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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

MARTINI, F., VIVEIROS, F., BEAN, C., CHIODINI, G., SACCOROTTI, G. (2008) – The role played by rainfall in seismicity patterns at Fogo volcano inferred by multiplets analysis and modeling. IAVCEI 2008 General Assembly, Reykjavik, 17 - 25 de Agosto (Poster).


Fogo is an active stratovolcano with a lake filled caldera, in the central part of S. Miguel Island, Azores. The most recent eruptions (a plinian intracaldera followed by a basaltic flank eruption) occurred in 1563 A.D. ; current activity is limited to hydrothermal manifestations such as active fumarolic fields, thermal and CO2 cold springs and soil diffuse degassing areas. It is affected by important active tectonic structures, with high seismic activity and practically continuous micro-seismicity, concentrated in very frequent swarms.


In this study, repeating events were identified within a catalogue of volcano- tectonic events recorded in 2003-2004 by stations around the Fogo volcano edifice and analyzed through a cross-correlation based technique. The results show a seasonal cycle of the seismic velocity patterns and exhibit an excellent correlation with the rainfall. A seasonal effect can also be broadly seen in the seismicity occurrence: some of the swarms recorded over the twoyear period occur in temporal and spatial association with higher than average rainfall.


These observations seem to point to the important role played by rainfall in the seismicity patterns in the Fogo area. Observed immediate response of the seismic velocity changes to the rain and lack of hypocenter migration and of main shock aftershock sequence exclude a process of fault lubrication but suggest that pressurization instead plays an important role in triggering the seismicity. The mechanism is more likely related to pressure changes at depth in response to surface rain and/or the interaction of the rainwater with the geothermal system. Interestingly, previous studies identified the geothermal fluids around Fogo massif as derived from meteoric water, which infiltrates through Fogo Lake and the volcano flanks. We model the effect of rainfall on hydrothermal fluid circulation and diffuse soil degassing with the TOUGH2 simulator, accounting for two-phase, two-component fluids (H2O and CO2) through a homogeneous porous matrix. The aim is to quantify consequent pressure variation at depth, and evaluate if those could be triggering the seismicity, at least when the system is critically stressed and in periods when a magmatic origin for the seismicity is most likely negligible.