Shallow moonquakes, which were discovered during observations following the Apollo lunar landing missions, may pose a threat to lunar surface operations. The nature of these moonquakes is similar to that of intraplate earthquakes, which include infrequent but destructive events. Therefore, there is a need for detailed study to assess the possible seismic risk before establishing a lunar base

Nakamura, Yosio

Oberst, Juergen

English

NASA Technical Reports Server

PRECEDING PPlGE BLAb!K NOT F_Lk_2_ 1
A SEISMIC RISK FOR THE LUNAR BASE
Jiirgen Oberst and Yosio Nakamura N g _ -
Institute for Geophysics and Department of Geological Sciences
The University of Texas at Austin
Austin TX 78713
]7439
Shallow moonquakes, which were discovered dunng observations following the Apollo lunar landing
missions, may pose a threat to lunar surface operations. The nature of these moonquakes is similar
to that of intraplate earthquakes, which include infrequent but destructive events, lberefore, there is
,a need for detailed study to assess tbe posmble seismic _ before establisl_'ng a lunar base.
INTRODUCTION
Contrary to the common belief of the pre-Apollo era, a belief
that unfortunately is still found in today's popular literature, the
Moon is by no means a seismically "dead" planet. It is an observed
fact that quakes occur on the Moon. Such moonquakes, if suffi-
ciently large, may damage man-made structures on the lunar
surface, may disrupt the activities of the lunar inhabitants, and may
cause injury or even loss of life.
From examination of the currently available observational data
on lunar seismic activity, we believe that there is sufficient reason
to be concerned about a possible seismic risk on the Moon. In
this paper, therefore, we will present some basic lunar seismic
data relevant to the question of a seismic risk and point out
deficiencies in our knowledge on lunar seismicity that need to
be resolved before we establish a base on the Moon.
LUNAR SEISMICITY
The data relevant to the seismic environment of the Moon were
acquired from 1969 to 1977 during the operational period of the
seismic station network established during the Apollo lunar
landing missions. Of the two types of seismographs used at each
station for covering two different spectral ranges of seismic waves,
the long-period seismographs recorded most of the seismic signals
relevant to the global seismicity. More than 12,000 seismic events
were detected by these instruments (Latham et al., 1970;
Nakamura etal., 1982). The majority of these events were tidally
induced moonquakes of body-wave magnitude mb less than 3
(energy release E < 106 J) deep in the lunar interior. These deep
moonquakes are not likely to pose any threat to lunar surface
operations. Next in abundance were the seismic events originating
from impacts of meteoroids on the lunar surface. The hazard of
these impacts and their effects on the lunar base activities require
.separate consideration and will not be evaluated in this paper.
The most important in terms of seismic hazard is a class of
moonquakes that occurs at shallow depths. They are believed to
be of tectonic origin and are likely to represent recurrent release
of thermoelastic stresses that have accumulated in the outer zone
by global cooling and contraction of the Moon (Nakamura et aL,
1979; Binder and Lange, 1980). Although less frequent than other
types of seismic sources, they constitute the most intense seismic
events on the Moon. The largest of them detected during the eight
years of observation had an estimated body-wave magnitude mh
greater than 5(E>6×lOmJ) (Nakamura et al., 1974, 1979;
Nakamura, 1977; Oberst, 1987).
We assign body-wave magnitudes to moonquakes by equating
the estimated total amount of energy released at the source in
the same way as for earthquakes: mh -----(log E + 1.2)/2.4 (Richter,
1958, p. 365). The effect of a moonquake on the lunar base,
however, may be quite different from what would be expected
for an earthquake of equal magnitude, as will be discussed later.
OCCURRENCE RATE OF
SHALLOW MOONQUAKES
Twenty-eight shallow moonquake events were identified during
the eight years of seismic observations. Based on the size
distribution of these 28 events and assuming that the shallow
moonquakes occur randomly distributed both in time and space
over the entire lunar surface, we estimate their occurrence rate
tobe
log N = -1.8 nab + 7.8 (1)
where N is the cumulative number of shallow moonquakes having
body-wave magnitudes greater than rnh occurring within an area
of 106 km 2 per year (Fig. 1). From this relationship we estimate
