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

A network of geodetic markers for high-precision Global Positioning System (GPS) surveying was established in 2001 in the island of Sao 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 Sao Jorge Island. Surface velocities estimated at 15 inland locations reveal that Sao 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 degrees +/- 6 degrees while the SE sector is moving at a rate of 1.4 +/- 0.3 mm/yr to N109 degrees +/- 11 degrees, 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. (C) 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.