Decade‐scale decrease inb value prior to the M9‐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.