that the chances for a lunar base at a randomly chosen site to
experience a shallow moonquake of, for example, nab greater than
4.5 (E > 4 × 109j) within a range of 100 km are approximately
one in 400 years.
The average seismic energy release for the whole Moon from
the shallow events is estimated to be about 10t4J per year
(Nakamura, 1980). This is several orders of magnitude lower
than the rate of seismic energy released for the entire Earth,
estimated to be about 10mJ per year (Kanamo_ 1977). How-
ever, this is not surprising, because the overwhelming majority of
earthquakes are concentrated along active plate margins, which
do not exist on the Moon. In fact, lunar seismicity is very similar
to the terrestrial seismicity if the seismically active regions along
the plate margins are excluded (Nakamura, 1980). The estimated
occurrence rate for shallow moonquakes [equation ( 1 )] is indeed
very similar to the approximate terrestrial intraplate seismicity for
the central U.S. (Fig. 1; Nuttl6 1979).
Much about a possible seismic risk on the Moon can be learned
from studies of these intraplate earthquakes. Intraplate earth-
quakes occur preferentially in regions of preexisting weakness in
the lithosphere (Sykes, 1978). However, because of their long
recurrence inter_-als, they often occur in places where .such
weakness is unrecognized, and thus large earthquakes are
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232 2nd Conference on Lunar Bases and Space Activities
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Seismicity of
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Moonquakes
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3.5 4.0 4.5 5.0 5.5
Body Wave Magnitude
Fig. 1. Magnitude-frequency relationships of shallow moonquakes (open
circles) and intraplate earthquakes (line). The moonquake body-wave
magnitudes are computed from the estimated energy release of the 28
events observed during the 8 years of operation of the lunar seismic
network. The number of observed events has been converted to the rate
of occurrence within a unit area and time. Sampling is incomplete at small
magnitudes, because amplitudes of small-magnitude events occurring at
far distances fall below the detection threshold of the seismometers. The
earthquake data are from Nutt_ (1979).
unexpected. Three of the strongest earthquakes ever reported
within the contiguous U.S. did not occur near the margin of the
Pacific plate but occurred instead in the Missi_ippi valley near
New Madrid, Missouri, in 1811 and 1812. These unexpected and
most devastating intraplate events had estimated body-wave
magnitudes of 7.1, 7.2, and 7.4 (Nuttl6 1973a). Another large
intraplate event occurred in Charleston, South CaroLina, in 1886,
and had a body-wave magnitude of 6.7 (Boilinger, 1983). We
cannot rule out the possibility that similar very large seismic
events also occur on the Moon but were not detected during the
short duration of our observations.
OTHER FACTORS THAT
AFFECT SEISMIC RISK
There are several properties of shallow moonquakes other than
the rate of occurrence that have direct bearing on the seismic
risk. For some of these, direct observational data are still lacking.
Moreover, the unique lunar environment that affects the propa-
gation of seismic waves through the Moon also requires special
consideration.
Spatial Distribution
We presume that certain regions of the lunar surface are more
prone to shallow moonquake activities than others. Such regions
may be recognized by statistically significant spatial concentration
of observed shallow moonquakes. It is important to identify such
regions before we set up a lunar base because the intensity of
seismic risk is directly related to the proximity of the base to such
concentrations of events.
Nakamura et al. (1979 ) note that moonquakes appear to occur
_- preferentially near the margins of large mare basins, which may
p
-.7_ represent zones of weakness in the lunar lithosphere. In contrast,
- Binder and Gunga (1985) and Binder and Oberst (1985)
- suggest that shallow moonquakes are associated with young thrust
= fault scarps in the lunar highlands. Unfortunately, the number of
_-- detected shallow moonquakes to date is not large enough to make
- a statistically meaningful correlation with any topographic or
_ selenologic feature.
= Focal Depths
The severity of ground motion and, cortsequently, the potential
danger to a lunar base of a shallow moonquake depend directly
on focal depth. Unfortunately, the observational data do not
provide accurate focal depths of any of the detected shallow
6.0 moonquakes. This is because all shallow moonquakes detected to
