Quote from kiwi_trader:
I suspect that magnitude 9 for multiple minutes translates into a huge motion in the sea floor which is what generates the wave pulse. I'm guessing that no-one expected it.
According to this piece: http://rogerpielkejr.blogspot.com/2011/03/roger-bilham-on-honshu-earthquake-and.html
"In 1896 a 33 m high tsunami drowned the Sanriku coastline 200 km to the north of Fukushima. A 23 m wave surged on the same coast in 1933, and in 1993 a 30 m wave swept over Okushira Island."
As I'm not a seismologist I surely don't know all the issues. It will be interesting to see what turns up in the inquiries into what will probably be the most investigated industrial accident of all time.