Japan, located within the Pacific Ring of Fire, is one of the most seismically active regions in the world, experiencing thousands of small earthquakes yearly. While predicting the exact timing of large earthquakes remains elusive, studying smaller seismic events helps researchers understand the conditions that lead to major earthquakes.

A recent study by researchers from Kyushu University and the University of Tokyo has explored seismic activity with unmatched detail, uncovering a connection between fault strength and the magnitude of earthquakes. The findings, published in Nature Communications, suggest that the strength of a fault influences the "b-value," a key seismological constant, which in turn affects the likelihood of a major earthquake occurring.

“The b-value is crucial in seismology because it defines the relationship between earthquake frequency and size,” explains Professor Satoshi Matsumoto, Director of Kyushu University's Institute of Seismology and Volcanology. “A low b-value indicates a greater likelihood of large earthquakes, while a high b-value points to more frequent smaller earthquakes.”

The b-value can vary across regions and change over time, often decreasing prior to a major quake. Previous studies have linked this decrease to growing stress on a fault. However, this new research highlights that fault strength is another important factor contributing to variations in b-value.

The team analyzed seismic data from the area around the epicenter of the 2000 Western Tottori Earthquake (magnitude 7.3). By deploying over 1,000 seismic stations, they captured highly accurate data, even detecting tiny aftershocks that continue to occur decades later.

"When the Earth's crust contains weak, randomly oriented faults, large earthquakes are less likely. But when stronger faults align in certain directions, the probability of a large earthquake increases," Matsumoto explains.

Using this data, researchers assessed the stress field, or the directions of force acting on each fault during seismic events. They categorized faults as strong or weak, discovering that stronger faults had lower b-values and were more likely to cause large earthquakes, while weaker faults exhibited higher b-values, indicating a reduced likelihood of significant quakes.

"These weak faults tend to slip before too much stress accumulates, which means they don’t release a large amount of energy," adds Matsumoto.

The researchers hope that by gaining a better understanding of b-values and fault strength, they can move closer to predicting seismic events. “While we may never predict earthquakes with precision, analyzing fault strength and b-values could help us gauge when a fault is nearing a critical point where a major quake could occur,” concludes Matsumoto.

This information is essential for improving earthquake preparedness and safety in seismically active regions.