The Sete Cidades region is dominated by a central volcano with caldera and has been affected by geological hazards throughout time, namely landslides, earthquakes and volcanic eruptions. To study the impacts of this kind of natural phenomena it was necessary to join information regarding earlier occurrences from both historical records and recent data.
In view of the accounts of historical submarine eruptions offshore of Sete Cidades area it is possible to anticipate the occurrence of significant seismic activity and the development of landslides associated to the onset of any volcanic eruption. As a result landslide susceptibility and earthquake vulnerability of dwellings were considered as critical for a pre-eruptive scenario.
Heavy rainfall periods, earthquakes or volcanic eruptions may trigger landslides in Sete Cidades region. Considering the different factors involved in the generation of these events and based in landslide dispersal mapping, lithology and slope steepness the areas exposed to this hazard were identified. It was possible to verify that in the study area about 73% of the mapped landslides took place in areas where pyroclastic fallout deposits are the dominant outcropping lithology and more than 77% occurred in areas where slope steepness equals or exceeds 20º. Adding the information about the distribution of inhabited areas, it is possible to evaluate that 37% of all dwellings were built in areas of high or very high landslide susceptibility.
Sete Cidades region was not affected in the last 150 years by seismic events reaching intensity degrees higher than VII (EMS-98), the maximum historical intensity assigned reached degree IX (EMS-98) (Silveira, 2002) in result of seismic swarms associated with volcanic eruptions.
In order to study the vulnerability of buildings to seismic events the dwellings were classified in different vulnerability classes (A to E). 75% of dwellings were classified as belonging to class A, the most vulnerable one. Taking into account the damage scale of EMS-98 and considering that the Sete Cidades region may again be affected by an earthquake or earthquake swarms that equals intensity IX (EMS-98), between 65% up to 80% of the dwellings are expected to collapse and the remaining ones (13% to 19%) shall be seriously damaged. This implies that 84% up to 93% of the population living in this area will need to be rehoused.
Considering the eruptive history of Sete Cidades Volcano three different eruptive scenarios may be expected: (1) a surtseyan eruption offshore Sete Cidades volcano; (2) a strombolian type event with associated lava flows on the flanks of the volcano; and (3) an intracaldera hydromagmatic event. The impact resulting from some of these volcanic hazards is presently under evaluation.