Molecular dynamics simulation of quartz deformation under slow earthquake background

JX Sun and QQ Guo and QL Hou, SCIENCE CHINA-EARTH SCIENCES, 68, 598-610 (2025).

DOI: 10.1007/s11430-024-1469-0

Slow earthquakes are the primary mechanism of slow energy release, and research on the focal mechanism has been inconclusive. Studies have primarily focused on the friction law based on physical mechanisms and have suggested that slow earthquakes are caused by brittle faults. However, the focal strength and structural characteristics of slow earthquakes in subduction zones provide evidence of plastic deformation. What is the role of plastic deformation in the focal mechanisms of slow earthquakes? Mechanochemical study have shown that mechanical forces can directly affect chemical bonds. In this study, we examine the storage and release of chemical energy during plastic deformation and consider a mechanochemical process in the focal mechanism of slow earthquakes. Combined with the Tersoff potential, molecular dynamics simulation on the shear deformation process of two alpha-quartz crystals show that the shear modulus of alpha-quartz is 18 GPa, and that the crystal model primarily exhibits atoms flowing and changing in the direction of chemical bonds during the steady-state flow stage. The molecular potential energy and stress vary in an oscillating up-and-down curve during shear, indicating that chemical energy can be stored and released during plastic deformation. This is consistent with the energy variation during slow earthquakes. Under the initial simple-shear loading condition, alpha-quartz crystals undergo general shear deformation instead of plane strain and the angle between the longest instantaneous stretching axis (ISA1) and the shearing direction is approximately 30 degrees, not 45 degrees. Both the deformation type and direction of ISA1 are contrary to basic deformation theory, which may provide clues for future research. This study reveals the process of quartz elastic- plastic shear deformation on an atomic scale. This information is useful for understanding focal mechanisms of slow earthquakes. This study is part of a series of investigations on tectonic stress chemistry.

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