The seismogenic area embracing Fogo and Congro volcanoes, on the central part of Sao Miguel Island, is characterised by a highly concentrated seismic activity, which increased significantly since the end of 2002 clustered in space and time, along the north-eastern flank of Fogo volcano. However the nature of the seismic source remains unclear: is it a pure tectonic source triggered by the main tectonic structures associated to the volcanic edifice, or is it magmatic/hydrothermal fluid driven?
In the framework of the European project VOLUME, the waveforms were analysed and cross-correlation of first arrivals for P and S waves was performed. About 3000 events were relocated using a double-difference approach, and 7 different swarms were found, 3 of which with more than 140 events. The swarms had been active throughout the entire time span, with significant increases in the activity during April and November 2003 and in January and October 2004. Correlation values rise up to values close to 1 and the events could be correlated to three subparallel features in depth levels ranging from 5 to 10 km.
Focal mechanisms were calculated for six of the seven swarms, using the FOCMEC software, based on the P waves polarities and SH/P amplitude ratios, for which a maximum of one erroneous polarity and one erroneous amplitude ratios were admitted. The directions obtained with the focal mechanisms are, in general, coincident with the tectonic alignments considered for the Fogo Volcano and Congro areas.