Ribeira Seca parish is located in the northern coast of S. Miguel island near the city of Ribeira Quente. On the 16th of April 1997 local people observed some vapour rising in the garden of a residence and alerted the Civil Protection about the occurrence.
In order to characterise this phenomena temperature and CO2 soil profiles were performed. At that time the weather was rainy and air temperatures rounded 19°C. The obtained results showed the existence of a thermal and geochemical anomaly affecting an area of approximately 250 m2 and extending along a WNW-ESE direction in the gardens of two residences. Measured soil temperatures reached 50°C while the maximum CO2 found 40 cm below the ground surface was about 13.5%. CO2 values slightly above the admissible limits for public health were detected in a narrow zone at the centre of the anomaly and protective measures were adopted to avoid the exposition of people and animals. Inside the residences the air temperature and composition didn't show any anomalous values.
During 1997 regular observations were made and field mapping carried out on July and September 1997 revealed that the anomaly pattern remained unchanged. However, on the 14th of March 1998 local people alerted for the increase of temperatures inside houses and some inhabitants reported symptoms of muscular weakness and headaches. Four families were immediately evacuated and their houses were used to develop a continuous monitoring programme. Obtained data showed that the air temperatures inside closed rooms attained 30-40°C while CO2 concentration varied between 1 and 8% in less than one hour. External factors like precipitation and wind showed an important influence in the results.
Temperature and CO2 soil profiles developed in the zone revealed the enlargement of the previously mapped anomaly, now affecting an area of about 10.000 m2 and trending NW-SE. Maximum temperatures of 50°C and CO2 levels of 40% were detected at that time. The increase of this anomaly is believed to be related with the seismic activity registered during Marsh in the north flank of Fogo Volcano, where the NW-SE Ribeira Grande graben is well expressed.