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?