Shallow Moonquakes Research Articles

An inquiry into the lunar interior: A nonlinear inversion of the Apollo lunar seismic data

This study discusses in detail the inversion of the Apollo lunar seismic data and the question of how to analyze the results. The well‐known problem of estimating structural parameters (seismic velocities) and other parameters crucial to an understanding of a planetary body from a set of arrival times is strongly nonlinear. Here we consider this problem from the point of view of Bayesian statistics using a Markov chain Monte Carlo method. Generally, the results seem to indicate a somewhat thinner crust with a thickness around 45 km as well as a more detailed lunar velocity structure, especially in the middle mantle, than obtained in earlier studies. Concerning the moonquake locations, the shallow moonquakes are found in the depth range 50–220 km, and the majority of deep moonquakes are concentrated in the depth range 850–1000 km, with what seems to be an apparently rather sharp lower boundary. In wanting to further analyze the outcome of the inversion for specific features in a statistical fashion, we have used credible intervals, two‐dimensional marginals, and Bayesian hypothesis testing. Using this form of hypothesis testing, we are able to decide between the relative importance of any two hypotheses given data, prior information, and the physical laws that govern the relationship between model and data, such as having to decide between a thin crust of 45 km and a thick crust as implied by the generally assumed value of 60 km. We obtain a Bayes factor of 4.2, implying that a thinner crust is strongly favored.

A new seismic velocity model for the Moon from a Monte Carlo inversion of the Apollo lunar seismic data

A reanalysis of the Apollo lunar seismic data and the subsequent application of an inverse Monte Carlo method to P and S‐wave arrival times has resulted in a more detailed lunar velocity structure than previously obtainable. The velocity is seen to increase from the surface down to the base of the crust at 45±5 km depth. The results furthermore indicate a constant velocity upper mantle extending to 560±15 km km depth, separated from a more complex high velocity middle mantle by an increase in velocity of 1.0 km/s. In addition, the moonquake locations have been improved. The shallow moonquakes are found to be located in the depth range 50–220 km. The majority of deep moonquakes are concentrated in the depth range 850–1000 km with an apparently rather sharp lower boundary.

Unusually high stress drops associated with shallow moonquakes

New estimates of stress drops and energy release associated with shallow moonquakes, believed to represent the only lunar seismic events of tectonic origin, have been made, taking into account (1) properties of source spectra, (2) radiation pattern of foci, (3) anelastic attenuation of seismic waves, (4) response of the seismometer, and (5) intense scattering of seismic signals. The results indicate that the three largest events show body wave magnitudes of mB > 5.5 and stress drops of Δσ > 100 MPa. The values for stress drops are high compared with those generally associated with most terrestrial events, but they are compatible with results predicted from thermoelastic models of the moon. Presence of high stresses in the lunar crust may indicate that at the beginning of its history the moon was originally molten not only in the outer surface region, but throughout its whole volume.

Shallow moonquakes and large shallow earthquakes: a temporal correlation

The structure and seismicity of the Moon is very different from that of the Earth. One point of similarity is that statistics of seismic energy release in both bodies are dominated by the largest shallow-focus events. This study presents evidence of a relationship linking the timing of the 28 known shallow moonquakes with the timing of the 26 largest shallow earthquakes occurring in the same years (1971–1976). Distributions of the geocentric ecliptic longitude of the Moon λ m for the time of occurrence of the lunar and terrestrial seismic events were analyzed both separately and in combination, for the entire 6-year period and for individual years, with the following results: (1)|Statistically significant clustering is present in the series of λ m for the times of the earthquakes and moonquakes for the entire sample period (random probability = 0.0036) and for 2 of the 6 individual years (random probabilities ⩽ 0.02). (2)|Phases of the separate lunar and terrestrial seismic event λ m distributions for the 6-year period differ by less than 7°. Phases of the earthquake and moonquake distributions are found in the same quadrant of the range in 5 of the 6 individual years. (3)|Annual levels of shallow-focus seismic activity in the Earth and Moon vary in concert in the years 1971–1976.

High stress shallow moonquakes: evidence for an initially totally molten moon

Thermoelastic stress calculations show that if the moon was initially molten only in the outer few hundred kilometers as in the magma ocean model of the moon, the highlands crust should be aseismic. In contrast, if the moon was initially totally molten, high stress (1 to ≳ 3 kbar), shallow (0 to ∼ 6 km deep), compressional moonquakes should be occurring in the highlands crust. Calculations of the minimum stress drops made for the 28 observed shallow moonquakes suggest that 3 of them probably have stress drops in the kbar range. Thus, these very limited seismic data are consistent with the model that the moon was initially totally molten.

