Thirteen feet was the maximum, eight feet was the average, over a stretch of 300 miles.
The eastward shift was documented by Japan’s Geonet network of GPS monitoring stations, based in Tsukuba, said Ken Hudnut, a geophysicist at the U.S. Geological Survey’s Earthquake Hazards Program in Pasadena, Calif. …
“When the earthquake occurs, the upper plate lurches eastward over the subducting plate. The oceanic plate that’s going down is relatively rigid, but the upper plate is like a wedge of material that’s more elastic. So picture that upper wedge as being almost like an accordion that’s being compressed between the times of earthquakes. It’s like a spring. You’re loading up the spring between earthquakes — in other words, you’re compressing the eastern edge of the spring toward the main island of Japan. The earthquake allows that material to spring out toward the east.”
There’s more in this excellent article. Well worth the click.