Very true, but there's not much of a shelf in Japan - water depth goes from 0 to 9,000 meters in 100-200 km. Having said that,  I just found this on the Earthquake Research Institute of Japan website. It shows that the tsunami first reached shore in 30 minutes

and that it took one hour and ten minutes for it to reach Sendai bay!

Seismographs would have picked up this event pretty much instantaneously which makes me wonder why there seemed to be so little done to get away from the tsunami? How could entire trains, much less towns be swept away?