Present-day deformation in Sa ˜o Jorge Island, Azores, from episodic GPS measurements (2001–2011) V.B. Mendes a,⇑ , J. Madeira a , A. Brum da Silveira a , A. Trota b , P. Elosegui c , J. Pagarete a a Faculdade de Cie ˆncias da Universidade de Lisboa, LATTEX/IDL – Laborato ´ rio Associado, Campo Grande, C8, Piso 2, 1749-016 Lisboa, Portugal b Universidade dos Ac ßores, Ponta Delgada, Ac ßores, Portugal c Institute for Space Sciences (ICE) and Institute of Marine Sciences (ICM), Spanish National Research Council (CSIC), Barcelona, Spain Available online 2 November 2012 Abstract A network of geodetic markers for high-precision Global Positioning System (GPS) surveying was established in 2001 in the island of Sa ˜o 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 Sa ˜o Jorge Island. Surface velocities estimated at 15 inland locations reveal that Sa ˜o 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. Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Azores triple junction; Sa ˜o Jorge Island; GPS; Tectonics; Volcanism 1. Introduction The Azores archipelago straddles the triple junction where the North America (NA), Eurasia (EU), and Nubia/Africa (NU) plates meet (Fig. 1). The islands are distributed in three groups. The western group (islands of Flores and Corvo) stands on the North America plate, west of the Mid-Atlantic Rift (MAR). The remaining seven islands form two groups, the central group (Graciosa, Terceira, Sa ˜o Jorge, Faial, and Pico) and the eastern group (Sa ˜o Miguel and Santa Maria, as well as the Formigas islets), and are located along the western sector of the Eurasia–Nubia plate boundary (Fig. 1). The tectonic regime in the area is characterized by dextral transtension in the Azorean sector of the EU–NU boundary, as a result of the faster eastward velocity and different spreading direction of EU relative to NU in this region (see inset in Fig. 1). The differential motion is absorbed on a wide shear zone characterized by active tectonics and volcanism (e.g. Lourenc ßo et al. (1998), Madeira and Brum da Silveira (2003)). The islands were formed by magmatic activity, which is spatially controlled by tectonics. As a result of this setting, the islands present frequent seismicity (Fig. 2), active faulting, and volcanism. For example, since the settlement of the Azores in mid-15th 0273-1177/$36.00 Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2012.10.019 ⇑ Corresponding author. Fax: +351 217500807. E-mail addresses: vmendes@fc.ul.pt (V.B. Mendes), jmadeira@fc.ul.pt (J. Madeira), antonio.brum@fc.ul.pt (A. Brum da Silveira), at@uac.pt (A. Trota), p.elosegui@csic.es (P. Elosegui), japagarete@fc.ul.pt (J. Pagarete). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research 51 (2013) 1581–1592