Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.u41c0831f&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #U41C-0831
Other
1221 Lunar And Planetary Geodesy And Gravity (5417, 5450, 5714, 5744, 6019, 5770 Tidal Forces, 6250 Moon (1221), 7223 Earthquake Interaction, Forecasting, And Prediction (1217, 1242)
Scientific paper
Understanding variations in the tidal response of deep moonquake (DMQ) nests may help us understand the origin of deep moonquakes. We have grouped nests of deep moonquakes into categories depending on how strongly their occurrence is influenced by the anomalistic or draconic month and the phase of this dependence. For well-located nests having 20 or more individual events, epicenters of some categories are geographically separated from one another on the Moon. The tidal potential caused by the Earth with mass M at distance R varies over the Moon's surface and is proportional to (3cos2θ - 1)M/R3, where θ is the angle between the point of interest and an axis E-M joining the line of centers. The solid tidal deformation causes the Moon's shape to be ellipsoidal with the long axis directed along E-M. The potential is zero when θ is 24° and thus along a small circle of this radius the tangential strain is zero. When θ is 55° the surface curvature of the tidal ellipsoid is unchanged and thus the radial strain is zero. For any fixed location on the Moon the tidal deformation changes because the E-M axis moves ±8° EW over the Moon's surface during the anomalistic month as the Earth-Moon distance changes and affects the angular rate at which the Moon orbits Earth. The E-M axis moves ±7° NS during the draconic month because the orbital plane doesn't coincide with the Moon's equator. The Sun affects the orientation of the long axis of the tidal deformation ellipsoid by less than 0.3°. We thus determine the angular distance θ between individual DMQ and the E-M axis, and search for features in the distribution of hypocenters near the nodes at 24° and 55°. We separate DMQ into three categories: wA - occurrence weakly correlated with anomalistic phase; sAp - occurrence strongly correlated with anomalistic phase; DMQ occur mostly as Moon approaches perigee; sAa - occurrence strongly correlated with anomalistic phase; DMQ occur mostly as Moon approaches apogee. DMQ in the sAp group are rare or absent at θ<~20°; DMQ in the sAa group are common at θ<~20°; the wA group doesn't appear to correlate with distance. Similarly, if we use draconic phase behavior as a classifier we find that DMQ strongly correlated with draconic phase are rare or absent at θ<~20° whereas DMQ weakly correlated with draconic phase do not appear to correlate with distance. We speculate that the different categories of DMQ may represent distinct mechanical phenomena. For example, the sAa group may represent the growth of faults as the shape of the Moon changes in response to its slow retreat from the Earth. The sAp group may come about because tidal shear stresses drive fluids that repeatedly shift blocks within the lunar mantle.
Fröhlich Carla
Nakamura Yoshifumi
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