Fogo Island is an active stratovolcano located in the Cape Verde Archipelago (North Atlantic) which has a nearly conical shape that rises 2928 m above sea level and ~ 6000 m above the surrounding seafloor. The volcano shape is truncated by a caldera of 9 km diameter, resulted from a massive lateral collapse which happened between 123 and 62 ky ago. The Island has a population of 37.000 which lives both inside and outside the caldera. Since the 15th until the 20th century the island had an intense volcanic activity with an average interval between eruptions of about 20 years. However, in the 20th century this rate has decreased and only two flank eruptions occurred in 1952 and 1995. In addition, after two centuries of activity inside and outside the caldera (i.e., 16th and 18th centuries), since 1785 the eruptions occurred only through flank vents inside the caldera.
In the frame of WP3 of MIA-VITA project (i.e., Work Package 3: Monitoring of targeted active volcanoes), it was initially established to install two broadband seismic stations for Fogo volcano. However, due to a significant price reduction of the broadband seismometer proposed by Güralp Systems Limited, four instruments were purchased. It was opted for the CMG-3ESPCD model, which is the most suitable for volcano monitoring. Furthermore, a CMG-EAM instrument is being used as data Gateway. For the telemetry, it was purchased four pairs of FreeWave Ethernet radios, FGRplusRE model. The power system of the station is composed by an 80 W solar panel, a 10 A charge controller and two 55Ah batteries. The acquisition of the equipment was done by IST (i.e., Instituto Superior Tecnico, Portugal), which also participated in the set up of the network.
At about the same time of the approval of MIA-VITA project (August 2007), the Government of Cape Verde has also financed a project for the seismic network of Cape Verde, particularly for volcano monitoring. Thus, with this last funding, four more seismic stations where installed near Fogo Volcano, namely two in Fogo Island and the two others in the neighbour Island Brava.
In order to draw the geometry of this network, it was taken into account the three main directions of dikes emplacement in Fogo Island. More precisely, it was required to: 1- reduce the azimuthal gap; 2- reduce the rms misfit locations of the hypocentres; 3- ensure the focal mechanism computation in such case. Figure 1 shows the geometry of the network. The two seismic stations in Brava were chosen not only for the very frequent seismic activity present in that island, but also because they will improve the location of the very first and deep seismic events produced by early stage of magma rise (it is well know that the last magma storage in Fogo is at the depth of at least 13 km).
Fig. 1 Geographical distribution of the six seismic stations of Fogo Volcano Monitoring Network.
The data recorded by each seismic station is transmitted in real time to a repeater by Ethernet radios. There are two repeaters in Fogo (FGMA and FGAF) linked to the data centre STMT at Santiago island (Fig. 2), where there is a CMG-EAM server connected to Internet leased line with fixed IP. As a consequence, the data recorded by each station can be retrieved in real time from any place.
Fig. 2 Map of the telemetry network of the Fogo Volcano seismic monitoring network.
At the present time, data recorded by the network is analysed manually on a daily bases. The current recorded signals are just the result of hydrothermal activity, which tends to rise in this season due to the increase of air humidity. Automatic data analysis is being set up, namely the rsam technique is already running. It is forecasted to have soon both an automatic analysis in spectral domain and an automatic seismic detector.