Seismic energy release of the Moon

Previous estimates of the seismic energy released by lunar events have not properly accounted for instrument bandwidth, variations in corner frequency, and the effects of intense scattering. In this paper, equations are developed that include all of these effects and give realistic estimates for source parameters. These equations are applied to seismograms and displacement spectra from near‐surface and deep moonquakes to obtain M0 (seismic moment), Ēs (seismic energy release), Eyr (seismic energy released annually by each lunar event class), Δσ (stress drop), and mb (body wave magnitude). The calculations yield M0 ∼ 3 × 1021 dyn cm, Ēs ∼ 2 × 1017 ergs, Δσ ∼ 400 bars, and mb ∼ 5.0 for the largest shallow moonquakes; and M0 ∼ 5 × 1020 dyn cm, Ēs ∼ 1 × 1013 ergs, Δσ ∼ 0.1 bars, and mb ∼ 3.0 for large deep events. The average energy released annually is Eyr = 2 × 1017 erg/yr and Eyr = 8 × 1013 erg/yr by shallow and deep events, respectively; overall, lunar energy release is dominated by the shallow events. The energy released by the deep events may be accounted for by tidal dissipation, and deep event stress drops are comparable to and smaller than the calculated tidal stresses. A comparison of the above values with those observed terrestrially (Eyr ∼ 1025 ergs) and with the energy available from heat flow and tidal dissipation emphasizes the importance of tectonic style (e.g., plate tectonics) in determining the characteristics of planetary seismicity.

Shallow moonquakes: Argon release mechanism

Apollo 17 surface mass‐spectrometer observations have shown that 1027 to 1028 mol. of 40Ar are released from the moon when shallow moonquakes occur. Model calculations, based on the seismic and strain energies released by the moonquakes, the volumes of their focal zones, and the surface areas of observed shallow thrust faults, indicate that > 1.5 × 1025 to ≲ 2 × 1028 mol. of 40Ar could be released by each shallow moonquake as a result of mylomization and possible frictional melting of the rocks within a few decimeters of the newly formed parts of the fault surface. The rapid escape of this 40Ar along the fractures of the fault would produce the observed argon release event shortly after each moonquake.

Lunar seismicity and tectonics

Seismic signals from 300–700 deep moonquakes and about four shallow moonquakes are detected by the long-period seismometers of two or more of the Apollo seismic stations annually. Deep-moonquake activity detected by the Apollo seismic network displays tidal periodicities of 0.5 and 1 month, 206 d and 6 a. Repetitive moonquakes from 60 hypocenters produce seismograms characteristic of each. 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 0.5-month periodicity. The 0.5- and 1-month activity peaks occur near times of extreme latitudinal and longitudinal librations and earth-moon separation (EMS). The 206-d and 6-a periodicities in moonquake occurrence and energy release characteristics are associated with the phase variations between the librations and EMS. Because of the exact relationship between tidal phases and the occurrence of deep moonquakes from a particular hypocenter, it is possible to predict not only the occurrence times from month to month, often to within several hours, but also the magnitudes of the moonquakes from that hypocenter. The predicted occurrence of large A 1 moonquakes in 1975, following a 3-a hiatus, confirms the correlation between A 1-moonquake activity and the 6-a lunar tidal cycle and implies a similar resurgence for all of the deep moonquakes. Because no matching shallow moonquake signals have been identified to date, tidal periodicities cannot be identified for the individual sources. However, shallow moonquakes generally occur near the times of extreme librations and EMS and often near the same tidal phase as the closest deep moonquake epicenters. With several possible exceptations, the deep-moonquake foci located to date occur in three narrow belts on the nearside of the moon. The belts are 100–300 km wide, 1,000–2,500 km long and 800–1,000 km deep and define a global fracture system that intersects in central Oceanus Procellarum. A fourth active, although poorly defined, zone is indicated. The locations of 17 shallow-moonquake foci, although not as accurate as the deep foci, show fair agreement with the deep-moonquake belts. Focal depths calculated for the shallow moonquakes range from 0–200 km. Deep-moonquake magnitudes range from 0.5 to 1.3 on the Richter scale with a total energy release estimated to be about 10 11 erg annually. The largest shallow moonquakes have magnitudes of 4–5 and release about 10 15–10 18 erg each. Tidal deformation of a rigid lunar lithosphere overlying a reduced-rigidity asthenosphere leads to stress and strain concentrations near the base of the lithosphere at the level of the deep moonquakes. Although tidal strain energy can account for the deep moonquakes in this model, it cannot account for the shallow moonquakes. The tidal stresses within the lunar lithosphere range from about 0.1 to 1 bar and are insufficient to generate moonquakes in unfractured rock, suggesting that lunar tides act as a triggering mechanism. The largest deep moonquakes of each belt usually occur near the same characteristic tidal phases corresponding to near minimum or maximum tidal stress, increasing tidal stress, and alignments of tidal shear stresses that correspond to thrust faulting along planes parallel to the moonquake belts and dipping 30–40°. With few exceptions, the shallow moonquakes occur at times of near minimum tidal stress conditions and increasing tidal stress that also suggest thrust faulting. The secular accumulation of strain energy required for the shallow moonquakes and implied by the uniform polarities of the deep moonquake signals probably results from weak convection. A convective mechanism would explain the close association between moonquake locations and the distribution of filled mare basins and thin lunar crust, the earth-side topographic bulge, and the ancient lunar magnetic field. The low level of lunar seismic activity and the occurrence of thrust faulting both at shallow and great depths implies that the moon is presently cooling and contracting at a slow rate.

