Moonquakes!

By Kassandra Del Greco

History: The Apollo Missions

• Starting in 1969 during the Apollo 12 mission, 6 seismic stations were installed

• In the next 10 years, thousands of small scale (2-3) earthquakes were recorded

• These earthquakes have since been classified into deep and shallow moonquakes

Moonquake or Meteoroid

• H-phase

• Time from beginning of signal to maximum signal amplitude

Internal Structure of the Moon

• Using moonquakes to determine there is mantle and core

• Lack of evidence of deep moonquakes originating on the far-side may indicate a molten interior/core

Deep Moonquakes

• 10 focus’ identified by 1971 although most moonquakes originated from the A1

• Depths of 600km-1000km (Moon’s radius 1737.4km)

• Periodicity of moonquakes indicates they are induced by tides

• Polarity reversal cycles can be attributed to a combination of tidal stress and a constant thermal stress

Tidal Stress

• Deep moonquakes occur in monthly cycles

• 7-month periodicities correlate to long-term gravity variations

• Moonquakes occur at depths of max tidal stress

Other Sources of Stress

• Slight expansion of the Moon by internal radiogenic heating or slight contraction on cooling

• Localized strains due to uncompensated masses

• Localized thermal stress

Sources of Stress

• Cumulative amplitude curves with linear slope could indicate 3 types of stress

• Thermal Stresses of possible volcanic origin

• Tectonic Stresses at low stress levels

• Dislocations along pre-existing fractures

Moonquake Swarms

• Distinctive sequence of moonquakes closely grouped in space and time

• Characterized by an abrupt beginning and ending of activity

• Semi-monthly periodicity indicates lunar tide trigger

• They do not have matching waveforms as regular deep moonquakes do

• Suggested relation to volcanism as they are found on earth

Moonquake Swarms

Evidence Against Tectonics

• Yearly seismic energy release in far below that of the earth

• Internal convection leading to lunar tectonism is most likely absent

• Much less heat is relased from the Moon than would be expected with convection

• This would indicate that swarms are signs of continuing minor adjustment to crustal stresses

Conclusion

• Moonquakes are cool (literally)• Unlikely tectonic association• Tidal stress• Studying the way seismic waves

travel can give insight on the interior composition of the moon

• More studies should be done

References

Latham, G. V., M. Ewing, J. Dorman, D. Lammlein, F. Press, N. Toksöz, G. Sutton, F. Duennebier, Y. Nakamura (1972), Moonquakes and lunar tectonism, Moon, 4, 373–382.

Nafi Toksoz, M., Groins, N.R., Cheng, C.H. (1977), Moonquakes: Mechanisms and Relation to Tidal Stresses, Science 27 May 1977: 196 (4293), 979-981. [DOI:10.1126/science.196.4293.979].

Nakamura, Y. (2005), Farside deep moonquakes and deep interior of the Moon, J. Geophys. Res., 110, E01001, [DOI:10.1029/2004JE002332].