Detection of the Lunar Deep Moonquake at Apollo 17 Site
Yasuyuki Saito1,2 , Shota Ono1,2 , Taichi Kawamura1,2 , Satoshi Tanaka2 ,Yoshio Nakamura3 , Axel Hagermann4 , Ki-iti Horai2
1

The University of Tokyo, Graduate School of Science
2
The Japan Aerospace Exploration Agency
3
The Texas University 4 The Open University

Abstract
Lunar earthquake called Moonquake had been observed by Apollo missions. This experiment was carriedout in a part of the ALSEP (Apollo Lunar Surface Experiments Package) of Apollo 12, 14, 15, and 16.
Network observation using all of the stations was carried out for about ﬁve years until September 30, 1977.
On the other hands, the Lunar Surface Gravimeter (LSG) experiment was carried out in Apollo 17 missionin order to detect gravitational waves and measure tidal deformations of the Moon. This experiment has
never obtained meaningful data for gravity, however, the instrument had been functioned as a seismometer.
In this study, we examined the LSG data and conﬁrmed that it was detecting seismic signals by using bandpass ﬁlter and spectrogram. These analyses enabled us to improve the accuracy of determination of arrival
times. The error of the arrival times of deep moonquakes are expected to be about 30 seconds. The analysisof the LSG data as a seismometer expands the observable area of Apollo seismic observation network. With
the expanded network, we may be able to determine seismic centers of unclassiﬁed deep moonquakes. If
seismic centers of the far side of the Moon can be identiﬁed, the observed signals may have passed thelunar core. We are expecting to obtain some new information of the lunar interior from the analysis of the
LSG data.

1

Introduction

Table 1: Cataloged seismic events
Type
Number of events
Artiﬁcial impact
9
Meteoroid impact
1,743
Shallow moonquakes
28
Deep Moonquake
7,245
3,533
Unclassiﬁed
Total
12,558The network observation of seismic signals was
carried out on the moon. Because the seismic
data provides the most direct method to estimate
the layer structure such as the crust and mantle,
an accurate determination of the thickness andelastic wave velocity of each layer are essential fora better understanding of the origin and evolution
Nakamura et al., 1981[11]; Nakamura, 2003[10]of the Moon.
Apollo seismic data provides us detailed inforLunar earthquake (called Moonquake) had been
observed during the Apollo missions. These ex- mation of the lunar interior. Some studies showedperiments were carried out as a part of ALSEP that the seismic discontinuities were found at about(Apollo Lunar Surface Experiments Package) of 60 km and 500 km in depth (Nakamura, 1982[9],Apollo 12 to 16 except for Apollo 13 mission for Goins et al., 1981[4], Lognonne et al., 2003[8], andabout eight years between November 19, 1969 and Khan et al., 2000[5]). It has been interpreted that
September 30, 1977 (Bates et al., 1979[1]). Apollo these discontinuity is boundary between crust andstations 12, 14, 15 and 16 were placed in an ap- mantle, and upper and lower mantle, respectively.
proximate equilateral triangle with distance be- It means that the Moon is a diﬀerentiated body,tween corners being about 1100 km and two of which implies that it experienced hot era and hasthem, Apollo 12 and 14, were placed only 180 a metallic core.
Since limited depth distribution of the deepkm apart in one corner. This triangle has been
moonquakes
and the absence of identiﬁable mooncalled the triangle network of Apollo seismic measurement. In the 8-year period, 12,558 events quake on the hemisphere opposite to that of thewere recorded (see Table 1). These events were triangle network, seismic velocity information was
found to diﬀer in signal characteristics and were precluded being inferred below a depth of aboutgrouped into four categories: deep moonquake, 1150 km. A lot of measurement was carried out
shallow moonquake, meteoroid impacts and ther- in order to estimate that there is metallic core inmal moonquake (Toksoz et al., 1974[15]; Naka- the Moon, for example mass and moments of inertia, induced dipole moment, lunar laser ranging,mura, 1982[9]; Nakamura, 2003[10]).
and so on. These measurements indicate that the1saito@planeta.sci.isas.jaxa.jp

Moon has partially molten region within deeper
than about 500 km in radius. If there is a seismometer at Apollo 17 landing station site, there
is a possibility that we ﬁnd opposite deep moonquake and direct evidence of a lunar core.
The seismic experiment had never been carried out at Apollo 17 landing site. On the otherhands, the Lunar Surface Gravimeter (LSG) experiment had been done. The LSG was set in order to detect gravitational waves and to measuretidal deformations of the Moon. Unfortunately,
this experiment failed and it was concluded thatno provision has been made to supply data fromthis experiment[7]. However, as a result of ourstudy (Saito et al., 2007[13], Ono et al., 2007[12]),
we succeeded in archiving the Lost Apollo data
and the LSG data was included in the data set.
In this study, we report the result of performance assessment of the LSG, and show that deep
moonquake signal obtained by the LSG can beused in seismic analyses in order to accomplish
the ultimate objective; revealing that if there is acore in the Moon or not.

