Joint Bayesian Modeling of Velocity Break Points, Noise Characteristics, and Their Uncertainties in GNSS Time Series: Far‐Field Velocity Anomalies Concurrent With Magmatic Activity in Iceland

Estimating the timing of velocity changes (break points) in Global Navigation Satellite System (GNSS) coordinate time series is required for understanding various Earth processes and how they may couple with each other. We jointly estimate break points, noise parameters and their uncertainties in GNSS time series using Bayesian interference. Synthetic data experiments demonstrate that time‐correlated noise can cause spurious estimates of break points for small velocity change and suggest the lower limits of velocity change for reliable estimates. A case study at the Krafla volcanic system shows increased rift‐perpendicular movementof 7.6–9.8 mm/yr, preceding by 23–77 days the 2014 Bárðarbunga rifting episode that occurred ∼120 km to the south (maximum likelihood estimates). These far‐field velocity anomalies occur in the same period as enhanced near‐field seismicity and deformation before the dike intrusion at Bárðarbunga, possibly indicating the coupling between the two volcanic systems through a deep partial melt zone. Plain Language Summary The velocity of ground movements can change due to Earth's activities. Satellite positioning techniques record these changes, while noise in the records may obscure their analyses. We developed a statistical method to detect velocity changes from these noisy records and give an estimate of uncertainty. Simulated data is used to examine the method, revealing the lower limits of velocity change for reliable estimates. Then the method is applied to detect velocity changes in the records at the Krafla volcanic system, Iceland. With maximum likelihood estimates, it is inferred that the ground moves 7.6–9.8 mm/yr faster, in rift‐perpendicular direction, before the start of the 2014 Bárðarbunga dike intrusion that occurred ∼120 km to the south. The ground movements resume to approximately the original velocities by a second velocity change in late 2014 or early 2015. The onset of the far‐field velocity anomalies occurs in the same period as enhanced near‐field seismicity and deformation rate before the dike intrusion at Bárðarbunga. In the context of enhanced seismicity at the Bárðarbunga volcano and rapid inflation at the Askja volcano before the Krafla rifting episode in 1975–1984, our result may again indicate coupling among the volcanic systems in Iceland. Key Points The lower limit of velocity change values, where reliable estimates can be derived, depends on noise characteristics of Global Navigation Satellite System