HFT events: Shallow moonquakes?

Physics of the Earth and Planetary Interiors, 14 (1977) 217—223 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands

HFT EVENTS: SHALLOW MOONQUAKES?

1

YOSIO NAKAMURA Geophysics Laboratory, Marine Science Institute, University of Texas, Galveston, Texas 77550 (U.S.A.)

(Accepted for publication February 8, 1977)

Nakamura, Y., 1977. HFT events: shallow moonquakes? Phys. Earth Planet. Inter., 14: 217—223 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 oftheir true identity.

1. Introduction

2. Characteristics

Among thousands of natural seismic events detected yearly by the Apollo lunar seismic network, there are a few distant events of distinctly high signal frequency. These are designated HFT (high-frequency teleseismic) events (Nakamura et al,, 1974). Though they constitute only a very small fraction of the total number of observed seismic events, they are quite significant because some of them represent the most energetic seismic sources in or on the moon. A total of 25 such events, listed in Table I, have been identifled to date, averaging about five events per year. In this paper, I present some of the major characteristics of these events including the spatial and temporal distributions of the sources, review hypotheses advanced for the identity of the sources, and discuss the significance of these events in relation to other current activities in the moon,

2.1. Signal characteristics

Marine Science Institute Contribution No. 145.

Seismic signals from HFT events are clearly distinguished from other types of signals primarily by their high-frequency content and relatively well-defined P- and S-wave arrivals. Fig. 1 shows a comparison of an HFT signal and two other major types of signals, namely those of a deep moonquake and a meteoroid impact. As seen in Fig. l,the recorded amplitude of the HFT signal is clearly larger on the short-period component (spz) than on the long-period (LP) components, demonstrating the high-frequency content of the signal. Power spectral analyses of the signals show that the spectra of HFT signals remain nearly constant up to about S Hz and fall off only very gradually above 5 Hz, in contrast to those of meteoroid-impact and deep-moonquake signals at comparable distances, which fall off sharply above about 1 Hz (Nakamura et al., 1974). The distinction in the frequency content of signals is so clear that no transitional cases have been observed. The higher-frequency content of

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signals may be attributed to higher-frequency sources for HFT events than for others, suggesting small source size for the amount of energy released. Alternately, it may be attributed to lower attenuation of high-frequency signals throughout the HFT signal transmission path than those of signals of other types. P and S arrivals of HFT signals are relatively well defined in contrast to poorly defined arrivals for most meteoroid-impact signals, as seen in Fig. 1. This property is similar to that of deep moonquakes. It suggests either that the HFT sources are located below the surface scattering zone (Iatham et al., 1972; Nakamura et al., 1975), or that the scattering zone is absent near the source region. If the former is the case, it implies that HFT events cannot be of external (impact) origin, but must be moonquakes. HFT

2.2. Depth The determination of depths of HFT hypocenters from travel times of seismic waves is not accurate enough to separate surface sources from those at finite shallow depths. This is because all the HFT events identified to date originated outside the seismic array, and only slight errors either in the assumed seismic velocities or in the readings of arrival times change the calculated depths of focus by an appreciable amount ranging easily up to a few hundred kilometers. Because of the complexity of lunar seismograms due to the intensive scattering of seismic waves, no depth phases, such as pP, have been identified. However, evidence exists that suggests that the events are quite shallow. Analyses of lunar seismic signals from meteoroid impacts indicate that there

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exists a large decrease of shear-wave velocity with depth at about 300 km depth inside the moon (Nakamura et al., 1976a). Because of this velocity variation, S-wave arrivals from surface sources are greatly delayed relative to P-wave arrivals at far distances, resulting in a P and S relative arrival time relation clearly different from that of deep moonquakes. P and S relative arrival times of HFT signals are found to show a relation that is not significantly different from that of meteoroid-impact signals. If the depths of the HFT sources approached 300 km, where the large S velocity decrease occurs, their P and S arrival time relation would be significantly different from that of meteoroid-impact signals. Therefore, HFT sources are most likely to be less than 100 km deep. Thus, there exists a large gap in seismic activity between the zones of HFT sources and deep moonquakes. 2.3. Spatial distribution ofepicenters Fig. 2. shows the spatial distribution of the epicenters of HFT events determined by using our latest ~

