Abstract
Many features became clear about deep moonquakes which are detected by seismological observation in the Apollo missions during 1969 - 1977. However, the details about behavior of tidal stresses in the focus region of deep moonquakes are not becoming clear, and it is only conjecture in the present which passed through a little more than 30 years from the Apollo missions.
In this study, in order to construct the first step of a new theory about the relationship between lunar tidal stress and deep moonquakes, we noted a behavior of tidal stress itself.
Calculation of tidal stresses has adopted a semi-analytical lunar ephemeris (ELP 2000/82) which is proud of the high precision given by Chapront and Chapront (1982) about the relative position of the Earth-Moon system, using the y function method given by Takeuchi (1950). A used lunar internal structure model is based on Nakamura et al. (1982) obtained by the analysis of the Apollo seismic data and a density structure uses Tanaka et al. (1990).
The size and amplitude of tidal six components which were calculated change with the depth or positions, even when the internal structure of the moon is considered as fixation. We picked up the largest tidal stress component at a depth of 900km at every 5deg. grid points from longitude 0 to 90deg. and latitude 0 to 80deg. for a model with a core of 400km in radius and plotted in the selenographical map, a tidal stress distribution will be qualitatively classified into six zones: (1) Low longitude and Low latitude zone, (2) Middle longitude and Low-to-middle latitude zone, (3) High longitude zone, (4) Low-to-middle longitude and Middle-to-high latitude zone, (5) Middle longitude and High latitude zone, and (6) Low longitude and High latitude zone. These zones are dominated by SIGMArr, SIGMArphi, SIGMAphiphi, SIGMArtheta, SIGMAthetaphi, and SIGMAthetatheta, respectively. If this is checked with the hypocenter distribution of deep moonquakes, the hypocenter will be concentrated on the SIGMArr- and the SIGMArphi-dominated zones, especially the SIGMArphi-dominated zone, and it will be thought that these components are working as a dominant trigger to an occurrence of deep moonquakes. Itagaki et al. (2003) concluded the possibility of the existence of a toroidal motion of the lunar interior, such as the east and/or west-ward flow in the lunar deep mantle. If the deep moonquake is controlled by the tidal shear stress, the observed depth distribution can be explained by a core model having the core radius of 400 to 600 km.
Consequently, although the focus which can be grasped clearly also exists that the SIGMArr component is a trigger in the SIGMArr zone which is next concentrating focus, the moonquake of the half which belongs to this zone has generated at the very deep point of a depth of 1100km or more, where the SIGMAthetatheta and the SIGMAphiphi components dominate. The result indicates the deep moonquakes which occur at points deeper than 1100km may be generated by different mechanism from the deep moonquakes which occur in the average depth of around 900km.
