Abstract

São Miguel, the largest island of the Azores, corresponds to one of the most seismically active areas within the archipelago. During historical times it was affected at least six times by destructive earthquakes, which were responsible for thousands of deaths. Several seismic crises are related both to volcanic eruptions and unrest episodes, being the cause of significant damage over more restricted areas. The central region of the island has been the most active in recent years and is characterized by the occurrence of seismic swarms, generally with low-magnitude events (M_D≤2), that are clustered both spatially and temporally. A significant relevant seismic swarm occurred between May and September 2005 and was associated with an episode of volcanic unrest. Another part of the seismicity recorded on São Miguel is associated with submarine volcano-tectonic structures that are located near the island, some of which are the source of the strongest earthquakes. The spatial distribution of b-values shows that high values (i.e. b>1) are mainly associated with the Fogo–Congro region, whereas low b-values (i.e. <1) are more common offshore. The spatial distribution of b-values in the Fogo–Congro region is suggestive of the existence of material and/or stress heterogeneities, and this is supported by a mixture of fault plane solutions.