Fogo volcano (Sao Miguel, Azores): a hazardous edifice

Abstract

Fogo volcano, the largest of the three active volcanoes of São Miguel Island in the Azores, presents a range of hazards which are related to non-eruptive processes. Often termed indirect volcano hazards, these are the focus of the present paper and on Fogo volcano are produced by interactions between the unstable volcanic edifice and processes controlled by seismic, hydrothermal, slope instability and hydrological processes. Many houses, roads and bridges are at risk should significant earthquake activity occur. Since the island was settled in the 15th century, earthquakes exceeding IX on the European Macroseismic Scale (i.e. EMS 98) have struck São Miguel in 1522, 1713, 1811 and 1935. An estimated 45,000 people live within the Fogo District and, without action to reduce vulnerability, future losses are inevitable. Gases are emitted from several locations on Fogo, and CO2 is a dangerous gas when it ponds in depressions. Concentrations of over 15% often lead to asphyxiation and death, and case studies of the hazards posed by gas discharge are presented. São Miguel has been affected by several destructive landslides and flash floods in the last five centuries, triggered by earthquakes, volcanic eruptions or periods of heavy rainfall. A large (X, EMS 98) earthquake in October 1522 generated a debris flow that buried and completely destroyed the town of Vila Franca do Campo. About 5,000 people were killed and an area of ~4.5 km2 was covered by millions of cubic metres of debris. Indirect volcanic hazards are an ever present threat to people living on the Fogo Volcano, yet until recently they have been little researched.