How do we know/how do we calculate that Megathrust earthquakes, such as the March 11, 2011 Megathrust earthquake in Japan, speed up the Earth’s rotation by a microsecond?

Question

How do we know/how do we calculate that Megathrust earthquakes, such as the March 11, 2011 Megathrust earthquake in Japan, speed up the Earth’s rotation by a microsecond?

In: Planetary Science

Answer 1

We have very accurate atomic clocks that can measure the time difference to that much precision and a vast collection of equally precise astronomical instruments on the Earth’s surface that use those atomic clocks to measure the planet’s position relative to objects so distant that they can be considered stationary.
That microsecond’s change wound up altering where astronomical objects appeared with those instruments. Nothing that the human eye could detect, but an immense difference as far as the computers were concerned.