Processing and initial analysis of the entire set of Apollo lunar seismic data collected continuously from 1969 through 1977 have now been completed. Recent results include: 1) better defined deep moonquake locations, which appear to be bounded rather sharply between about 800 km and 1000 km depths with concentrations near both boundaries; and 2) middle mantle (∼500 to 1000 km depth) seismic velocities of Vp, = 8.3 ± 0.4 km/sec and Vs = 4.6 ± 0.2 km/sec, which are significantly higher than previous estimates and represent an increase of velocities from the upper mantle as opposed to a decrease in previous estimates.

We have reexamined the structure of the lunar mantle by using data on the decay of shear wave amplitude with distance and the relative arrival times of P and S waves. The new analysis confirms our previous lunar model based primarily upon travel times and more closely defines certain properties of the lunar mantle. A negative shear wave velocity gradient of 0.0013 (km/s)/km, shear wave Q of 4000, and Poisson's ratio of 0.250 4-0.025 are found for the upper mantle, which lies between the depths of 60 and 300 km. A rapid decrease of shear wave velocity with increasing depth ig observed starting at around a 300-km depth, associated with a lower Q for shear waves. Poisson's ratio in the lower part of the middle mantle, which extends to a depth of about 1000 km, is estimated to be 0.36 4-0.02.

We reexamined the horizontal‐to‐vertical amplitude ratios of the long‐period seismograms to determine the shear wave velocity distributions at the Apollo 12, 14, 15, and 16 lunar landing sites. Average spectral ratios, computed from a number of impact signals, were compared with spectral ratios calculated for the fundamental mode Rayleigh waves in media consisting of homogeneous, isotropic, horizontal layers. The shear velocities of the best fitting models at the different sites resemble each other and differ from the average for all sites by not more than 20% except for the bottom layer at station 14. The shear velocities increase from 40 m/s at the surface to about 400 m/s at depths between 95 and 160 m at the various sites. Within this depth range the velocity‐depth functions are well represented by two piecewise linear segments, although the presence of first‐order discontinuities cannot be ruled out.

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Seismometers on the moon have detected several brief periods of enhanc ed meteoroid-impact activity, believed to represent encounters of the moon with "c louds" of objec ts in the kilogram range. The latest and most active encounter, in June 1975, is interpreted as a meteoroid c(loud of diameter 0.1 astronomical unit and total mass 10(l3) to 10(14) grams.