what+caused+the+earth+quake?

http://news.yahoo.com/s/yblog_thelookout/20110314/ts_yblog_thelookout/japans-earthquake-shifted-balance-of-the-planet

The 9.0 magnitude quake (the [|fourth-largest recorded since 1900] ) was caused when the Pacific tectonic plate dove under the North American plate, which shifted Eastern Japan towards North America by about 13 feet (see NASA's before and after photos at right). The quake also shifted the earth's axis by 6.5 inches, [|shortened the day by 1.6 microseconds], and sank Japan downward by about two feet. As Japan's eastern coastline sunk, the tsunami's waves rolled in.