Simple jelly beads on a liquid surface reveal the secrets of slow earthquakes

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Cross section of the subduction zone and source areas of slow earthquakes

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Credit: Yuto Sasaki

Osaka, Japan – Slow earthquakes that exhibit slow, long-lasting and unusually small slips have been discovered next to normal earthquakes where catastrophic shaking is sometimes felt (Figure 1). However, no one knows why slow earthquakes exhibit such strange properties. In a study published in the scientific journal Nature Communications, researchers at Osaka University successfully reproduced multiple features of slow earthquakes ly in the laboratory (Figure 2), and proposed their grain-scale origin based on their direct observations.

More than 20 years after the discovery of slow earthquakes, which cause weak or imperceptible shaking but are associated with slow sliding for about a year maximum, there is no unified explanation of the characteristics of slow earthquakes. To date, most studies reproducing slow earthquakes have focused only on slow fault slip. However, what makes slow earthquakes more distinct from normal earthquakes is rather their statistical features revealed by seismic observations (Figure 2, upper panels). However, these statistical properties of slow earthquakes have rarely been reproduced or explained in laboratory experiments.

“Slow earthquakes have traditionally attracted attention due to their slow slip rate, but the statistical properties of their duration and frequency relative to earthquake magnitude have so far only been addressed in a few limited experiments,” says study author Yuto Sasaki. “It goes without saying that experiments using actual rocks and natural fault grains are of great interest; however, we recognized the necessity of a simpler physical equivalent, which enables us to directly observe internal structures during deformation. This is a great advantage of our system, as such direct internal observation is usually very difficult in rock deformation experiments.”

The researchers prepared a very simple “raft of jelly beads” (Figure 3). Since the fault zone of slow earthquakes is expected to be rich in fluids and soft grains, they envisioned a physical counterpart of soft jelly grains in a liquid solution. “This tabletop system is available even in your home, but it shows an amazing variety of behaviors and provides us with a wealth of fascinating evidence about slow underground earthquakes,” says Sasaki. “If you put hard glass beads in a dry cup and mix them slowly, you can feel rapid, intermittent scratching. In fact, these scratching events show statistics similar to regular earthquakes.” However, the mixture of gel beads and liquid solution shows significantly different features with longer and smaller events, as is the case with slow earthquakes (Figure 2, bottom panels). “You can feel the slow, intermittent glide when mixing bubble tea, but the two phenomena appear to be fundamentally different.”

In contrast to hard dry beads, soft wet beads are ineffective in transmitting force and deformation. This property is likely to lead to longer, isolated microslips. “In retrospect, it seems that this system was well suited for studying fault systems of slow earthquakes, whereas our previous target was the deeper part of tectonic plates,” Sasaki says. “We expect a similar report from high-temperature and pressure experiments using rocks and fault materials.”

Slow earthquakes often occur near the source areas of normal and destructive earthquakes (Figure 1). “Based on the results, the observed slow earthquake statistics can be interpreted as fault states,” says Sasaki. This would contribute to probabilistic assessments of earthquakes. Moreover, the long-standing mystery of the mechanisms behind slow earthquakes is expected to be addressed not only through laboratory experiments, but also through observational studies and geological analyses. “This result will serve as a starting point for further contributions from a wide range of fields, ultimately advancing our understanding of slow earthquakes and enabling better assessment of their impact on conventional and destructive earthquakes,” says Sasaki.

Since the setup used in this study is simple, the observed results could also represent the general properties of the mixture of fine and liquid beads. “By analyzing the detailed relationship between microscopic grain rearrangements and macroscopic slip, fundamental aspects of discrete soft matter systems can be revealed, as well as the origin of the characteristic features of slow earthquakes. Nothing excites me more than the realization that benchtop experiments in soft matter can solve the mysteries of both fundamental soft matter physics and geological phenomena,” says author Hiroaki Katsuragi.

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The article, “The origin of slow earthquake statistics in low-friction smooth intergranular shear,” is published in Nature Communications at DOI: https://www.doi.org/10.1038/s41467-025-65230-z

About Osaka University

Osaka University was founded in 1931 as one of the Seven Imperial Universities of Japan and is now one of Japan's leading comprehensive universities with a wide range of disciplines. This strength is coupled with a unique drive for innovation that spans the scientific process, from basic research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world. Now, Osaka University leverages its role as a designated national university institution selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human well-being, sustainable development of society and social transformation.

Website: https://resou.osaka-u.ac.jp/en

magazine

Nature Communications

Search method

Experimental study

Research topic

Not applicable

Article title

The origin of slow earthquake statistics in soft, low-friction intergranular shear

Date the article was published

1-December-2025

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