Researcher
Aron Mirwald
研究者名
Enescu, Bogdan Dumitru
Overview
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.
To address this question, a team of researchers from Kyoto University and ETH Zurich set out to analyze the Japanese earthquake catalog, examining how b-values vary before, after, and around large earthquakes. The study was led by Aron Mirwald, formerly a short-term visiting student at Kyoto University and a doctoral student at ETH Zurich, together with Bogdan Enescu of Kyoto University and Leila Mizrahi and Stefan Wiemer of the Swiss Seismological Service (SED) at ETH Zurich.
The research team examined large earthquake sequences in Japan using the Japan Meteorological Agency earthquake catalog from January 2000 to July 2025. They carefully restricted the data by depth, distance from the coast, and space-time windows in order to reduce catalog incompleteness and possible contamination from other large earthquakes.
Their results revealed no clear evidence for a systematic decrease in b-values before large earthquakes, or for a systematic increase afterward. Instead, the team found that large earthquakes tend to occur in regions where b-values are slightly lower than in the surrounding areas. Moreover, differences in b-values among earthquake sequences could be explained mainly by the location of the mainshock.
“Our results suggest that, at broad spatial scales, b-values are controlled more by where a large earthquake occurs than by systematic changes before or after the event,” says Enescu. “This highlights the importance of local geological and stress conditions in shaping earthquake size distributions.”
These findings suggest that b-values may reflect local geological and physical conditions, such as rock properties and stress state. Although this conclusion does not imply that individual earthquakes can be predicted, it provides a stronger statistical basis for interpreting b-values around large earthquakes and may contribute to future studies of long-term earthquake hazard.
