A new study has suggested that Earth’s gravity may be responsible for deep moonquakes, earthquakes taking place inside our lunar companion.
Moonquakes are odd in that the Moon doesn’t have tectonic activity like our own planet, which causes our earthquakes. Thus, scientists had sought another explanation.
Between 1969 and 1977, seismometers taken to the Moon by the Apollo missions recorded deep moonquakes about 800 to 1,200 kilometers (500 to 745 miles) under the surface every 27 days. A re-analysis of this data, published in the Journal of Geophysical Research: Planets, suggests Earth is the culprit.
The team, led by Taichi Kawamura from the National Astronomical Observatory of Japan, studied more than 100 deep moonquake events. They looked at the fault characteristics, and found that tidal stresses caused by Earth’s gravity could mimic the cause of earthquakes on our planet.
Known as solid-body tides, these tidal stresses can form faults or cracks inside the Moon. As the stress builds up, these can rub against each other and eventually form moonquakes. The idea has been mooted before, but this study provides more data on the odd phenomenon.
“The correlation between the deep moonquake occurrence and tidal stress has been pointed out and our study supports the idea that the tidal stress is not only triggering the deep moonquakes but also responsible of the whole stress release of the seismic activity,” the team write in their paper.
However, they noted that previous studies had underestimated the stress that was released, because they only used readings from one of the seismometers. By combining signals from different instruments, this study found that the buildup of tidal stresses took place over a month.
The 27-day regularity of moonquakes also supports the idea they are caused by Earth’s gravity, as this is the same time it takes the Moon to orbit Earth. On our planet, a particular fault line only experiences an earthquake decades or centuries apart, caused by shifting plates.
“Moonquakes also last longer than earthquakes, which typically cease within a few minutes,” Madeleine Jepsen noted for the American Geophysical Union. “Since the moon is much drier and cooler than Earth, the vibrations carry for longer, whereas Earth’s more compressible structure acts like a sponge to absorb vibrations.”
Quite how moonquakes affect the Moon isn’t known yet, but the researchers note they may point to the Moon’s mantle being colder than thought.