What was the Japan 2011 earthquake measure on the Richter scale?

It is not a matter of size: a tsunami is very different from an ordinary wave. A tsunami wave is not wind-driven but is usually caused by an earthquake or landslide. They have a much longer wavelength, and so carry far more water than an ordinary wave of similar height. In open ocean a tsunami may be a couple feet high an hundreds of miles long, becoming higher in the shallow water near shore. While a storm wave washes in and out in a matter of seconds, a tsunami comes in as a continuous surge for several minutes.