Moon: Seismicity | SpringerLink

Moon: Seismicity
As a terrestrial planet evolves, its internal temperature changes. The initial accretional energy and the energy released by differentiation and by decay of radioactive materials dissipate via radiation to space. This causes strain to accumulate inside the planet. The strain may eventually be released as materials experience brittle failure; energy is released suddenly as stress waves, producing seismic events. In addition to these seismic events of internal origin, there are other types of seismic events induced by external causes, such as gravitational and radiational effects of the Sun and other planets, and impacts of small interplanetary objects. Quakes due to all of the above causes are observed on the Moon.
At present the only observational data concerning the seismicity of the Moon are from the network of seismic stations established on the lunar surface during the Apollo missions (q.v.; Plate 14). Using the data recorded between 1969 and 1977, researchers identified four types of seismic activities: deep moonquakes, shallow moonquakes, meteoroid impacts and thermal moonquakes. Figure M95 shows seismograms of the first three types.
Deep moonquakes
A large number of small moonquakes having nearly identical waveforms occur at great depths (700–1200 km) at highly regular monthly intervals (Lammlein et al., 1974). The identical waveforms imply that they occur within a small hypocentral region. From a cross-correlation analysis of waveforms, Nakamura (1978) estimates that the hypocentral region is no greater than about 1 km in diameter. More than 3000 such events belonging to 109 separate hypocentral regions were identified (Nakamura, Latham and Dorman, 1982). The equivalent body-wave magnitude of individual events is estimated to be less than 2.
The focal depths of these moonquakes range from 700 km to 1200 km, about halfway to the center of the Moon and deeper than any known deep-focus earthquakes. Their epicenters appear to cluster within a few seismic belts, and they are nearly absent in the southeast quadrant of the Moon, which is mostly highland (Figure M96). All but one of the known epicentral regions are on the frontside of the Moon. This may indicate that there are few deep moonquakes on the mostly mountainous farside of the Moon, or may simply reflect an observational bias because all the Apollo seismic stations were on the frontside of the Moon. The deep interior of the Moon below the level of the deepest moonquakes hypocenters is thought to be partially molten (Figure M97), and this zone attenuates shear waves from moonquakes on the far side. The absence of shearwave arrivals prevents us from positively identifying and locating farside moonquakes even if they exist.
Figure M96
Moonquake epicenters. Each of the closed circles represents an epicentral region within which many deep moonquakes occur at monthly intervals. Open circles represent epicenters of observed shallow moonquakes. Those shown outside the rim of the Moon indicate epicenters located on the farside of the Moon.
The monthly periodicity of deep-moonquake activity suggests a strong relationship between deep moonquakes and tides raised on the Moon by the Earth and the Sun. At present, it is uncertain whether the tide merely triggers a release of accumulated tectonic strain, or if the tide is wholly responsible for these quakes. However, several lines of evidence favor the latter hypothesis. (1) There is a positive correlation between moonquake magnitudes and tidal amplitudes. (2) The orientation of the deep moonquake slip vector rotates with tidal phases (Nakamura, 1978). (3) Deep moonquake stress drops (∼ 10 kPa; Goins, Dainty and Toksöz, 1981) are significantly lower than the maximum tidal stress inside the Moon (∼ 100 kPa; Cheng and Toksöz, 1978). (4) The total energy release by all deep moonquakes is much less than the amount of energy tidally dissipated from the Earth—Moon system (see Tidal friction).
Shallow moonquakes
Rare but relatively strong moonquakes occur at shallow depths (Nakamura et al., 1979; Nakamura, 1980). These events are characterized by having unusually high frequencies for recorded seismic waves, compared with other lunar and earthquake events. Only 28 shallow moonquakes were identified during the 8 years of observation. Unlike deep moonquakes, their occurrence is not correlated with tides, and no multiple events have been detected from a single source region. Even though much fewer in number than deep moonquakes, their magnitude is much larger. The estimated body-wave magnitudes of the three largest ones observed in 8 years exceed 5.5.
The exact depths of shallow moonquakes are as yet unknown. This is because all those observed occurred outside the Apollo seismic array. Also, intense near-surface scattering of seismic waves prevented the identification of depth phases, the surface-reflected seismic arrivals which give a direct measure of focal depth for earthquakes. However, indirect evidence suggests that most of them occur in the upper mantle of the Moon, between 50 km and 200 km depths. Their epicenters (Figure M96) appear to occur near boundaries between dissimilar surface features, such as the edges of major impact basins. However, the paucity of the observed events precludes definitive conclusions.
