Earth's surface is covered by more than a dozen tectonic plates, and in subduction zones around the world—including the Japanese Islands—plates converge and dense oceanic plates sink into the Earth's interior. These regions, especially plate boundaries, are known for frequent seismic activity. In recent years, scientists have increasingly emphasized that water plays a crucial role in earthquake phenomena in subduction zones, and thus conducted active research to investigate the influence of water on various processes occurring within earthquake source regions.
When water is supplied, peridotite—the primary constituent of the upper mantle—can transform into serpentinite. This process is thought to occur extensively in the mantle wedge, the mantle region on the overriding-plate side above a subducting oceanic plate. The transformation of peridotite into serpentinite involves chemical reactions that alter the mineral assemblage of the rock. Since individual minerals have distinct physical properties, such as their deformability, serpentinization is expected to result in significant changes in the physical properties of the whole rock. However, while the deformation mechanisms of peridotite have been relatively studied over a long history of research, those of serpentinite are still under active investigation. As a result, serpentinite has become one of the key research targets for understanding the physical properties of plate boundaries in subduction zones.
Furthermore, it has been proposed that dislocation creep dominates deformation in antigorite, the main mineral of serpentinite. As deformation progresses, a crystallographic preferred orientation (CPO) develops, in which the crystal orientations become aligned. The deformation of antigorite by dislocation creep produces an "A-type" CPO pattern, in which the crystallographic a-axes of antigorite are preferentially aligned parallel to the shear direction. However, in addition to the A-type, multiple antigorite CPO patterns occur in nature, and the formation mechanisms of these different CPO patterns have not yet been fully understood. This discrepancy suggests that antigorite in the Earth's interior may deform by mechanisms other than dislocation creep.
Motivated by this puzzle, a team of researchers, led by Associate Professor Takayoshi Nagaya from the Department of Earth Sciences, Waseda University, Japan, including Professor Simon R. Wallis from The University of Tokyo, Japan, has demonstrated that grain boundary sliding (GBS) can form the most common natural CPO pattern, known as the "B-type," in which the crystallographic b-axes of antigorite are preferentially aligned parallel to the shear direction. Their findings have been published in Volume 13, Issue 4 of the Progress in Earth and Planetary Science journal on January 21, 2026.
In this study, the team used natural serpentinite samples collected from the Besshi and Shiraga areas in Shikoku, Japan, to investigate the deformation mechanisms of serpentinite at plate boundaries within the Earth's interior. Their finding that antigorite deforms via GBS suggests that serpentinite deformation at plate boundaries is associated with aseismic slip that generates little to no seismic waves and does not produce felt earthquakes.
Nagaya and Wallis highlight the real-life implications of their fundamental work. "Our study improves methods for inferring how rocks deform to form shear zones, enabling a more advanced understanding of rock deformation mechanisms and contributing to insights into deformation processes and earthquake generation in the Earth's interior, particularly in subduction zones."
"Moreover, in recent years, slow earthquakes have attracted attention because of their potential links to great megathrust earthquakes. And our findings may help understand the relationship between slow earthquakes and large earthquakes from the standpoint of materials science," concludes Nagaya.
