Contemporary 3D Velocity Field of Eastern North America based on 14 Years of Continuous GPS Observations

English Abstract:

The Saint Lawrence Valley in eastern North America is under intraplate surface deformations and has experienced strong historical and instrumental earthquakes. This shows the probability of occurring earthquakes all along the valley which is an important issue from the seismic hazard point of view. The area is further influenced by the glacial isostatic adjustment (GIA). We study this region using observations of all the available continuous GPS (CGPS) stations in the area as well as some more CGPS stations to the west and south of the valley in order to delineate the GIA process in eastern North America. The data comprise 14 years of continuous GPS daily solutions (8 years on average) from 112 CGPS sites. We use Bernese GNSS software to process the GPS data and our GITSA software for time series analysis to obtain the contemporary three-dimensional velocity field of the eastern North America and their uncertainties. The results clearly show the counter-clockwise rotation of the North American plate with an average velocity of 16.3 mm/yr constrained to ITRF 2008. The repeatability of results is at the level of 1.2 mm/yr for horizontal and 4.7 mm/yr for vertical velocities at the best sites assuming the white noise model. The accomplished noise analysis on the post-fit residuals using both spectral and maximum likelihood estimation methods brings us to obtain more realistic uncertainties for the estimated velocities and distinguish non stable GPS stations. The results show the white plus flicker noise model is the best, and uncertainties are increased with the order of 5~30 times. We discuss limitations in the computed velocities, especially when the possible instability over time causes non-linear behavior in the time series. In this way, 13 stations (out of 112 sites) were identified as stations with non-stable monuments. We also present an ongoing uplift model in the study area using the variogram analysis. At the best sites, the vertical velocities agree with the ICE-5G (VM2) model to within 0.02 mm/yr. However, the model shows a systematic tilt of ±6 mm/yr compared to the ice model in the east-west direction with a vertical hinge line approximately located to the west of Ontario province.