Fogo volcano is a polygenetic volcano located in the central part of S. Miguel Island (Azores archipelago). From 2002 to 2005 several seismic swarms took place in the area involving Fogo and Congro volcanoes. The visible hydrothermal manifestations are all found in the northern flank of Fogo volcano and comprise low temperature fumarolic fields, thermal and CO2 cold springs.
Mapping soil CO2 degassing is relevant not only for the seismovolcanic monitoring but also for public health risk assessment as the maps obtained should be used on land-use planning. A total of 2647 soil CO2 measurements were performed in an area with 240 km2 in order to define the soil diffuse degassing structures. Soil temperature was also measured in almost all the surveyed points. Soil CO2 concentration values oscillated between 0 and 99.2% vol. and soil CO2 efflux values varied from a minimum of 0 to a maximum value of 11203 g/m2/d. Soil temperature at 20 cm depth ranged from 11.9 ºC to 98.6 ºC.
The most significant soil diffuse degassing areas are correlated with the three main fumarolic fields and with Lombadas CO2 cold spring. Inside Fogo caldera the CO2 values are quite low. The more significant degassing anomalies at Congro volcanic system are located in the area around Congro lake and in the vicinity of Furnas central volcano. The soil temperature anomalies come out at the fumarolic fields and no temperature anomaly is observed at Congro volcanic system.
Several degassing lineaments observed in the surveyed area are correlated with some tectonic structures that cross these volcanic systems. Two tectonic lineaments have been inferred from the degassing structures. The main seismogenic areas are coincident with an almost complete absence of soil diffuse degassing. A possible explanation may be the presence of a buffer structure at depth that not allows the gas to arise on the surface, as could be for instance some secondary rock alteration that sealed the fractures and changed the permeability on that area.