Site Characterization of São Sebastião Volcanic Crater Using Geophysical Tests

The Azores archipelago is located at the North America, Eurasia and Africa triple junction showing high seismicity and volcanism typical of an active plate boundary. Terceira Island has been affected by several damaging earthquakes and the 1980 January 1st earthquake (M=7,2) was the strongest in Azores during the last century. The São Sebastião volcanic crater, located at SE of Terceira Island, is characterized by an amplification of ground movements inside the crater with respect to the surroundings. Inside this volcanic crater, is located the village of São Sebastião that has shown an anomalous seismic behaviour in a more local expression during the past earthquakes. The local geology and geotechnical characteristics of the soil deposit play a strong role in the seismic ground shaking at the site. Some previous geophysical studies with information merely qualitative were not able to perform a rigorous and well-defined model of the crater. A recent survey added new information that allows a better comprehension of the damage distribution, since it indicates inside the crater a basaltic lava layer controls the seismic behaviour of that area, serving as a protection to the buildings in the SW region of the crater.