Figure 1: Schematic diagram of the Lunar Surface
Gravity sensor. The beam balance between two ﬁxed
capacitor plates. The LSG employed the type of
Lacoste-Romberg gravimeter. This type of gravimeterhas high sensitivity for a long-period vibration phenomenon. Figure from Giganti et al. (1973)[3].

tions in the frequency range from 0 to 16 Hz. A
zero-length spring is one in which the restoring
force is directly proportional to the spring length;
such a spring is very useful in obtaining a longperiod sensor.
2 Lunar Surface Gravimeter ExWhen the LSG was emplaced, it was impossiperiment
ble to balance the beam by sending commands to
add or subtract mass because 1/6-g mass weights
The primary objective of the LSG was to use the were too light[1]. However, as a result of reconﬁgMoon as an instrumented antenna to detect graviuration, beam was centered and the instrument
tational waves predicted by Einstein’s general relwas apparently behaving like a gravimeter withativity theory[3]. If gravitational waves of sufresonance at 1.5Hz and possibly at a much lower
ﬁciently high intensity covering certain bands offrequencies are incident on the Moon, internal vi- frequency. In addition, the seismic data output
brations of the Moon will be excited. These vibra- during a several minute period associated with
tions may cause oscillatory surface accelerations[3]. lunar sunrise is shown in Apollo 17 preliminary
Approximate estimates suggest that present pro- report[3]. This suggests that the instrument funccedure have a fair chance of observing real eﬀects tioned as observation equipment for gravimeterby using the Moon because of the relative quiet and seismometer.
Finally, Lauderdale and Eichelman (1979) conof the lunar environment. In addition, the LSG
was also designed to measure the tidal eﬀects on cluded that no provision has been made to supply
the Moon and to serve as a one-axis seismometer data from this experiment because they could notwith seismic mode[16]. Thus, the LSG had three ﬁnd out the event signals of gravitational wave
bands: tide, free, and seismic mode. Tide mode with the LSG[7] and no one could do until now,
covers the frequency range from 0 to 0.048 Hz, too. Many people have believed that the LSG exfree mode covers the range from 0.00083 to 0.048 periment failed. Thus, the LSG have never been
Hz, and seismic mode covers the range from 0.05 analyzed as seismic data in detail. Here, we show
to 16[16].
the results of the LSG data evaluation as seismic
A schematic diagram of the spring-mass sus- data.
pension system is shown in Figure 1. The LSG
used the Lacoste-Romberg type of spring-masssuspension to sense changes in the vertical component of local gravity. This instrument is a sensitive balance with a mass, spring, and lever system
and with electronics for observation of accelera-3.1

Evaluation of the LSG data as
seismic data
The LSG data in the seismic channel and its sensitivity

The LSG data was contained by six parametersin 36 ALSEP main frame words; 31 samples ofseismic data, 1 sample of tidal data, 1 sample offree mode data and 2 status and 2 housekeeping
words per ALSEP main frame[14]. Each ALSEP
data record is 5,776 bytes and it has 60 data parts
which size is 96 bytes. A data part contains 64
ALSEP words and each ALSEP word is a 10-bit
word. At 1060 bps normal data rate, the nominal duration of a logical record is 10x64/1060 =603.77 ms. The seismic channel is contained in 31
words in 603.77 ms, the sampling rate of seismic
data was 603.77/31 = 19.5 ms.
Figure 2 shows the response curve of the seismic mode of the LSG copied from Giganti et al.
(1977)[2]. The LSG was designed to perform withhigh response around 0.05Hz to 16 Hz. However, the reconﬁguration had to be carried out
because of the malfunction of the beam discussed
previously. Eventually, the beam was recenteredand the LSG carried out the observation withthe response shown in Figure 2[2]. The ﬁgureshows that the response of the LSG was kept
high in frequency range from 1Hz to 16 Hz witha peak around 1Hz to 2Hz. Since the lunar seismic signals are known to have characteristic peaks
around 1Hz and 10Hz, the observation of the seismic mode is likely to have detected seismic signals.

