Reassessment of the historical seismic activity with major impact on S. Miguel Island (Azores)

Abstract

On account of its tectonic setting, both seismic and volcanic events are frequent in the Azores archipelago. During the historical period earthquakes and seismic swarms of tectonic and/or volcanic origin have struck S. Miguel Island causing a significant number of casualties and severe damages. The information present in historical records made possible a new macroseismic analysis of these major events using the European Macroseismic Scale-1998 (EMS-98). Among the strongest earthquakes of tectonic origin that affected S. Miguel Island, six events were selected for this study. The isoseismal maps drawn for these events enabled the identification of areas characterized by anomalous values of seismic intensity, either positive or negative, to constrain epicentre locations and to identify some new seismogenic areas. Regarding seismic activity associated with volcanic phenomena six cases were also selected. For each of the studied cases cumulative intensity values were assessed for each locality. The distribution of local intensity values shows that the effects are not homogeneous within a certain distance from the eruptive centre, the area of major impacts relates with the eruptive style and damages equivalent to high intensities may occur in Furnas and Sete Cidades calderas. Combining all the historical macroseismic data, a maximum intensity map was produced for S. Miguel Island.