Lunar Seismic Tomography and the Early Earth

Seismic tomography reveals significant lateral heterogeneities in the lunar interior. A correlation is found between S-wave velocity tomography and distribution of thorium. The area with a high distribution of thorium exhibits a distinct low S-velocity, which extends to a depth of similar to 300 km below the Procellarum KREEP Terrane (PKT), perhaps reflecting a thermal and compositional anomaly beneath the PKT. The distribution of deep moonquakes shows a correlation with tomography in the deep lunar mantle, which is similar to earthquakes affected by structural heterogeneities in the terrestrial crust and upper mantle. The occurrence of deep moonquakes and seismic-velocity heterogeneities implies that the lunar interior may contain fluids; therefore, it is still thermally and dynamically active at present. Because there are no plate tectonics in the Moon, the lunar surface and interior structure formed at an early stage of the Moon's history have been preserved until today. Consequently, the results of lunar tomography provide useful information for our understanding of the Hadean Earth.