The Azores Plateau (Portugal), in the North Atlantic Ocean, was formed by the complex geodynamic setting of the tectonic triple junction between the American, Eurasian and Nubian plates. The Azores islands are situated along the Terceira Rift, the third branch of the triple junction. The obliquity of this segment of the Eurasian–African boundary results in differential rates of seafloor expansion, as well as intense volcanic and seismic activities. Between 2019 and 2021, a dense temporary seismic deployment in Terceira Island allowed a significant increase in detection capability and spatial coverage. From the collected dataset, we obtained the first local earthquake tomography 3D model of the island's crust. We also analyzed shear wave splitting and focal mechanisms to obtain information on crustal stress. Two major features were imaged, beneath the main volcanic structures of the island: Santa Bárbara volcano to the west, and the couple Guilherme Moniz-Pico Alto volcanoes in central Terceira. They both exhibit high Vp/Vs values and a roughly North-South segmentation, characterised by distinct Vp values and seismic activity. Seismicity also tends to be concentrated along the borders of these features, which are interpreted as corresponding to the plumbing system or magmatic chamber that feeds the volcanoes. A very shallow high Vp area, coincident with a volcanic cone alignment, marks the inland surface expression of the Terceira Rift, the Vp/Vs depth pattern suggests that the main feeder is Santa Bárbara volcano. Focal mechanisms and shear wave splitting point to a radial stress pattern around Santa Bárbara volcano, but a more complex pattern in the Guilherme Moniz-Pico Alto volcanoes. Although some fluctuations occur, the orientation of the fast polarisation direction and the SHmax tend to mark a SW-NE direction, ~90° to the regional orientation, indicating the transition in the Terceira Rift orientation from NW-SE to the WNW-ESE. • A temporary 29-station seismic network allowed a high-resolution study of the crust beneath Terceira Island, Azores. • Combined seismic methods provided new insights of the crust beneath Terceira Island, Azores. • First 3D seismic velocity model obtained through local earthquake tomography on Terceira Island. • First shear wave splitting analysis (SWS) performed on Terceira Island. • Low-magnitude inland seismicity reveals clustering around major volcanic structures. • High Vp beneath main volcanoes interpreted as partially crystallized magmatic intrusions, possibly magmatic chambers. • Shear-wave splitting reveals local stress rotation influenced by volcano-rift interactions.
Dias et al. (Sun,) studied this question.