The U.S. Geological Survey has updated the magnitude of the March 11, 2011, Tohoku earthquake in northern Honshu, Japan, to 9.0 from the previous estimate of 8.9.
Independently, Japanese seismologists have also updated their estimate of the earthquake's magnitude to 9.0. This magnitude places the earthquake as the fourth largest in the world since 1900 and the largest in Japan since modern instrumental recordings began 130 years ago.
The USGS often updates an earthquake's magnitude following the event. Updates occur as more data become available and more time-intensive analysis is performed. There are many methods of calculating the energy release and magnitude of an earthquake. Some methods give approximate values within minutes of the earthquake, and others require more complete data sets and extensive analysis. Due to inherent uncertainties in the modeling of energy and magnitude, the results from different agencies often vary slightly. These magnitude discrepancies arise from the use of different data and techniques.
For more information on why magnitudes change, see the Earthquake Hazards Program FAQ website at: http://earthquake.usgs.gov/learn/faq/?faqID=297
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