Plate Boundary Deformation and Volcano Unrest at the Azores Triple Junction Determined From Continuous GPS Measurements, 2002–2017

Abstract

AbstractGround deformation in the Azores, at the triple junction between the Eurasian, Nubian, and North American plates, has been mapped with continuous Global Positioning System (GPS) geodetic measurements to improve tectonic motion estimates and for understanding volcanic unrest. We compute daily positions, spanning almost 17 years (2000–2017), from 18 continuous GPS stations. The GPS time‐series are analyzed by searching for discontinuities and periodic functions. Results show that Flores and Graciosa islands have displacements close to predicted North American and Eurasian plate motions, respectively, while São Miguel, Terceira, São Jorge, Faial and Pico islands have displacements in between predicted Eurasian and Nubian plate motions. The Eurasian‐Nubian plate boundary in the Azores behaves as a diffuse ultra‐slow oblique spreading center with focused deformation found in the central group (CG) and São Miguel islands. The velocity field is modeled by approximating segments of the Eurasian‐Nubian plate boundary with vertical dislocations with right‐lateral motion and opening below a locking depth. Best fitting models have deep motion in the range of 2.0–2.3 mm yr−1 directed N (82.3–88.7)°E. Such displacement accounts for almost half of predicted Eurasian‐Nubian relative plate motion. The modeling results suggest that the locking depth in the CG islands is at least 20 km while in São Miguel it is about 2 km. We find transient deformation at Fogo volcano, São Miguel Island, due to unrest activity mainly during 2003–2006 and 2011–2012, and local continuous subsidence in Terceira Island, attributed to a deflation source centered on the island.