Lunar seismicity, structure, and tectonics

Abstract

Natural seismic events have been detected by the long‐period seismometers at Apollo stations 16, 14, 15, and 12 at annual rates of 3300, 1700, 800, and 700, respectively, with peak activity at 13‐ to 14‐day intervals. Repetitive moonquakes from 41 hypocenters produce seismograms characteristic of each. About 90% of the long‐period signals are from these and other numerous, less active hypocenters, and meteoroid impact signals account for the remainder. At each hypocenter, moonquakes occur only within an active period of a few days during a characteristic phase of the monthly lunar tidal cycle. An episode of activity may contain up to four quakes from one hypocenter. Nearly equal numbers of hypocenters are active at opposite phases of the monthly cycle, accounting for the 14‐day peaks in total lunar seismic activity. A period of about 206 days in the seismic activity of several of the hypocenters is superimposed on a strong one‐to two‐year trend where the signal amplitudes decrease to the instrumental detection threshold. A 206‐day period with no secular decrease in amplitude is also observed in the total lunar seismic activity, suggesting that the total number of active hypocenters does not vary appreciably with time. Moonquake magnitudes range between 0.5 and 1.3 on the Richter scale with a total energy release estimated to be about 1011 ergs annually. With several possible exceptions, the moonquake foci located to date occur in two narrow belts on the near side of the moon. Both belts are 100–300 km wide, about 2000 km long, and 800–1000 km deep, and they lie along great‐circle arcs. Seismic data from a far‐side focus and a large far‐side meteoroid impact define the base of the lunar lithosphere at a depth of about 1000 km. In our present model the rigid lithosphere overlies an asthenosphere of reduced rigidity in which present‐day partial melting is probable. Tidal deformation presumably leads to critical stress concentrations at the base of the lithosphere, where moonquakes are found to occur. The striking tidal periodicities in the pattern of moonquake occurrence and energy release suggest that tidal energy is the dominant source of energy released as moonquakes. Thus, tidal energy is dissipated by moonquakes in the lithosphere and probably by inelastic processes in the asthenosphere. The low level of seismicity and the absence of shallow seismicity implies that the moon is neither expanding nor contracting at an appreciable rate. The secular accumulation of strain implied by the uniform polarities of moonquake signals may result from weak convection in the asthenosphere or from secular recession of the moon from the earth.