Your first thought when describing a rock formation likely isn't a mille-feuille, but there are actually certain types composed of many thin layers that bring the flaky pastry to mind. Not only that - but these rocks can quite literally fold under pressure. These formations have the interesting ability to fold under compressive forces and form sharply localized bends known as kink bands. For many years, these kink bands were believed to reduce the mechanical strength of crustal rocks, weakening the Earth's outer shell.

However, a research group led by Professor Hiroyuki Nagahama, Professor Jun Muto, and Ph.D. Candidate Hiroaki Yokoyama at the Department of Earth Science, Tohoku University, has provided new experimental evidence that challenges this assumption. Their findings may improve our predictions of earthquake risk assessment to help protect communities in seismic zones.

Using biotite, a common mineral found in the Earth's crust that peels easily like a stack of paper due to its layered structure, the researchers conducted deformation experiments under varying pressure conditions. They found that when kink bands satisfy a specific geometric condition known as a rank-1 connection, the material exhibits strengthening rather than weakening. The rank-1 connection ensures smooth continuity between two differently deformed regions within the rock.

The group observed that kink bands with symmetric tilt boundaries led to a consistent increase in material strength. This phenomenon, known as kink strengthening, has been proposed in the field of materials science, but this study is among the first to clearly demonstrate it in natural geological materials.

"This study not only bridges concepts between materials science and geology, but also offers new insight into the mechanical behavior of the Earth's crust," says Nagahama.

To further strengthen the evidence of their findings outside of a controlled laboratory setting, the researchers also identified kink bands with similar geometric characteristics in real-world rock formations. These range from microscopic features within minerals to large-scale folds spanning kilometers, often referred to as mega kinks. The results suggest that such structures may locally enhance the strength of the crust and influence the distribution or location of earthquake ruptures.

"This finding may contribute to more accurate seismic hazard assessments in the future," says Yokoyama. "The more we understand about these rock formations, the better our predictions about earthquakes become. As such, this research could have a real positive effect on people living in earthquake-prone regions."

The findings were published in Scientific Reports on September 26, 2025.