The most detailed study yet of seismic activity links fault strength to the likelihood of large earthquakes.

Researchers from Kyushu University deployed 1,000 seismic stations in the area surrounding the epicenter of the 2000 Tottori West Earthquake, allowing for unprecedented earthquake monitoring. Copyright: Satoshi Matsumoto, Kyushu University

Located on the Pacific Ring of Fire, Japan is one of the most earthquake-prone countries in the world, with thousands of small earthquakes occurring each year and the constant threat of a “big one.” Currently, it is not possible to predict when major earthquakes will occur, but by studying the many small earthquakes that do occur, seismologists in Japan hope to understand more about the processes in the Earth’s crust that lead to major earthquakes.

Now, researchers from Kyushu University and the University of Tokyo in Japan have studied seismic activity in unprecedented detail, and identified a link between fault strength and earthquake size. The study, published in Nature Communications, suggests that fault strength affects the value of b — and thus the likelihood of a large earthquake.

“The b value is a very important constant in seismology, as it characterizes the relationship between earthquake frequency and magnitude. A low b value means there is a higher proportion of large earthquakes, while a high b value means there is a higher proportion of smaller earthquakes,” explains Professor Satoshi Matsumoto, the study’s first author and director of the Institute of Earthquake and Volcanology at Kyushu University.

The value of b can vary between different locations as well as over time, and is often said to decrease before a major earthquake. A previous study suggested that the decrease in b was due to increased stress forces exerted on the fault. Now, this study suggests that the strength of the fault is also a contributing factor.

In the study, the research team analyzed seismic activity in the area surrounding the epicenter of the 7.3-magnitude West Tottori Earthquake in 2000. By installing more than 1,000 seismic stations in the area, the researchers were able to conduct seismic observations with an unprecedented level of accuracy.

“Even after two decades, hundreds of small aftershocks are still occurring, most of them too small to be felt,” Matsumoto says.

When the Earth's crust has many weak faults in random directions, large earthquakes are unlikely to occur. However, when the Earth's crust has many strong faults oriented in a distinct direction, there is a greater chance of a large earthquake. Image credit: Satoshi Matsumoto, Kyushu University

Using this large number of sensors, researchers were able to detect small movements of faults, as well as the direction of each fault within the Earth's crust.

Using this massive amount of data, the team was able to estimate the stress field (the different directions of stress forces applied to each crack at the time of failure) and allowed them to characterize failures as strong or weak.

“Under certain stress conditions in each tectonic system, there is a suitable direction for the fault plane to slip. When faults are in the wrong directions, this indicates that they are weak faults that can slip more easily. On the other hand, strong faults require more stress to slip, and have a more distinct direction,” Matsumoto explains.

Through stress field calculations, the researchers were also able to estimate the b value for the set of events classified by fault strength. They found that stronger faults had smaller b values, indicating that large earthquakes were more likely to occur, while weaker faults had larger b values, indicating that large earthquakes were less likely.

Simply put, these weak faults are more likely to slip before a significant amount of stress builds up, which means they are unable to release a significant amount of force, Matsumoto says.

By gaining a deeper understanding of the factors that influence b values, researchers hope they can get closer to the “holy grail” of earthquake prediction.

“I don’t think we’ll ever know exactly when an earthquake will strike, but looking at data like the direction and magnitude of the fault, and calculating b values, can help us estimate when the fault reaches a critical point — where it only takes a small extra push of force to cause the fault to slip,” Matsumoto concludes. “This information is vital for knowing how to prepare for major earthquakes.”

For more information: Satoshi Matsumoto et al, Strength dependency of frequency–magnitude distribution inquases and effects for stress state criticality, Nature Communications (2024). DOI: 10.1038/s41467-024-49422-7

Presented by Kyushu University

Citation: Most detailed study yet of seismic activity links fault strength to likelihood of large earthquakes (2024, September 9) Retrieved September 9, 2024 from https://phys.org/news/2024-09-seismic-links-fault-strength-likelihood.html

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