Decade‐scale decrease in b value prior to the M 9‐class 2011 Tohoku and 2004 Sumatra quakes

The Gutenberg‐Richter frequency‐magnitude distribution of earthquakes has become well established in seismology. The slope of the relation between frequency and magnitude ( b value) is typically 1, but it often shows variations around 1. Based on an analysis of seismicity prior to the 2011 Tohoku and 2004 Sumatra earthquakes (both in magnitude ( M ) 9 class), we show that the pronounced decade‐scale decrease in b value was a common precursor to both mega‐quakes around their hypocenters. This is the first report on M 9‐class quakes to confirm a change in b value, which has been predicted based on the results of laboratory experiments. We propose that the b value is an important indicator of an impending great earthquake, and has great potential in terms of predicting a future large quake off the Pacific coast of Hokkaido, Japan.