How are Earth's rotational changes due to large earthquakes calculated? According to NASA (and many other sources), Japan Quake May Have Shortened Earth Days, Moved Axis and in the answers to the question here What geophysical events can (temporarily) increase the Earth's rate of rotation?, that major earthquakes result in a slight shift in the length of the day, from NASA:


  calculations indicate that by changing the distribution of Earth's mass, the Japanese earthquake should have caused Earth to rotate a bit faster, shortening the length of the day by about 1.8 microseconds (a microsecond is one millionth of a second).


Past great earthquakes, such as the 8.8 in Chile in 2010 and the 9.1 Sumatra in 2004 have caused similar rotational 'wobbles'.  The NASA article states that the earthquake's magnitude, location and dynamics result in the orbital 'wobbles'.

How are Earth's rotational changes due to large earthquake calculated, and especially distinguished from oceanic and atmospheric effects?
Earthquakes, specially those at subduction zones, result in redistribution of mass (as one plate slides on top of the other). This change in distribution of mass causes changes in gravitational field that can be measured by satellites such as GRACE.

Now angular momentum is conserved. If we know the change in mass then we can calculate the change in moment of inertia and thus the change in the angular velocity.