According to the Christian Science Monitor, the uneven mass distribution on Earth means gravity has different magnitudes in different places. The earthquake was strong enough to produce a measurable difference because it thinned out the crust around Japan.
The researchers found the Tohoku-Oki quake reduced the gravity field there by an average of two- millionths of a gal by slightly thinning the Earth's crust. In comparison, the strength of the gravitational pull at the Earth's surface is, on average, 980 gals. (The gal, short for Galileo, is a unit of acceleration; one gal is defined as one centimeter per second squared).
That may be a small amount, but still, effecting gravity at all is insane. [CS Monitor]