Lunar Seimology

Physics

Scientific paper

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Scientific paper

Seismology has the highest resolving power of all geophysical methods used to study the Earth's structure, and, for this reason, it carries the responsibility of determining many parameters critically important to our understanding of the dynamic behaviour of the Earth. This was probably the main scientific motivation for NASA to deploy seismometers on the lunar surface during the Apollo missions. In the period from 1969 to 1972 the US Apollo missions landed seismographs on the lunar surface. These, of which four of the landed stations constituted a seismic array, were positioned in an approximate equilateral triangle with distances between stations being about 1100 km. The array recorded more than 12000 events in the period 1969-1977 which were continuously signaled to Earth. Subsequent examination of the seismograms revealed a highly complex wave train unlike anything observed on the Earth, hampering at times even the detection of the first-arriving P and S-waves, due to intense scattering in a highly porous regolith. The seismic events constitute man-made impacts, meteoroid impacts, shallow moonquakes and deep moonquakes, of which the latter are by far the most numerous. The deep moonquakes are found to occur half-way toward the center of the moon and are believed to be correlated with the tides raised on the moon by the Earth and the Sun. The shallow moonquakes occur in the depth range from 50-220 km and are thought to be akin to intraplate earthquakes. Generally, the Apollo-era studies were successful in determining the gross features of the lunar interior which resulted in the recognition of the Moon as being a differentiated body with a crust and a mantle whose lower parts were thought to be partially molten. However, details remained perfunctory with questions concerning seismic velocity variations and possible discontinuities in the mantle left unanswered. In the present study we show how we have obtained somewhat more detailed information on the lunar interior by inverting first-arrivals of P and S-waves using a Monte Carlo sampling method.

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