Plate boundary deformation at the Azores triple junction determined from continuous GPS geodetic measurements, 2002-2017

Ground deformation in the Azores, at the triple junction between the
Eurasian, Nubian, and North American plates, has been mapped with
continuous GPS (Global Positioning System) geodetic measurements to
improve tectonic motion estimates and for understanding volcanic unrest.
We compute daily GPS 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 and São Miguel Island. 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 2.4-2.7 mm/yr directed N(76.5-78.8o)E. Such displacement accounts
for more than half predicted Eurasian-Nubian relative plate motion. The
modeling results suggest that the locking depth in the Central Group is
about 17 km while in São Miguel is about 2 km. We found 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.