Kyoto, Japan -- Japan is well known for its large earthquakes, but not all regions show the same patterns of earthquake activity. One way to understand which places tend to experience large or small earthquakes is the b-value, a key statistical measure long used by researchers for understanding seismicity and assessing earthquake occurrence patterns.

The b-value describes the relative numbers of small and large earthquakes in a given area. A high b-value means that small earthquakes are comparatively more frequent, whereas a low b-value indicates a relative increase in the proportion of larger earthquakes.

Many previous studies have suggested that b-values may decrease before -- or increase after -- large earthquakes, possibly reflecting changes in stress before and after rupture. Other studies have proposed that large earthquakes tend to occur in areas where b-values are low. Until now, however, it has remained unclear whether time-dependent changes or spatial differences are more important when comparing many large earthquake sequences.