They hadn't released richter scales when I was driving back, only that it was substantial and that tsunami warnings had been issued. First report I read when I got home said Japan’s Meteorological Agency measured the quake at magnitude 8.5 -that's a pretty severe earthquake, and some large waves can be expected.
Anything over 8 is considered to be a 'great' earthquake, and the Pacific Tsunami Warning Center said the quake had the "potential to generate a widely destructive tsunami in the ocean or seas near the earthquake."
the same reason as for any scientific use of logarithmic scales. If you are comparing wind forces, they only occur over quite a small range, so it is possible to draw linear graphs where low measurements can be plotted on the same scale as large ones.
Earthquakes however occur over such a huge range, that a graph of high level measurements, say 8 or 9 Richter, lower level earthquakes wouldn't even appear as a squiggle, making comparison difficult. By plotting on a logarithmic scales, you can compare different earthquake events (such as after shocks) on one graph.