3.2

Signal-to-Noise (S/N) ratio

Background noise levels of the LSG data is higher
than those indicated for the Moon by the Passive Seismic Experiment and for Taurus-Littrow
by the Lunar Seismic Proﬁling Experiment. J.
Weber dealt with noisy seismic data by ﬁlteringof free-mode data by smoothing the power spectrum through stacking multi data records and averaging out noise. Since our objective is to provethat the LSG data is useful for analysis of deep
moonquake with certainty, we compared the seismic signal with background noise by using thecatalog of lunar seismic data[11].
Deep moonquake has characteristic frequency
near 1 Hz (0.4–2 Hz). Figure 3 shows that thecomparison of the background noise with deep

1012
1010
LSG pre-launch

108
Sensitivity,DU/cm

3

106
4

10

LP

102

LP-peaked

010

LSG on the Moon

-2

10

SP
10-4
-3
10

10-2

10-1
100
Frequency,Hz

101

102

Figure 2: Response curve of the seismic mode of the
LSG . The solid line indicates the response of the LSG
on the Moon and the dotted line indicates that ofpre-launch. The response curves of other Apollo seismographs are superposed for comparison. For Apolloseismographs SP means Short Period mode and LP
means Long Period mode, which has ﬂat mode andpeaked mode. The LSG has high response comparable to other seismographs from 1Hz to 16Hz. The
response curve of the LSG was cited from Giganti etal. (1977)[2], and that of other Apollo seismographs
are cited from Latham et al. (1972)[6].

moonquake signal detected on March 9th, 1976
in frequency ﬁeld by using Discrete Fourier Transform (DFT). Nakamura et al. (1981)[11] identiﬁed this deep moonquake as A7 group. Figure
3 shows that S/N ratio of this seismic event ishigher than 3 at about 1.6 Hz. It implies thatnoise reduction using band-pass ﬁlter around thisfrequency may be eﬀective.
Not all deep moonquake signal level are always
larger than noise, we succeeded in detecting thedeep moonquake signals of A1, A6, A7, A22 groupwith ratio higher than 40%. We could ﬁnd out
only one deep moonquake signal for A1 group.
This may be because of the long distance between
Apollo 17 site and A1 site. For group site otherthan A1 group, about 64% of the deep moonquake
signals have S/N ratio higher than 2.
Figure 4 shows the comparison of the raw datawith band-pass ﬁltered signal. Seismic signals of
frequency from 1.4Hz to 1.7Hz was selected. The
envelope of ﬁltered seismic signal is clearer thanthe one of non-ﬁltered signal. Nakamura (1992)[9]showed that the deep moonquake (occurred on
March 9th, 1976) arrived at the nearest station
at AM 3:10[11]. Since the location of the A7 site
is closer to Apollo 17 site compared to other seismic stations, the arrival time is likely to be earlier
than AM 3:10. This seems to be consistent withthe signal observed with the LSG, whose arrival

30

0.20

20
10
Signal, DU

Signal, DU

0.25

0.15

0.10

0
-10

0.05

-20
0.00
0.5

1.0
Frequency, Hz

1.5

1.7

Figure 3: Characteristics frequency of A7 group deep
moonquake between 0.5 Hz and 2 Hz. The gray region shows the background noise and the black region
shows the seismic signal, respectively. We can see a
prominent signal around 1.6Hz. Since this frequency
is not dependent of the group or time, it may be occurred by characteristic of the instrument.

time read from the ﬁltered signal is expected to
be around 3:09 or earlier. This implies that bandpass ﬁltering is eﬀective for improving the accuracy of determination of arrival times.

3.3

-30
2:50

2.0

3:00

3:10
Time

3:20

3:30

Figure 4: The comparison of the raw data (black)with band-pass ﬁltered signal (white) by using the frequency between 1.4 and 1.7 Hz. The envelope of theseismic signal is much clearer than raw data. The ﬁgure shows that the LSG detected certain signal around
3:09. There is no signal at both ends of the ﬁltered signal because of the eﬀect of the window function.

another method to determine the arrival time.

