“Moonquake”, is it an earthquake on the moon?

Have you heard of moonquakes ?

When we look up at the starry sky, the moon is undoubtedly the most eye-catching celestial body. In addition to its bright surface and unique halo, the moon also hides many mysteries, one of which is moonquakes .

What is a moonquake?

Moonquakes, as the name suggests, are seismic activity that occurs on the surface of the Moon. Like earthquakes on Earth, moonquakes are caused by geological activity inside the Earth. Moonquakes vary in frequency and energy range, with some triggered by impacts in craters, while others are caused by tectonic activity inside the Moon.

Moonquake (Image source: Baidu)

Apollo Missions

As early as the 1960s, the Apollo 11 mission, the first human landing on the moon, began studying moonquakes.

Passive moonquake experiments were conducted at Apollo 11, 12, 14, 15, and 16 stations, and the first type of active moonquake experiment was located at Stations 14 and 16. The second type of active experiment, also known as the Lunar Surface Profiling Experiment, was located at Station 17. Station 17 also included a lunar surface gravimeter that provided additional passive moonquake information. (Modified from Nunn et al., 2020)

(Modified from Nunn et al., 2020)

Astronauts installed seismometers on the lunar surface and recorded a large number of earthquake events. These data have become an important basis for us to understand the internal structure and activities of the moon.

"That's one small step for man, one giant leap for mankind" (Image source: Baidu)

Classification and characteristics of moonquakes

According to the type and source of moonquakes, we can divide moonquakes into two categories: one is shallow moonquakes , which are usually caused by meteorite impacts; the other is deep moonquakes , which are caused by internal tectonic activities of the moon.

Examples of deep moonquakes, meteorite impacts, shallow moonquakes, and artificial impact events (modified from Nunn et al., 2020). These events were recorded on the three components (MHZ, MH1, and MH2) of the Apollo 12 station.

Deep moonquakes are the most numerous natural events, distributed at depths of 700 to 1,200 km. They have highly repeatable waveforms, indicating that they originate from adjacent areas. Deep moonquakes have more persistent energy and last longer , and these characteristics help scientists infer the nature and evolution of the lunar interior structure.

Schematic diagram of moonquake (picture from Baidu)

Future research and exploration

In recent years, with the widespread application of new data processing technologies in lunar exploration, including receiver functions, noise cross-correlation, seismic coda analysis, array-based waveform superposition methods, and waveform analysis technology based on spatial seismic wave field gradients , our understanding of the lunar interior has been significantly improved.

These methods have further revealed the structure and activity of the lunar interior, however, we are still faced with the challenge of insufficient understanding of the lunar interior. Let us look forward to the performance of the future Chang'e mission!

References:

[1] Zhang Junce, Hu Caibo, Shi Yaolin. Study on the causes of thermal event moonquakes using finite element thermal stress calculation method[J]. Journal of University of Chinese Academy of Sciences, 2024, 41(01): 50-64.

[2] Tao Sha, Shi Yaolin, Zhu Bojing. Preliminary analysis of the relationship between thermal stress and moonquake mechanism during lunar cooling[J]. Chinese Journal of Geophysics, 2023, 66(09): 3730-3746.

[3] Zhang Xiang, Zhang Jinhai. Progress and prospects of moonquake research[J]. Reviews of Earth and Planetary Physics, 2021, 52(04): 391-401. DOI: 10.19975/j.dqyxx.2021-032.