Preliminary analysis of the relationship between the thermal stress of the Moon in cooling process and moonquake mechanisms

Numerous studies have shown that the periodicity of deep moonquakes is caused by the tidal force from the earth to the moon. However, the maximum tidal stress is too small to cause rock fracture for the occurrence of deep moonquakes, which indicates that there should be other background stresses. Due to the lack of plate activity, the mechanism of shallow moonquakes also remains to be resolved. We developed a parameterization scheme to simulate the cooling history of the moon. The thermal stress accumulated after a short period of time is calculated, as well as the long-time thermal stress with relaxation. We pay special attention to the characteristics of recent stress accumulation in the deep and shallow moonquake zone. The study shows that thermal stress in the shallow moonquake area causes mainly horizontal extrusion, resulting in overthrust fault moonquakes, which is consistent with observation. Although the thermal stress concentrates in the deep moonquake area either, it is not sufficient to cause the failure of dry rocks. We speculate that the fluid from the part melt layer may be a possible origin of deep moonquakes. The results in this article preliminarily reveal the relationship between the thermal stress and the mechanism of deep and shallow moonquakes, which may be helpful for future analysis of these two types of moonquakes.