Spectrogram

In the previous section, we showed that the LSG
had S/N ratio high enough to read arrival timeby using the DFT technique. If seismic signal has
characteristic frequency, it is likely that we canobserve some kind of change in spectrum typicalto seismic events. A change in frequency withtime can be well understood by using a spectrogram. First, a certain time is determined as areference, a constant time period is extracted andrun the DFT. A constant time period is determined by requested accuracy of resolution in timeand frequency. At least 10 seconds data need to
be extracted for identiﬁcation of 0.1 Hz resolution in frequency. Figure 5 shows the spectrogram of A7 group on March 9th, 1976. Data of
30 seconds was used in the DFT and this value
is proportional to the time resolution of the spectrogram. The seismic signal clearly detected near
1.5 Hz between about 3:09 and 3:25. At least, it
is may be able to read the arrival time to an accuracy of 30 seconds. As shown in ﬁgure 5, spectrogram is constructive for detecting small deep
moonquake signal, however, there is not high resolution in time ﬁeld. Then, we have to develop

Figure 5: Spectrogram constructed from the seismic
signal of the deep moonquake on March 9th, 1976..
This Moonquake is expected to be around 3:08 - 3:09,
and carry on until about 3:25. Data of 30 seconds wasused in the DFT to make the spectrogram.

4

Discussion

Observations of more than three seismic stations
are needed to determine the seismic source of aseismic event. Since Apollo 12 site and Apollo 14
site are relatively close in the global observation
network, and the number of the seismic stations
is limited, the observable area of the network was
restricted. In addition, all of the stations are located at the near side of the moon. Therefore, todetect the seismic event from the farside, the sig-

Opposite Deep Moonquake
nal has to be relatively strong. These made theA01, A06, A07 or A22
determination of seismic centers at the opposite
Deep Moonquakeside of the observation network diﬃcult. Figure 6
shows the projection to equatorial plane of seismic
source of deep moonquakes identiﬁed by Nakamura et al. (1981)[11] and Nakamura (2003)[10].
Most of these are located at the near side and8 out of 77 deep moonquakes’ seismic sources are
identiﬁed as ’opposite deep moonquakes’. By adding
the LSG to the observation network, the observable area expanded and detection of more ’opposite deep moonquakes’ may be possible. In theﬁgure, the tangential line from Station 12 andStation 15 to the central region with radius 300 Figure 6: Projection on the equatorial plane of the
km is drawn. This central region represents a hy- seismic source of deep moonquakes identiﬁed in Nakapothetical sphere lunar core, and seismic signals mura et al. (1981)[11] and Nakamura (2003)[9]. The
eight stars means opposite seismic centers, four circles
from the region more to the far side than the tan- means the analyzed seismic group by this study, andgental line have a possibility of passing the lunar the others. Two lines drawn in this ﬁgure represent
core. Therefore, if a seismic event occurred at tangential line drawn to the central area of 300 km in
the region between the two lines, which is 150 ± radius; the solid line indicates the tangential line from
Apollo 15 Station and the dotted line indicates that15 degrees in longitude, signal through the lunar from Apollo 12. .
core may be detected at Station 12 or 14 while
signal that did not pass the lunar core may be is at the near side or far side. We have alreadydetected at Station 15 to 17. If we can determine conﬁrmed that the LSG detected the signal fromthe seismic source in that region, we may be able A33 site, which is the seismic source farthest fromto obtain information of the lunar core from the the near side among the reported seismic sources
signals observed at Station 12 and 14.
of deep moonquakes. This implies a possibility
Assuming that the core-passing signal is ob- that the LSG was detecting seismic signals fromserved at Station 12 and 14 while signal that did the region opposite to Station 12 or 14. Within
not pass the core is observed at Station 15 to 17 these seismic signals from the far side, there may
for a seismic event. By comparing the amplitude be signals that passed through the central regionof the two types of signals, we can speculate the of the Moon. If we can detect this kind of signals,
state of the lunar core. Heavily attenuated core- discussion on the lunar core will be possible. We
passing signals implies viscous, or may be par- are expecting to obtain new information of the
tially molten core and vise versa. If we can distin- lunar interior from further analyses.
guish P-wave from S-wave, we can obtain further
implication of the lunar core. Strong P-wave andReference
attenuated S-wave may imply a molten core.

5

Conclusion

It was believed that the LSG was unable to perform meaningful observations, however, the seriesof reconﬁgurations made it possible. The responseof the LSG implied that it functioned as a seismograph and we succeeded in proving this. We also
showed that the arrival time of deep moonquakes
are expected to be read within error 30 seconds.
Assuming that the velocity of P-wave in the mantle is 9 km/s, this error corresponds to an errorof 270 km for determinations of seismic sources.
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