I don't know how they even measure such a minute change. I mean they must need reference to compare against a stable point in space. What is that reference and what point of city you compare against that stable reference?
This is simply their calculation.
And it's absolutely possible to measure this, see: https://www.thoughtco.com/measuring-plate-motion-1441107 and https://www.youtube.com/watch?v=S1m1tAGbfL4