Plate boundary deformation at the Azores triple junction determined from
continuous GPS geodetic measurements, 2002-2017
Abstract
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
Sao 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-1 on segments 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,
S\~{a}o 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.