The unusually high frequency content of seismic signals from shallow moonquakes suggests exceptionally high stress concentration at the source. Stress drops associated with large shallow moonquakes are estimated to exceed 100 MPa (Oberst, 1987). These events have seismic moment on the order of 1015 Nm. The high stress drop is consistent with a model in which thermoelastic strain within a thick lithosphere is released by occasional shallow moonquakes as the Moon cools.
Meteoroid impacts
Because the Moon has no atmosphere, meteoroids colliding with the Moon reach the surface without ‘burning out’; the seismic waves generated by their impacts constitute yet another type of seismicity. Only a very small fraction of a meteoroid's kinetic energy is converted to seismic energy, because the majority is spent excavating a crater and heating the material. Nevertheless, many impacts have sufficient energy to be detectable at seismic stations. The long-period seismometers at Apollo stations detected more than 1700 such seismic events, representing meteoroids of masses of roughly 100 g to 100 kg, during the 8-year observation period. Those generated by impacts of smaller objects and detected only by short-period seismometers were too numerous to be counted.
Observations of impact-generated seismicity provide a unique census of the population of small objects in interplanetary space, and are complementary to Earth-based observations of meteors and meteorites. Two distinct types of objects crossing the Earth—Moon orbit have been identified from the seismic data: cometary and asteroidal (Oberst and Nakamura, 1991). The former objects are mostly lighter and less energetic, and many of them occur in clusters identifiable with known meteor showers. The latter include many massive objects, in mostly prograde orbits, and show a lesser degree of clustering. However, the asteroidal objects also include some prominent swarms, which may be concentrated debris from relatively recent disintegration of asteroids in Earth-crossing orbits (see Near-Earth object).
Thermal moonquakes
The Apollo seismometers detected numerous very small seismic events originating from many isolated locations within a few kilometers of each seismic station (Duennebier and Sutton, 1974). Similar to deep moonquakes, these events also possessed nearly identical waveforms repeating at regular monthly intervals. However, most of these micromoonquakes occurred during daylight hours, with high activity starting about 2 days after sunrise.
The clear diurnal periodicity suggests that these moonquakes are either induced or triggered by thermoelastic stresses due to temperature variations on the lunar surface. It is possible that they represent slumping on lunar surface slopes triggered by thermal stresses.
Comparison with the Earth and other planets
The types of seismicity we observe on the Moon are different from those observed on the Earth. Both tidally induced earthquakes (the possible counterpart of the deep moonquakes) and thermal earthquakes may occur inside the Earth. However, even if they do, their magnitudes are too small to be detectable above the Earth's microseismic background noise. These two types of seismicity are observable on the Moon simply because the Moon's surface is so seismically quiet relative to the Earth. This is because the Moon has neither atmosphere nor ocean (the major sources of microseisms on the Earth). Practically all meteoroids encountering the Earth disintegrate as they enter the Earth's atmosphere, and those which reach the ground (meteorites) have decelerated sufficiently that they rarely generate globally detectable seismic waves.
Shallow moonquakes appear to be the only truly tectonic quakes in the Moon. However, even though the overwhelming majority of earthquakes are of tectonic origin, there are significant differences between shallow moonquakes and earthquakes. Most earthquakes occur along lithospheric plate boundaries, where large stresses caused by the relative motion of plates are concentrated. In contrast, there are no surface geologic features on the Moon that indicate the existence of moving lithospheric plates. Apparently, the extraordinary thickness of the Moon's lithosphere (Figure M97) precludes the occurrence of plate-like segmentations. Thus it is not surprising that no moonquakes similar to typical interplate earthquakes have been observed. On the other hand, shallow moonquakes do have many similarities with intraplate earthquakes. These are the earthquakes that occur in the interiors of lithospheric plates, usually along some pre-existing weaknesses, and include some of the largest earthquakes ever observed. As the Moon has a single lithospheric plate, all moonquakes are ‘intraplate’; thus such similarities are not surprising.
There is a significant difference in the overall level of seismicity between the Earth and the Moon. The total energy annually released by earthquakes is about 1018 J (1025 erg); in contrast, moonquakes annually release about 1012 J (1019 erg), mostly from shallow moonquakes. Because the Moon is smaller than the Earth, its interior has cooled faster and thus there is significantly less tectonic activity at the present time. However, the Moon is by no means a dead planet.
Comparisons with other planets are not straightforward because seismicity on other planets is yet to be observed. Limited observations on Mars during the Viking mission set certain limits on its seismicity, but these data only indicate that Mars is not as active seismically as the Earth. Whether it is more or less active than the Moon is not yet known. The present level of seismicity of various terrestrial planets may be estimated, based on analysis of surface geologic features, but the validity of such estimates needs to be verified with future direct observations.