At a very early age I found,
That nothing at all was ever sound,
Subject to disease, rust and decay,
It burned, or moved or fell away,
In order to gain peace of mind,
A solid rock I strived to find,
In Earth's landmass I gained my lift,
But then came Continental Drift.
c. JonnyBoy12 (1992)
Sorry, JJ, just wondered off there, in response to your question I do not know how they measured the speed, but they just said it is moving at that speed.
See above for my vain poetic attempt to avert this threat on our supposed very symbol of permanence.