Seismometer operation for 21 days at Tranquillity Base revealed, among strong signals produced by the Apollo 11 lunar module descent stage, a small proportion of probable natural seismic signals. The latter are long-duration, emergent oscillations which lack the discrete phases and coherence of earthquake signals. From similarity with the impact signal of the Apollo 12 ascent stage, they are thought to be produced by meteoroid impacts or shallow moonquakes. This signal character may imply transmission with high Q and intense wave scattering, conditions which are mutually exclusive on earth. Natural background noise is very much smaller than on earth, and lunar tectonism may be very low.
One year of ambient ocean noise data, 0.4 to 30 Hz, from the Wake Island hydrophone array in the northwestern Pacific are compared to surface wind speeds, 0–14 m/s (0–28 kn). Between 0.4 and 6 Hz, noise levels increase with wind speed at rates of up to 2 dB per m/s until a saturation is reached having a slope of about −23 dB/octave and a level of 75 dB relative to 1 μPa/√Hz at 4 Hz. This noise saturation, called the ‘‘Holu Spectrum,’’ likely corresponds to saturation of short-wavelength ocean wind waves. It is probably a worldwide constant. Between 4 and 30 Hz, noise also increases with wind speed at rates of up to 2 dB per m/s, but no saturation level is observed and the slope increases to about 4 dB/octave. This may be acoustic noise from whitecaps. On a hydrophone less than 3 km from Wake, noise between 0.5 and 10 Hz increases with wind speed at a rate up to 2 dB per m/s, but absolute noise levels are significantly higher than levels on the other hydrophones more distant from Wake, and no saturation is apparent. Surf breaking against the shore of the island is the probable source of this noise.
Unusually long reverberations were recorded from two lunar impacts by a seismic station installed on the lunar surface by the Apollo 12 astronauts. Seismic data from these impacts suggest that the lunar mare in the region of the Apollo 12 landing site consists of material with very low seismic velocities near the surface, with velocity increasing with depth to 5 to 6 kilometers per second (for compressional waves) at a depth of 20 kilometers. Absorption of seismic waves in this structure is extremely low relative to typical continental crustal materials on earth. It is unlikely that a major boundary similar to the crustmantle interface on earth exists in the outer 20 kilometers of the moon. A combination of dispersion and scattering of surface waves probably explains the lunar seismic reverberation. Scattering of these waves implies the presence of heterogeneity within the outer zone of the mare on a scale of from several hundred meters (or less) to several kilometers. Seismic signals from 160 events of natural origin have been recorded during the first 7 months of operation of the Apollo 12 seismic station. At least 26 of the natural events are small moonquakes. Many of the natural events are thought to be meteoroid impacts.
Many thousands of seismic events have been recorded by the short‐period components of the Apollo passive seismic network. The majority of these events appear to be small local moonquakes triggered by diurnal temperature changes. These events, termed thermal moonquakes, are recognized by the repetition of nearly identical signals at the same time of each lunation. Forty‐eight different types of thermal moonquakes have been identified at the station 14 site, and 245 different types at the station 15 site. Thermal moonquake activity starts abruptly, about 2 days after lunar sunrise and decreases rapidly after sunset. Two possible source mechanisms for thermal moonquakes are suggested: fracturing or movement of rock along zones of weakness and slumping of soil on lunar slopes triggered by thermal stresses. The latter mechanism is favored, since characteristics of the signals imply that motion is always in the same direction and that some thermal moonquake sources change position from one event to the next. The observations presented provide the first data on an active mechanism for degradation of the lunar surface by non‐meteoritic effects. The relative efficiency of thermal erosion and impact erosion is yet to be determined.
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