The Azores region comprises a zone of anomalously shallow topography, often related with the presence of a mantle plume, lying in the triple junction between North American (NA), Eurasian (EU) and Nubian (NU) tectonic plates. The location of the Mid-Atlantic Ridge (MAR) is well defined, characterizing the boundary between the NA and EU/NU plates. In contrast the boundary between EU and NU is more complex in this western segment of the Azores-Gibraltar Fracture Zone, showing evidences of a transtensional regime, in which the deformation is accommodated by a large number of active faults, along which significant seismic and volcanic activity occur. This segment is named Terceira Rift (s.l.) by several authors.
Bathymetric, seismic and tectonic data show that the Terceira Rift is controlled by two sets of conjugated faults with oblique motion: the normal dextral system trending WNW-ESE to NW-SE and the normal left lateral system trending NNW-SSE. These data are compatible with a clockwise rotation of the stress field from the northwestern domain towards the southern one. For this reason, the former models consider that this deformation area acts like a transfer zone between the MAR and the dextral Gloria Fault, as it accommodates the differential shear between the EU and NU plates.
S. Miguel Island is located in the southeastern part of the Terceira Rift. Along its history it has been affected by several earthquakes and seismic/volcanic swarms that caused a high number of casualties and severe damages. Several tectonic studies have been carried on in order to recognize the main volcano-tectonic structures and characterize the present-day deformation pattern of this area.
In the eastern and older part of S. Miguel Island, the Povoação-Nordeste region, three groups of faults with different directions were identified: the dominant ones have NW-SE to WNW-ESE directions, while the less representative ones have NNW-SSE and NE-SW directions. The kinematical data reveal two distinct groups of conjugated faults with oblique motion, indicating the presence of two different stress fields separated in time or in space: WNW-ESE normal dextral faults conjugated with NNW-SSE normal left lateral faults and WNW-ESE to NW-SE normal left lateral faults conjugated with NE-SW normal dextral faults.
The first conjugated fault set is in agreement with the present transtensional stress field proposed to Azores region, with a maximum horizontal compressive stress axis (σ1) trending NW-SE, a maximum horizontal tensile stress axis (σ3) in the NE-SW direction and a intermediate vertical compressive stress axis, generating strike slip faults, which may alternate, by permutation between σ1 and σ2, with normal faulting.
The second fault set is incompatible with the former one, indicating horizontal σ1 in the E-W direction, horizontal σ3 trending N-S and a vertical compressive σ2 that can permute with σ1.
The temporal relation between these two stress fields is yet to be established. As such, the study has been enlarged to the entire S. Miguel Island, to get more information on the local volcano related and regional stress fields. To obtain more data paleoseismicity studies are being performed on faults with recent surface rupture. The expected results and the Azores seismicity catalogue will contribute to determine deformation patterns, main seismogenic zones and to evaluate the seismic risk of the S. Miguel Island.