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No, PTWC didn't raise the magnitude. The Global CMT project did. They are generally regarded as the best source for earthquake magnitudes among seismologists.
The USGS has not yet adopted the higher 9.1 magnitude.
From Quick CMT catalog
201103110546A NEAR EAST COAST OF HONSH
Date: 2011/ 3/11 Centroid Time: 5:47:32.8 GMT
Lat= 37.52 Lon= 143.05
Depth= 20.0 Half duration=70.0
Centroid time minus hypocenter time: 69.8
Moment Tensor: Expo=29 1.730 -0.281 -1.450 2.120 4.550 -0.657
Mw = 9.1 mb = 7.9 Ms = 7.9 Scalar Moment = 5.31e+29
Fault plane: strike=203 dip=10 slip=88
Fault plane: strike=25 dip=80 slip=90
The Japan earthquake was the fourth most powerful ever recorded with a magnitude of 9.1, twice more powerful than the initial estimate of 8.9, Gerard Fryer, geophysicist of the Pacific Tsunami Warning Center, said this morning.
Three others that were more powerful since the late 1800s when seismometers started measuring ground motions were in 9.5 in Chile in 1960, 9.2 in Alaska in 1964 and 9.1 in Sumatra in 2004, according to Fryer.
The new magnitude was adjusted based on the impact of the quake throughout the Pacific, he said. "It fits all measurements, including in Hawaii," Fryer said.
The U.S. Geological Survey estimate of the quake's magnitude is still 8.9.
It is not uncommon for scientists to estimate different magnitudes immediately after an earthquake.
Originally posted by PinkAndBlack
Uh I don't think 9.1 is twice 8.9, but definitely a powerful quake none the less.
Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; in terms of energy, each whole number increase corresponds to an increase of about 31.6 times the amount of energy released, and each increase of 0.2 corresponds to a doubling of the energy released.
Well, technically, it's a logarithmic scale...that's why a 9.0 is 10 times as powerful as an 8.0, and it means that a 9.1 is 1.6 times as powerful as an 8.9. Not quite 2, but close enough.