Following the devastating 2011 Tōhoku Earthquake and Tsunami, Japanese seismologists have made significant progress in understanding how large earthquakes develop. A new study suggests that small clusters of seismic activity and so-called “repeating earthquakes,” once considered insignificant, may hold the key to predicting major disasters.

This was reported by The Mainichi.

Clusters as Precursors to Disaster

In 2025, researchers identified two active earthquake clusters north of the 2011 epicenter. One was a swarm of quakes up to magnitude 6.9 off the coast of Sanriku, while the other was a magnitude 7.5 event near Aomori Prefecture. Both were accompanied by sequences of smaller tremors.

Ryo Hino, a professor at Tohoku University, explained that such clusters gradually expand the будущего очага (future rupture zone). He noted that a similar pattern had been observed before major earthquakes in 1968, 1994, and 2011.

He stated that clusters of magnitude-6 earthquakes tend to expand the rupture area over time, potentially leading to a large seismic event.

A New Fault Model

Modern understanding of earthquakes has shifted significantly. The older “asperity model,” which assumed fixed strong patches along a fault where stress accumulates, is being replaced. Scientists now view tectonic plate boundaries as a complex mosaic of many such zones.

Failure of a single zone may trigger a moderate earthquake, while simultaneous rupture of multiple zones can produce a catastrophe on the scale of the 2011 event.

Slow slip events—nearly imperceptible movements of tectonic plates—also play a crucial role. According to Hino, seismic activity expands in sync with these slow-slip zones, representing two aspects of the same underlying process.

“Repeating Earthquakes” as Warning Signals

Special attention is being paid to so-called repeating earthquakes—series of very small tremors that occur at the same point on a fault at regular intervals.

Naoki Uchida of University of Tokyo suggested that an increase in the frequency of such micro-events may indicate accelerating plate motion and rising stress in the Earth’s crust.

He indicated that if the frequency of repeating tremors begins to rise, it could serve as a warning sign of an impending major earthquake.

Prospects for Real-Time Monitoring

Japanese researchers are already developing a system to track these subtle signals in real time. Thanks to the country’s dense seismic monitoring network, even very weak tremors can be detected.

However, scientists acknowledge that despite significant progress, there is still no guarantee that all precursors can be reliably captured.

Hino emphasized that monitoring has improved considerably, but researchers are not yet certain they can detect all the processes that occurred before the 2011 disaster.

In Brief

Japanese seismologists have found that clusters of moderate earthquakes and increasing frequencies of “repeating” small tremors may act as precursors to major earthquakes. Such patterns were observed before events in 1968, 1994, and the 2011 Tōhoku Earthquake and Tsunami. A new model treats plate boundaries as a mosaic of stress zones, with slow slip and microseismic activity acting as interconnected signals of energy buildup. Scientists are working on real-time monitoring systems, though precise prediction of major earthquakes remains a challenging task.