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lunar structural model and assuming surface sources (Table I). The distribution appears quite random, even when allowances are made for uncertainties in epicentral determinations. There is no clear regularity in the distribution; there is certainly no concentration into narrow belts like the earthquake belts, suggesting that they are not caused by relative movements of large plates. Attempts to correlate these epicenters with surface features, such as rims of mare basins, have been fruitless. Correlation with the distribution of deep-moonquake epicenters (Lammlein et a!., 1974; also at this symposium) is also poor. The only similarity between distribution of HFT epicenters and deep-moonquake epicenters is that the HFT activity is relatively low in the southeast quadrant of the moon, in which no deep moonquake epicenters have been located to date. Fig. 3 is a histogram showing the number of HFT epicenters located in each octant of the moon. There are more events observed in the northeast and southwest octants than others, but these peaks are not statistically significant. The trough of the distribution in the south and southeast octants is more significant, though statistically not highly so. The southeast quadrant of the moon is mostly highlands. Therefore, the lower level of both HFT and deep-moonquake activities may be representative of highland regions. 2.4. Temporal distribution

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Fig. 2. Distribution of HFT epicenters. Crosses indicate locations of the active seismic stations: Apollo 12, 14, 15 and 16. The base map shows the entire lunar surface in an equal-area projection. The coordinate grids are spaced at every 30°.HFT magnitudes are defined in a footnote to Table I. .

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Fig. 4 shows the time history of the occurrence of events. The occurrence is rather sporadic with no apparent patterns. No apparent regularities, such as the monthly and 7-month cycles of the deep-moonquake activity, are observed. This suggests that, in contrast to deep moonquakes, their occurrences are not strongly controlled by the tides. In Fig. 5, I have borrowed the technique used by Cameron (1972) to plot the number of observations of HFT events relative to the phase of the anomalistic period. To my surprise, there appears a rather statistically significant peak at a phase angle of 0.7. The location of this peak happens to coincide with that of certain types of lunar transient events found by Cameron (1972). Thus, there may be some correlation between these two phenomena. The peak, howHFT

ever, does not correspond to any particular area on the moon. In fact, out of seven observations repre-

220 TABLE I List of HFT events with estimates * 1 Year

Day

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Year

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Epicenter

Magnitude *2

1971 1971 1971 1972 1972 1972 1972 1973 1973 1973 1973 1974 1974

197 (Apr. 17) 140 (May 20) 192(Jul.11) 002 (Jan. 2) 261 (Sep. 17) 341 (Dec. 6) 344 (Dec. 9) 039 (Feb. 8) 072(Mar.13) 171 (Jun. 20) 274 (Oct. 1) 054 (Feb. 23) 086(Mar.27)

07 0050 17 26 10 132440 22 29 30 14 35 50 23 08 30 0350 10 2252 10 065620 2022 00 03 5800 2116 50 091050

47°N33°E 39°N24°W (*3) 55°N97°E 13°N43°E 43°N42°E 29°S74°W 43°N28°E 82°S113°W 0° 68°W 35°S27°W 34°N15°W 49°S103°W

2.8 2.0 1.9 1.9 1.0 1.4 1.2 0.8 3.2 2.2 1.1 0.7 1.6

1974 1974 1974 1975 1975 1975 1975 1975 1975 1975 1976 1976

109 (Apr. 19) 149 (May 29) 192(Jul.ll) 003 (Jan. 3) 012 (Jan. 12) 013 (Jan. 13) 044 (Feb. 13) 127 (May 7) 147(May27) 314 (Nov. 10) 004 (Jan. 4) 012 (Jan. 12)

13 35 20 2042 10 004620 01 41 50 03 14 10 00 2640 2203 50 06 3840 232900 07 53 00 111850 08 18 10

37°S 38°E (*4) 21°N86°E 30°N96°W 64°NSlow 1°S 48°W 18°S23°W 44°S47°W 5°N55°W 7°S60°E 47°N30°E 34°N41°E

0.9 0.6 2.7 3.2 1.7 1.1 1.4 1.3 1.4 1.8 1.8 1.1

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~ Origin times and epicenters were estimated using Model LM-752 and assuming surface focus. *2 Lunar HFT magnitude defined by log (SPZ envelope amplitude in DU reduced to ~ = 60°). *3 43°N, 47°Wor 42°S,60°W. *4 30°from station 16 on east side of station.

senting the peak, two occurred in the east octant and a single event occurred in each of the north, northeast, southeast, southwest and west octants. The absence of observed events near apogee is not statistically significant.