date occurred far outside the seismic network, while accurate
determination of focal depth requires observations at near ranges.
Once we identify any region of high seismic activity on the Moon,
we need to set up local seismic stations in order to determine
precisely the focal depths of individual shallow moonouakes there.
Character of Ground Motion
Even on Earth it is difficult to predict the level of ground
motion for an earthquake of given magnitude occurring at a given
range (see Heaton and HartzeU, 1988, for a review). The
difficulty is further compounded for the Moon because of the
vastly different environment. The observed hmar seismic data
clearly show that the character of ground motion caused by a
shallow moonquake is significantly different from that of an
earthquake of equal magnitude at equal range for several reasons.
The source spectra of shallow moonquakes generally contain
much more energy at high frequencies than those of earthquakes
of comparable total energy release (Oberst, 1987). Since the
response of some man-made structures to the ground motion near
the epicenter is highly dependent on frequency, a significant
difference in potential damage to the structures is expected
between earthquakes and moonquakes.
Seismic waves are much less attenuated in the Moon than in
the Earth (Nakamura and Koyama, 1982). As a result, seismic
energy from a moonquake is likely to spread much farther from
the epicenter, thus affecting a wider area of the Moon than would
be expected for an earthquake of comparable magnitude. Such
a difference is observed between earthquakes occurring in the
eastern and western parts of the U.S. At a given magnitude,
earthquakes near the east coast are often felt over an area up to
100 times larger than those in the western U.S., since in the east
the more stable tectonic setting allows more efficient transmission
of seismic waves (Nuttlt_ 1973b).
Another distinct difference between the lunar and the terrestrial
seismic environment is the more intense scattering of seismic
waves in the highly fractured near-surface zone of the Moon than
on Earth. Seismic signals on E,-,rth are relatively short and
impulsive. In contrast, seismic signals from moonquakes show a
very gradual increase of intensity within the first several minutes
followed by an extremely slow decay, often lasting for several
hours. The dispersion of seismic energy over a very long time span
may reduce potential structural damage to the lunar base.
The intense seismic scattering in the near-surface zone and the
consequent destruction of coherent surface reflections prevent
ellicient long-range propagation of surface waves on the Moon.
Oberst and Nakamura: A seismic risk for the lunar base 233
Since surface waves, which decay more slowly with distance than
body waves, are often primarily responsible for earthquake
damage, the absence of surface waves may be of benefit to the
lunar base.
CONCLUSIONS
In spite of considerable scientific literature on lunar seismicity,
the general public is not fully aware that the Moon is tectonically
quite active. The seismic data now available suggest that shallow
moonquakes may pose a significant risk to the operation of the
lunar base. Studies of the seismic environment are common
practice before starting important construction projects such as
bridges, dams, and power plants on Earth. Such a study may also
be indispensable for the Moon before establishing a base there.
The currently available data are not sufficient to fuanly establish
the seismic risk for the lunar base. In particular, further studies
are needed in two general areas: (1)a better characterization of
the shallow moonquake sources, including their focal depths and
spatial distribution; and (2) the effect of the moonquake ground
motion, which is significantly different from earthquake ground
motion, to structures and inhabitants. For the former, establish-
ment of a global network of seismic stations deployed by, for
example, penetrators or soft landers may be required. Seismic
observations during the initial phase of the lunar base operations
will be beneficial for furthering our knowledge on the seismicity
and seismic risk.
Acknowledgt_,nt& We wish to thank C. Frohlich and S. Davis for
helpful discussion and for thorough review of the manuscript. We also
appreciated the critical comments by L Hood. This work was supported
by NASA grant NAGW-I064. University of Texas Institute for Geophysics
Contribution No. 769.
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A seismic risk for the lunar base

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