Cause of Deep Moonquakes

Details Cause of Deep Moonquakes Cause of Deep Moonquakes

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p31b1407n&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #P31B-1407

Physics

1221 Lunar And Planetary Geodesy And Gravity (5417, 5450, 5714, 5744, 6019, 6250), 6250 Moon (1221), 7230 Seismicity And Tectonics (1207, 1217, 1240, 1242)

Scientific paper

It is well known that the occurrence of deep moonquakes is highly correlated with the solid tides raised by Earth and the Sun. However, it has long been debated whether the tides are simply acting as a triggering mechanism for a release of accumulated tectonic stresses in the Moon's interior or the tides themselves are responsible for their generation, releasing tidally dissipated energy in the form of moonquakes. One way to test which of these two hypotheses is correct is to see when deep moonquakes occur relative to the long- term tidal amplitude variations caused by the changing position of the Sun relative to the eccentric orbit of the Moon around Earth. If the tides are simply acting as a trigger mechanism, deep moonquakes are more likely to occur when the tidal stress amplitudes are increasing, while if the tides are the main cause of deep moonquakes, they are more likely to occur shortly after the peaks in tidal stress amplitudes. We thus examined the frequency of deep moonquake occurrence as reported in the recently updated lunar event catalog relative to the difference between the anomalistic and synodic phases. Tidal stress amplitude reaches maxima when this phase difference is 0°, i.e., a new moon coincides with a perigee crossing, and when it is 180°, i.e., a full moon coincides with a perigee crossing. The result shows a general trend of maximum activity shortly following each of the tidal amplitude maxima, supporting the tidal generation hypothesis. However, an additional peak activity is found shortly before the tidal amplitude maximum at phase difference of 180°, also supporting the tidal triggering hypothesis in certain restricted cases. This secondary peak is limited only to some, but not all, deep moonquake nests. These trends are independent of whether the deep moonquake epicenters are located in either of the E-W hemispheres and in which of the tidal stress regimes as determined by the distance to the sub-earth point. Thus it appears that deep moonquakes generally represent release of tidally dissipated energy with additional triggered release of accumulated tectonic stress at a limited number of specific locations. However, this conclusion is provisional because the Apollo data did not cover the entire 18-year tidal cycle of the Moon.

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