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, affecting Gravity at all is insane. [CS Monitor] Note: Headline originally read "Japan Earthquake Weakened Earth's Gravity" which, as Bob points out, is a little misleading. The article itself explains the Japan localisation, and the headline has been updated to reflect this. Meanwhile Bob (and all readers) -- we're happy for you to point out stuff like that. Just please don't be a dick about it.