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25. Source magnitude A good understanding of the seismic energy transmission through the lunar interior is required to estimate the energy released at the source of an HFT event. With the present knowledge of the structure of the lunar interior and of the transmission of seismic energy, particularly in the region near the surface, all we can accomplish is a very rough estimate. One such estimate (Nakamura et al., 1974) assigns a Richter body-wave magnitude 4 for a large HFT event. This is

equivalent to a seismic energy release of about 3 1015

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Fig. 3. A histogram showing the distribution of observed HFT epicenters is~octants of the lunar surface. North (N), east (E), south (S), and west (13’) octants are indicated,andthediagonaloctantsarenotlabeled.Thefirst two octants are repeated. The numbers in parentheses are the levels of significance of peaks and troughs.

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Fig. 4. Occurrence history of HFT events.

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nitudes are about 1 less than the Richter body-wave magnitudes. As mentioned earlier, some of the strongest natural seismic events observed are HFT events, though they constitute only a very small fraction, in number, of

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all the teleseismic events observed. This makes the b value, the slope of the log-cumulative frequency—magnitude distribution, of HFT events quite small. The b value of thetodistribution 6 is about This is in contrast the b valuesinofFig. greater than 0.5. 1 .0 found for meteoroid-impact signals (Duennebier et al., 1975) and for deep-moonquake signals (Lammlein et al., 1974). Although little is known about their real sigmficance, earthquake b values of less than unity are often associated with high stress concentration in the

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Fig. 5. A histogram showing the distribution of observed HFT events with respect to the phase of the anomalistic month. The first two tenths of the period are repeated. P andA mdicate perigee and apogee, respectively. The numbers in parentheses are the levels of significance ofpeaks and troughs.

source region. Therefore, the low b value for HFT events may suggest higher stress concentration at these sources than at those of deep moonquakes.

erg. The magnitudes listed in Table I are the lunar HFT magnitudes defined in the footnote to the table, and are used simply for convenience. The relationship between the lunar HFT magnitude and the Richter body-wave magnitude is subject to great uncertainty due to our lack of detailed information concerning seismic energy transmission through the lunar interior. The above estimate indicates that the lunar HFT mag100

3. Working hypotheses We have advanced earlier the following three working hypotheses for the identity of the HFT signal sources (Nakamura et al., 1974): (1) Ordinary meteoroid impacts on an unusually

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competent surface zone which is grossly different from the normal regolith-covered lunar surface. (2) Some impacting objects that achieve unusually deep penetration into more competent material beneath the heterogeneous surface zone. (3) Shallow moonquakes. The first two hypotheses postulate an external (impact) origin for the events, while the third hypothesis of these postulates hypotheses an internal can explain (moonquake) many oforigin. the characEach teristics of HFT signals, but no evidence appears to be conclusive. The third hypothesis, however, seems most probable at the moment. Many of the properties of HFT signals, including the presence of distinct and strong shear-wave arrivals, the lack of transitional cases to normal meteoroid-impact signals, the possible nonrandomness in spatial and temporal distributions, and the low b value, seem to support this shallow-moonquake hypothesis more strongly than the others, though not conclusively.

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4. Discussion The HFT events thus seem to be moonquakes occurring at depth shallower than 100 km. If they are truely moonquakes, what is their significance in relation to other current activities in the moon? Fig. 7 schematically shows our current model of the lunar interior inferred from seismic data, showing also sites of located moonquake activities. Deep moonquakes are concentrated in a depth range of 800— 1,000 km, just above the boundary between the middie mantle and the lower mantle. They are interpreted as being due to the high concentration of tidal stresses caused by the grossly different elastic properties of the two zones: the solid middle mantle and partially molten lower mantle. In contrast, HFT’s, if they are truely moonquakes, occur in the shallow zone where the material is at temperatures well below the solidus. The poor correlation of the occurrence of HFT events with the tidal cycle suggests that they are not strongly controlled by tides, as deep moonquakes are. This implies that a secular accumulation of strain of some sort must occur in the outer shell of the moon. The nature of the strain must be different from that which exists in the lithosphere of the earth, because HFT epicenters are not concentrated in narrow belts like the earthquake belts and no surface manifestations mdicating existence of plate motions are visible on the SEISMIC STATIONS

