Present-day deformation in São Jorge Island, Azores, from episodic GPS measurements (2001-2011)

Surface velocities estimated at 15 inland locations reveal that São Jorge is moving neither with Eurasia nor with Nubia and presents two sectors with different behavior. The NW sector of the island is moving at a rate of 2.2 ± 0.3 mm/yr to N82° ± 6° while the SE sector is moving at a rate of 1.4 ± 0.3 mm/yr to N109° ± 11°, when compared to the velocity of Nubia predicted by ITRF2008-PMM. This velocity field cannot be explained by slip along the active faults that cross the island, nor is compatible with volcanic deformation on the active volcanic rift zones. It is suggested that the velocity differences that are measured represent deformation related to local sub-surficial magmatic/volcanic processes occurring near the island. This intra-island deformation may also be related to the stress field and seafloor spreading occurring in an area situated on the western sector of the Azores Plateau, reflecting the presence of WNW-ESE volcanic structures, along which magma intrusion produces NE-SW dilatation, bounded to the East by a NE-SW trending structure, probably of tectonic nature, working as a dextral transfer zone.