By analogy with earthquakes on the Earth, there are so called »moonquakes« on the Moon. By that, we are referring to tremors on the Moon. The astronauts of the five Apollo missions in 1969 to 1972, left seismographs on the Moon, to measure these seismic waves. Afterwards, all data had been sent back to Earth until 1977 and analyzed.
The results of this data analysis show that there are different types of moonquakes:
- vibrations, which are caused by the impact of meteorites on the Moon
- deep moonquakes, which happen in approx. 700 km (435 mi) depth, below the Moon’s surface
- thermal moonquakes that are caused by the expansion of the frigid moon crust, when they are being warmed up by the sun
- shallow moonquakes that take place 12–18 mi below the Moon’s surface
The first three are usually short and of mild intensity. The shallow earthquakes, however, last up to 10 minutes (in comparison: an earthquake lasts normally about 10–30 seconds) and reach a magnitude of 5.5 on the Richter scale (this correlates to a moderately severe earthquake with damage to property).
For more than 40 years, scientists have been trying to find an explanation for these quakes. Because the Moon does not have moving tectonic plates like the Earth, there have to be other physical laws and causes that trigger these moonquakes. It is assumed that the quakes might be set off by Earth’s gravity, similar to the tides of the oceans, which are caused by the Moon. This thesis is confirmed by the fact that moonquakes happen in steady, chronological rhythm, which shows in the measurements.
Fortunately, moonquakes do not cause any damage. And whoever is under the impression, when gazing at the Moon that it might be trembling, has an impressive explanation on hand. However, in this case, it is more likely that they are actually the knees of the observer that are trembling.