited 3-4.5 cm of medium to coarse pale grey to white rhyodacitic ashfall on the city of Bariloche, Argentina (population ~120,000), located 100 km SE of the vent. Despite both cities receiving similar thicknesses of ashfall, impacts on urban infrastructure were very different. This poster will present a comparison of impacts on critical infrastructure sectors (electricity, water supplies, wastewater management, transport networks) in both locations to identify other important variables influencing the nature and severity of impacts. This information may then be incorporated into ashfall hazard models for a more complete risk assessment.