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lunar surface. The existence of shallow moonquakes might be an indication that the entire moon is either uniformly expanding or contracting at sufficiently low rate to leave no apparent surface manifestations. The gap in seismic activity between the zones of HFT’S and deep moonquakes, indicating the lack of moonquakes of intermediate depths, may be interpreted either as a lack of secular accumulation of strain in this depth range, or as accumulation of strain sufficiently slow to allow creep of material in the zone without sudden release of strain energy through moonquakes. The significance of HFT events in relation to the lunar transient phenomena (LTP) is still to be demonstrated. Spatial correlation of HFT epicenters with locations of LTP’S is difficult owing to the uncertainty of epicenter determinations. Concerning the temporal correlation, no simultaneous observations of an HFT and an LTP have been reported to date. The significance of HFT events in relation to the gaseous emissions detected at the lunar surface by ALSEP instruments appears to be more realistic. Possible temporal correlation between occurrence of HFT events and observations of increased Ar in the lunar atmosphere has been reported (Hodges and Hoffman, 1975; also at this symposium). If proven, it provides strong evidence that HFT’s are truely moonquakes. The time delay between the occurrence of an HFT event and the observations of increased Ar may be interpreted as representing diffusion of Ar from some depth to the surface of the moon. Longterm observations of gaseous emissions at multiple stations are necessary to provide more positive data on the possible correlation. The potential energy of the trapped Ar, estimated to be about 2 1015 erg/year (Hodges and Hoffman, 1975), is not sufficient to cause moonquakes of the size of HFT’s (3 1015 erg for a large HFT) as estimated earlier, when we consider that very small fraction of the available strain energy is normally converted to seismic energy by a quake. The escape of Ar into the lunar atmosphere, therefore, will be a consequence of a moonquake rather than a cause.

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5. Conclusions

CRUST

Fig. 7. Schematic diagram of the lunar interior inferred from seismic data (Nakamura et al., 1976b).

HFT events, which occur on or near the surface of the moon, are likely to be shallow moonquakes,

223

though the evidence is not yet conclusive. Further investigation of these events in relation to other observable phenomena in and on the moon is expected to be fruitful in elucidating the current activity in and the state of the moon.

Hodges, R.R. and Hoffman, HJ., 1975. Implication of atmo40Ar escape on the interior structure of the moon. spheric

Acknowledgements

Nakamura, Y., Toksöz, N., Lammlein, D. and Duennebier, F., 1972. Passive seismic experiment. In: Apollo 15 Prelim. Sci. Rep. NASA SP-289, Sect. 8. Nakamura, Y., Dorman, J., Duennebier, F., Ewing, M., Lammlein, D. and Latham, G., 1974. High-frequency lunar teleseismic events. In: Proc. 5th Lunar Sci. Conf. Geochim. Cosmochim. Acta, Suppl., 5: 2883—2890. Nakamura, Y., Dorman, J., Duennebier, F., Lammlein, D.

I wish to thank Drs. Gary V. Iatham and AbouBakr K. Ibrahim for critically reviewing the manuscript and offering constructive suggestions. This work

was supported by National Aeronautics and Space Administration contract NAS 9-14581.

In: Proc. 6th Lunar Sci. Conf., Geochim. Cosmochim.

Acta, Suppl., 6: 3039—3047. Lammlein, D.R., Latham, G.V., Dorman, J., Nakamura, Y. and Ewing, M., 1974. Lunar seismicity, structure, and tectonics. Rev. Geophys. Space Phys., 12: 1—21. Latham, G.V., Ewing, M., Press, F., Sutton, G., Dorman, J.,

and

Latham, G., 1975. Shallow lunar structure determined

from the passive seismic experiment. In: Origin and

References Cameron, W.S., 1972. Comparative analyses of observations of lunar transient phenomena. Icarus, 16: 339—387.Duennebier, F., Dorman, J., Lammlein D., Latham, G. and

Nakamura, Y., 1975. Meteoroid flux from passive seismic experiment data. In: Proc. 6th Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl., 6: 2417—2426.

Evolution of the Lunar Regolith, Moon, 13: 5 7—66. Nakamura, Y., Duennebier, F.K., Latham, G. and Dorman, H.J., 1976a. Structure of the lunar mantle. J. Geophys.

Res., 81: 4818—4824. Nakamura, Y., Latham, G.V., Dorman, H.J. and Duennebier, F.K., l976b. Seismic structure of the moon — A summary of current status. In: Proc. 7th Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl., 7: 3113—3121.