Abstract: Research on the slow-earthquake focal mechanism was mainly based on brittle deformation. However, high ratios of P-wave to S-wave velocity, and anomalously high Poisson’s ratios indicate that the source material of slow earthquakes is more viscous and more prone to plastic deformation. Traditional studies suggest that plastic deformation begins with dislocation, and is permanent. Therefore, the plastic strain energy cannot be released. However, it may be different when talked about on a microscopic molecular scale. The lithosphere is mainly constructed of silicate minerals, especially of tetracoordinated compounds with [SiO₄] tetrahedrons as fundamental units. That is, within minerals, there are chemical bonds binding various atoms together. Therefore, the dislocation mechanism of plastic deformation may first start with the change of chemical bonds, and then the chemical bonds begin to break off and rebond, forming sub-grains and grain size reduction. Mechanochemical study shows that the mechanical force can directly act on the chemical bond via stretching and rotating, change the bond length and angle, and finally break off the chemical bond. Quantum chemical calculations on molecular fragments of the crystalline structure in coal indicate that the carbon bond breaks off at high energy levels, when the hydroxyl group of the 6-membered benzene ring falls off to form CO and 5-membered rings. During plastic deformation, the energy conversion may be work done from mechanical energy by external forces firstly to internal energy as atomic potential energy, and then plastic strain energy. However, not all atomic potential energy could transform into plastic strain energy for the reconstruction of the chemical bond may release energy. As a result, a little energy may be released during the plastic deformation. Whether this is related to slow earthquakes is a scientific proposition worth exploring. It may be a possible way to explore the focal mechanism of slow earthquakes by using atomic-scale quantum chemistry calculations to establish a finer energy change process of crystal plastic deformation and comparing the source parameters of slow earthquakes.

Key words: slow earthquake, focal mechanism, microscopic mechanism of plastic deformation, plastic strain energy, tectonic stress chemistry