The Richter scale works in an exponential way sort-of - so this was a much smaller quake compared to the previous one (7.4 vs. 9). To make it easy - every point on the scale is x31 in size. So a quake of 9.0 scale is 31 times more powerful than a quake of 8.0 scale. This is not good - but this is a much much smaller risk than the previous one. The difference in size between a 9 and a 7.4 is around 80-83 times - so the last one was much, much bigger than this one.
I think it's a lot bigger difference than that. A quick wiki check says the a 9.0 is about 1000X stronger than a 7.0.
Not saying it was a minor quake though. If this hit anywhere else, it'd be considered quite big. Just in comparison it's a lot weaker.
Look at the under "Examples" http://en.wikipedia.org/wiki/Richter_magnitude_scale A 7.5 quake had 6.6 megatons of energy, a 9.0 had 476, 476/6.6 = 87 - so I should have said somewhere around 90-100 - which is bigger than the 80-83 estimate I gave earlier (cut me some slack, it's early in the morning and I am multi-tasking). Bigger than my estimate, but not a tons bigger. FWIW - The US measurements had it at 7.1 - which would make it much smaller. I went with 7.4 because that's what the Japanese are reporting.
Yeah, in my head the difference was a lot bigger, but I guess not. Still a big quake, regardless if it's 7.4 or 7.1.
From the USGS WWW site: http://pubs.usgs.gov/gip/earthq4/severitygip.html Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.