Chapter 4 Earthquakes and volcanic eruptions in the Azores region: geodynamic implications from major historical events and instrumental seismicity

Since the settlement of the archipelago, in the fifteenth century, 31 destructive earthquakes and 28 volcanic eruptions have been registered in the Azores. Major earthquakes occurring in historical times have reached magnitudes >7, often triggering landslides and even small tsunamis. In the same period, subaerial volcanic eruptions have ranged from Hawaiian to sub-Plinian, sometimes with a hydromagmatic character, while submarine eruptions have been Azorean to Surtseyan in style. The temporal and spatial distributions of major historical events are presented and their impacts summarized. The instrumental seismic activity registered since 1980 is discussed taking into account the main volcano-tectonic structures. These seismological data allow us to improve the characterization of the present-day boundary between the Eurasia and Nubia lithospheric plates, herein defined as the East Azores Volcano-Tectonic System. The seismological data also suggest that the location of the Azores Triple Junction is to the west of Faial Island, at about 38 degrees 50' N, 30 degrees 25' W, in agreement with proposals made by other authors using aeromagnetic data. A natural seismic gap, centred in the Sao Jorge structural alignment, is recognized and is interpreted as a zone of stress accumulation with the potential to generate a high-magnitude earthquake similar to that of 1757.