Abstract

The coexistence of two earthquake groups, each following the Gutenberg–Richter (GR) relation with different b-values (hereafter called the hyper-GR group), is discussed, and an algorithm is developed to recognize them. I show how the η-value proposed in 1978 by Utsu, who introduced the parameter representing an upward or downward convex curvature of frequency-magnitude distribution (FMD), differs significantly between GR and hyper-GR groups. The spatial variation of the FMD of earthquakes down to M 2.2 in and around the Japanese islands is studied using the algorithm and the η-value. The η-value for one year preceding the M 7.4 Kii-Hanto-Oki earthquake of 2004 was high, displaying an upward convex curvature of FMD in and around the source region. The hyper-GR group was found to be a significantly better distribution model than the GR group for the seismicity greater than M 2.15, but hyper-GR was not significantly better for seismicity greater than M 3 of greater sampling radius. Further study is required to judge if it is a precursor to the event.

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