HFT events: Shallow moonquakes?

A few large distant seismic events of distinctly high signal frequency, designated HFT (high-frequency teleseismic) events, are observed yearly by the Apollo lunar seismic network. Their sources are located on or near the surface of the moon, leaving a large gap in seismic activity between the zones of HFT sources and deep moonquakes. No strong regularities are found in either their spatial or temporal distributions. Several working hypotheses for the identity of these sources have been advanced, but many characteristics of the events seem to favor a hypothesis that they are shallow moonquakes. Simultaneous observations of other lunar phenomena may eventually enable the determination of their true identity.

A survey of lunar transient phenomena

Periodic and morphological characteristics of lunar transient phenomena (LTP) are described. Some early historical records are reviewed. Most sites of LTP's show evidence of internal disturbance either initiated from within or by volcanic modification of impact formations. Some individual crater sites are described. The probability of the reliability of at least some, and probably most, reports has been increased by the later discovery of the periodic nature of the deep-focus and shallow moonquakes; spurious reports would not have produced the periodicity of LTP's, but random errors would increase the noise level. The conclusion to be drawn from the inadequacy of external energy sources is that some energy must be injected from within the moon, probably in the excitation of gases, but this must remain speculation until better data is obtained. The underlying problem now is still the difficulty of obtaining permanent instrumental records; only twenty heterogeneous reports are at present known.

Lunar seismicity, structure, and tectonics

Interpretation of lunar seismic data results in a lunar model consisting of at least four and possibly five distinguishable zones: (I) the 50-60 km thick crust characterized by seismic velocities appropriate for plagioclaise rich materials, (II) the 250 km thick upper mantle characterized by seismic velocities consistent with an olivine-pyroxene composition, (III) the 500 km thick middle mantle characterized by a high Poisson’s ratio, (IV) the lower mantle characterized by high shear-wave attenuation, and possibly (V) a core of radius between 170 and 360 km characterized by a greatly reduced compressional wave velocity. The Apollo seismic network detects several thousand deep moonquake signals annually. Repetitive signals from 60 deep moonquake hypocentres can be identified. The occurrence characteristics of the moonquakes from the individual moonquake hypocentres are well correlated with lunar tidal phases and display tidal periodicities of 1 month, 7| months, and 6 years. With several possible exceptions, the deep moonquake foci located to date occur in three narrow belts on the near side of the Moon, and are concentrated at depths of 800-1000 km. The locations of 17 shallow moonquake foci, although not as accurate as those of the deep foci, show fair agreement with the deep moonquake belts. Focal depths calculated for the shallow moonquakes range from 0-300 km. The moonquakes of a particular moonquake belt, or a region within a belt, tend to occur near the same tidal phase suggesting similar focal mechanisms. Deep moonquake magnitudes range from about 0.5 to 1.3 on the Richter scale with a total energy release estimated to be about 10 11 ergs (10 4 J) annually. The largest shallow moonquakes have magnitudes of 4-5 and release about 10 15 -10 18 ergs (10 8 -10 11 J) each. Tidal deformation of a rigid lunar lithosphere overlying a reduced-rigidity asthenosphere leads to concentrations of strain energy near the base of the lithosphere. Although tidal strain energy can account for the deep moonquakes in this model, it cannot account for the shallow moonquakes. Tidal stresses within the lunar lithosphere range from about 0.1 to 1 bar (10 4 -10 5 Pa). This low level of tidal stresses suggests that tides act as a triggering mechanism. The secular accumulation of strain implied by the uniform polarities of the deep moonquake signals probably results from weak convection. A convective mechanism could explain the distribution of moonquakes, the Earth-side topographic bulge, the distribution of filled mare basins, and the ancient lunar magnetic field.

Transient lunar phenomena, deep moonquakes, and high-frequency teleseismic events: possible connections

The positions of the epicentres of the recently discovered, high-frequency teleseismic (h.f.t.), shallow moonquakes have been compared with those of the surface phenomena known as lunar transient phenomena (l.t.p.). About 300 sites for the latter and about 17 shallow moonquake epicentres have been recorded. Most of the epicentres are within 5° of at least one l.t.p. site and the depths are given by Nakamura et al. (1974) as up to 265 km. The present paper considers some of the implications of the correlations between the two classes of sites and between the l.t.p. and the epicentres of the deep moonquakes at levels of about 800-1000 km below the surface. It appears likely that channels produced by the outrush of gases from the interior of the Moon are associated with the sites of the h.f.t. and of the l.t.p. that are almost vertically above them, and that release of gases through poorly consolidated soils at an early stage of the Moon’s history may have led to the formation of at least some of the craters on the Moon.

Passive Seismic Experiment

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.