Fogo Volcano is one of the three central volcanoes of S. Miguel Island (Azores). Fumarolic fields, thermal and CO2 cold springs and several soil diffuse degassing areas are the main hydrothermal manifestations in this volcanic system.
Since the end of 2002 several energetic seismic swarms occurred at Fogo Volcano and in the nearby Congro volcanic area reaching a maximum peak on September 2005, when thousand of low magnitude earthquakes were registered. At that time more than 200 events were felt in the central part of S. Miguel Island. The strongest earthquake occurred on September 20th with a magnitude (ML) of 4.3 and a maximum intensity of VI/VII (MMS). A few number of houses suffered minor damages and several landslides were triggered in the area.
In order to interpret the meaning of such increase of activity, a multidisciplinary monitoring programme was developed, including geophysical, geodetic and geochemical observations.
Gas geochemistry continuous monitoring started at Fogo Volcano in February 2002 with the installation of a permanent CO2 soil flux station (GFOG1) in the northern flank of the volcano being the mean CO2 flux value at this site around 600 g/m2/d. During May 2005 a second permanent CO2 flux station (GFOG2) was installed inside Fogo Volcano Caldera in an area where the flux values are around 7 g/m2/d. Despite the high number of earthquakes that affected the area no CO2 flux anomalies were observed.
In general, gas chemical analyses from fumarolic discharges didn’t show significant differences when compared with data obtained in previous campaigns. However, minor changes in the H2/CH4 ratio have been noticed in the Caldeira Velha fumarole, revealing a possible slight increase of pressure. Additionally to these studies, a regular sampling profile was defined along the main seismic area to measure CO2 flux, CO2 concentration and temperature. A CO2 concentration gas analyser, performing measurements every hour, was also installed in Monte Escuro seismic cave, in October 2005.
Despite the different gas monitoring methodologies applied during the 2005 Fogo-Congro seismic crisis no significant gas geochemistry anomalies were detected in the area.