A network of geodetic markers for high-precision Global Positioning System (GPS) surveying was established in 2001 in the island of São Jorge, at the Azores triple junction, where the North America-Eurasia-Nubia plates meet. The aim was to monitor tectonic, volcanic, and landslide-induced surface deformation in an active tectono-volcanic region. The network consisted of 17 stations, and was occupied in 2001, 2004, and 2010. GPS observations from the three episodic campaigns were analyzed using the latest geophysical models and geodetic procedures to generate a velocity field for São Jorge Island. 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

This work was supported by Fundac ão para a Ciência e a Tecnologia (FCT) projects SHA-AZORES (PTDC/CTEGIX/108637/2008), DISPLAZOR (POCTI/1999/CTA/32444), and PEST-OE/CTE/LA0019-2011/2012.We thank the institutions that provided the receivers (IGeoE, IGP, IH, FCTUC, FCUP, and Estereofoto) and all survey participants, in particular Rita Rodrigues, Paulo Amaral, Paulo Sousa, and Gonc alo Prates. We thank Fernando Carrilho, from Instituto de Meteorologia, for providing the seismic catalogue for the 2001-2011 period. We thank Zuheir Altamimi for providing information for the ITRF2008-PMM and Donald Argus for the ITRFVEL- M model. We very much appreciate a thorough review by Donald Argus, which helped much to improve the manuscript. Carefully reviews by Rui Fernandes, an anonymous reviewer, and the Editor are also much appreciated

12 pages, 9 figures, 1 table

Peer reviewed