A three‐axis short‐period seismometer has been operating on the surface of Mars in the Utopia Planitia region since September 4, 1976. During the first 5 months of operation, approximately 640 hours of high‐quality data, uncontaminated by lander or wind noise, have been obtained. The detection threshold is estimated to be magnitude 3 to about 200 km and about 6.5 for the planet as a whole. No large events have been seen during this period, a result indicating that Mars is less seismically active than earth. Wind is the major source of noise during the day, although the noise level was at or below the sensitivity threshold of the seismometer for most of the night during the early part of the mission. Winds and therefore the seismic background started to intrude into the nighttime hours starting on sol 119 (a sol is a Martian day). The seismic background correlates well with wind velocity and is proportional to the square of the wind velocity, as is appropriate for turbulent flow. The seismic envelope power spectral density is proportional to frequency to the −0.66 to −0.90 power during windy periods. A possible local seismic event was detected on sol 80. No wind data were obtained at the time, so a wind disturbance cannot be ruled out. However, this event has some unusual characteristics and is similar to local events recorded on earth through a Viking seismometer system. If it is interpreted as a natural seismic event, it has a magnitude of 3 and a distance of 110 km. Preliminary interpretation of later arrivals in the signal suggest a crustal thickness of 15 km at the Utopia Planitia site which is within the range of crustal models derived from the gravity field. More events must be recorded before a firm interpretation can be made of seismicity or crustal structure. One firm conclusion is that the natural background noise on Mars is low and that the wind is the prime noise source. It will be possible to reduce this noise by a factor of 103 on future missions by removing the seismometer from the lander, operation of an extremely sensitive seismometer thus being possible on the surface.

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.

In this study, unconfined samples of basalt, sintered and bonded perlite, insulating firebricks, and paraffin were subjected to thermal gradients ranging from 15øC/cm to 100øC/cm. Microshocks produced by thermoelastic stress relief were detected by means of miniature accelerometers. Histograms of microshock occurrences have similar shapes for all tests. Activity begins and increasesabruptly following onset of heating or cooling of the sample. It reaches a maximum, and then decays hyperbolically, in approximate correlation with the time dependence of the thermal gradient. There are later swarms of activity apparently initiated by small thermal fluctuations in the sample. The log-log plots of cumulative number of microshoCks versus amplitude were constructed and show kinks or knees. The value of the negative slope b for the linear approximations to these plots ranges from 1.2 to 2.7. These values are greater than those for normal earthquakes series and are similar to those observed for volcanic B-type earthquakes. The effects of a cellular crack pattern, nonuniformity of stress due to the thermal gradient, and sample inhomogeneity may explain these results. shocks were observed. Lamont-Doherty Geological Observatory Contribution No. 1528.In the first part of this paper we describe the model study and our observed results. In the second part we compare our results to those obtained in earthquake, microearthquake, and microfracture studies. TEST SAMPLESTest materials included basalt, insulating firebrick, paraffin, and perlite. Specimens were generally brick size. The smallest sample tested was approximately 12 x 4 x 3 cm. Specimens used in the vacuum chamber tests, as described below, were approximately 15 X 15 X 7.5 cm.The perlite specimens were manufactured from commercial perlite (a crushed and heattreated perlitic obsidian). The perlite grains are vesicular, irregularly shaped, and microfractured. The test samples were made by two methods: (1) by compressing and sintering the loose grains into bricks, and (2) by bonding the grains with sodium silicate.The insulating firebricks are commercial refactory bricks principally composed of silica and alumina, in a 56/40 ratio by volume. The paraffin samples are polycrystalline and not amorphous. The sample of basalt is aphanitic and nonvesicular with nodules of olivine. The basalt sample was obtained from Vulcan's Throne in the Grand Canyon. 4455 4456 WARREN AND LATHAM

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.

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.