The first of the two recent Japanese earthquakes (which has now been upgraded to a 9.1 magnitude, making it the fourth largest recorded since 1900) was caused when the Pacific Plate moved under the North American Plate.
This, in turn, nudged Japan 13 feet closer towards North America (see image, showing before-and-after, courtesy of NASA). The earthquake also shifted Earth on its axis by 6.5 inches, and shortened the day by 1.6 microseconds by making the Earth spin faster, Most disturbing of all, Japan sank downward into the ocean by at least two feet.
Soil liquefaction, which has always been a problem in mainland Japan, is also exacerbated by the ongoing disaster. Tokyo Disneyland closed because of water seeping up from the ground in the immediate wake of the quake. And now with the entire population on radiation fallout alert, the future of the theme park is up in the air. Many parts of Japan's coastline are actually artificial, built on reclaimed land extended into the sea by man-made means. The side effects of this are only now coming back to haunt the Japanese people, as nature seems to be reclaiming it back.
The idea of Japan completely sinking into the ocean is one that's been heavily explored in science fiction, such as Nihon Chinbotsu. While the reality is not quite this dire - yet - the potential for further and even greater tectonic movement is indeed possible.
Update: as we were preparing this post, the alert came over the USGS wire that a third earthquake has taken place in Japan. At this time, the quake is said to be a 5.7 and is just off the coast of Honshu.
Developing....
