Annelise Miska and Anne-Sophie Berg
EPSC 330
MOONQUAKES
From 1969 to 1971, the National Aeuronatic and Space Administration (NASA) led Apollo 15,
12, and 14 missions to the Moon. During these missions, they installed three geophysical
stations in an equilateral triangle shape spanning a total of 1100km, which would record
thousands of seismic events on the lunar surface over the following ten years. The installation of
the Apollo seismic network on the Moon resulted in the collection of important geophysical data
and gave insight into the Moon’s interior structural elements through the interpretation of
seismic event data.
What do the seismic stations record?
The seismic signals recorded by the seismographs are a result of natural and anthropogenic
causes. Man-made signals originate from both the impacts and boosters of space shuttles
(Apollo 12 and 14). The natural seismic signals arise from meteoroid impacts and moonquakes.
The Moon’s Structure + Seismic Waves
Through techniques such as laser ranging and electromagnetic sounding, a small metallic liquid
core has been identified at the center of the moon (definitive proof has yet to be discovered). It
represents about 1% to 3% of the total body’s mass.
Moonquakes are characterised by a long duration (ex: signals of man made impacts continue
for up to 4 hours) and the absence of successions of body or surface waves ordinarily found on
Earth during seismic events. Scientists have explained this by a thin (~20 km deep)
heterogenous layer around the surface of the Moon. This layer only contains minor amounts of
fluid causing intense scattering of seismic waves. Below this zone is another layer which is able
to transmit waves sufficiently without any significant scattering.
How do lunar seismic signals differ from terrestrial signals?
The lunar moonquakes are distinguished from terrestrial quakes by the difference in energy
released during seismic events. Apollo 12 seismic stations recorded a release of energy
amounting to 10x10^3 - 10x10^15 J per year. Comparatively, this is significantly lower than
seismic energy released from earthquakes, which amounts to about 50x10^17 J per year.
How are seismic signals originating from moonquakes distinguished from those caused
by meteroid impacts?
The natural seismic signals are distinguished by:
(1) the ‘H-Phase’ which is a recorded incoming group of waves with distinct boundaries;
(2) the rise time (time elapsed between the beginning of the signal and its maximum amplitude).
Moonquakes are characterized by the presence of an H-Phase and a short rise time. ‘C-events’,
designated as man-made and meteorite impacts, are distinguished by their longer rise times,
‘emergent’ beginnings and exceedingly long durations (up to 4 hours).
Moonquake facts
The recorded moonquakes are proposed to have originated from ten small foci approximately
10 km in width, with 80% of the seismic energy being accounted for by a single originating focus
estimated to be at 800 km depth. Except for a small amount of shallow events, most
moonquakes originate at depths of about 600 to 1000 km inside the moon. The periodicity of
these moonquakes recurs at 27-day intervals, 206-day intervals and also at 6-year intervals.
The recorded moonquakes are relatively small, registering on the Richter scale at magnitudes
between 2 and 3.
Different types of Moonquakes
(1) Periodic Moonquakes: with few exceptions, the periodic moonquakes occur at monthly
intervals near times of perigee and apogee and show correlations with longer-term lunar gravity
variations (~ 206 days).
(2) Swarm Activity: a period of episodic seismic events generating up to 8 - 12 moonquakes per
day. Characterized by an abrupt onset and termination of seismic activity. Although the source
of this swarm activity is not precisely known it is proposed to be correlated to lunar tides due to
the semi-monthly periodicity.
(3)Thermal Quakes: the crust of the moon becomes very brittle after spending 2 weeks in lunar
night, shaded from any sunlight. After this period, comes lunar day at which point the crust
quickly warms and experiences rapid thermal expansion.
How do lunar tides create earthquakes?
It has been proposed that the periodicity of moonquakes is related to tidal forces acting on the
moon. These tidal forces are a consequence of the gravitational interactions between the Earth
and the moon. The centripetal force drives these two bodies to rotate around their common
center of mass. As the moon’s gravitational field acts upon the Earth, and vice-versa, the two
bodies experience differential strain across their respective surfaces.
The tidal stresses in the moon have been calculated to explore this possibility of a causality.
The periodicity of tidal stresses is marked by subtle changes, nevertheless these slight stress
variations would be enough to generate small magnitude moonquakes. A correlation was found
between the tidal shear stress maxima and the depth at which the majority of moonquakes
occur.
Polarity reversals
Additionally, some moonquakes have been recorded to experience polar reversals. That is, the
direction of motion registered at the focus underwent a reversal, maintained this reversal for 3
years and then switched back to the original direction of motion. This reversal is likely an
expression of changing fault slip motion caused by a component of the tidal stresses and a
constant ambient tectonic stress of unknown origin.
References
1. Latham, G., M. Ewing, J. Dorman, D. Lammlein, F. Press, N. Toksoz, G. Sutton, F.
Duennebier, and Y. Nakamura. "Moonquakes." Science 174, no. 4010 (1971): 687-92.
2. Latham, Gary, Maurice Ewing, James Dorman, David Lammlein, Frank Press, Naft Toksoz,
George Sutton, and Fred Duennebier. "Moonquakes and Lunar Tectonism." In Conference of
Lunar Geophysics. Houston, Texas, 1971.
3. Nakamura, Yosio. "Farside Deep Moonquakes and Deep Interior of the Moon." Journal of
Geophysical Research 110 (2005).
4. Toksoz, M. Nafi, Neal R. Goins, and C.H. Cheng. "Moonquakes: Mechanisms and Relation to
Tidal Stresses." Science 196, no. 4293 (1977): 979-81.