Lunar seismicity, structure, and tectonics

Abstract

Interpretation of lunar seismic data results in a lunar model consisting of at least four and possibly five distinguishable zones: (I) the 50-60 km thick crust characterized by seismic velocities appropriate for plagioclaise rich materials, (II) the 250 km thick upper mantle characterized by seismic velocities consistent with an olivine-pyroxene composition, (III) the 500 km thick middle mantle characterized by a high Poisson’s ratio, (IV) the lower mantle characterized by high shear-wave attenuation, and possibly (V) a core of radius between 170 and 360 km characterized by a greatly reduced compressional wave velocity. The Apollo seismic network detects several thousand deep moonquake signals annually. Repetitive signals from 60 deep moonquake hypocentres can be identified. The occurrence characteristics of the moonquakes from the individual moonquake hypocentres are well correlated with lunar tidal phases and display tidal periodicities of 1 month, 7| months, and 6 years. With several possible exceptions, the deep moonquake foci located to date occur in three narrow belts on the near side of the Moon, and are concentrated at depths of 800-1000 km. The locations of 17 shallow moonquake foci, although not as accurate as those of the deep foci, show fair agreement with the deep moonquake belts. Focal depths calculated for the shallow moonquakes range from 0-300 km. The moonquakes of a particular moonquake belt, or a region within a belt, tend to occur near the same tidal phase suggesting similar focal mechanisms. Deep moonquake magnitudes range from about 0.5 to 1.3 on the Richter scale with a total energy release estimated to be about 10 11 ergs (10 4 J) annually. The largest shallow moonquakes have magnitudes of 4-5 and release about 10 15 -10 18 ergs (10 8 -10 11 J) each. Tidal deformation of a rigid lunar lithosphere overlying a reduced-rigidity asthenosphere leads to concentrations of strain energy near the base of the lithosphere. Although tidal strain energy can account for the deep moonquakes in this model, it cannot account for the shallow moonquakes. Tidal stresses within the lunar lithosphere range from about 0.1 to 1 bar (10 4 -10 5 Pa). This low level of tidal stresses suggests that tides act as a triggering mechanism. The secular accumulation of strain implied by the uniform polarities of the deep moonquake signals probably results from weak convection. A convective mechanism could explain the distribution of moonquakes, the Earth-side topographic bulge, the distribution of filled mare basins, and the ancient lunar magnetic field.