Volcanic hazards related to N-S Poás-Río Cuarto-Hule volcanic complexes (Costa Rica)

ALVARADO INDUNI, G.; SALANI, F. M.

Pucon

Congreso; General Assembly 2004 IAVCEI; 2004

IAVCEI

A volcanological study in the north side of the Costa Rica Central Volcanic Range, near the Poás- Río Cuarto-Hule complexes was carried out in order to assess the volcanic hazards and risk for the construction of two hydroelectric projects. The Poás, is an andesite shield volcano with three main volcanic foci: Botos cone (active 7000 years ago), the main active crater and the Von Frantzius cone (active 40 000 years B.P), built up upon an older volcanic massif (PaleoPoás) all of them controlled byregional N-S tectonic features. Different minor centres are aligned along the same direction: towards the south of the main crater the Sabana Redonda pyroclastic cones (actives 38 000 years ago) and towards the north of the main crater the Congo volcano (active 6000 years B. P.), Hule maar (active 2800 years ago) and Río Cuarto maar (active a few thounsand years ago). Lava flows, lahar deposits and particularly pyroclastic rocks, 40 000 to 2000 years B.P. in age, are the common products related to these complexes. The tephrochronostratigraphy analysis and volcanic hazards maps point out that the projects are within middle to low hazard area. We estimate a 3% maximum risk for the hydroelectric construction although it could be less than 0.2%. The lapilli and ashfall would be the main hazards and eventually block fall during the largest eruptions. In view that the studied area has a tropical rainy climatic regime, slumps, debris flows and lahars should not be ruled out.