INSIGHTS OF DEEP MOONQUAKES CAUSAL PHYSICAL MECHANISM

Abstract:

Fifty years after the Apollo 11 mission, some questions on the geophysical characteristics of the Moon still remain unanswered. One of them involves the casual mechanism of deep moonquakes, which are low magnitude seismic events (M ≤ 2) that ranges from 700 km to 1200 km in depth. Although the interpretation of seismological data from the Apollo Missions has provided essential information on the composition and structure of the Moon, a relationship between this structuration and how the gravitational forces in the Earth-Moon-Sun system influence deep moonquakes is still unclear. Previous studies have attributed periodicities of deep moonquakes occurrence to gravitational solid tides produced by the motion of the Moon around the Earth, such as the synodic, anomalistic and sidereal periods. However, cyclic tidal stress is not sufficient to cause rupture at confining pressures where deep moonquakes occur. In addition, some of maximum tangential tidal stress do not coincide with periods of occurrence of deep moonquakes. In this context, this study aims to improve the understanding of deep moonquakes mechanism, exploring the causalities due to combinations of orbital periods of the Moon-Earth-Sun system and gravitational forces. We analyzed a time series of deep moonquakes with the number of seismic events per day, and calculated correspondent orbital parameters, such as variations in distance, angular and temporal occurrence between the Moon and the Sun-Earth orbital planes. This analysis was performed using Machine Learning and Wavelet approaches. In contrast to previous studies, our preliminary results suggest that the orbitals elements subject to solar disturbances do not significantly influence deep moonquakes. Alternately, we suggest that deep moonquakes are mainly related to orbital parameters that describe the position of the Moon in its orbital plane, in relation to the coordinate system referenced by the ecliptic and equinox of the J2000.0 epoch.