The Moon and the Sun are not the primary causes of earthquakes, but their gravitational pull can, under certain conditions, help move faults that were already under stress.
A study titled “Theoretical limits on the triggering of slow earthquakes by tidal forces” shows that the gravitational pull of the Moon and the Sun can, in specific circumstances, trigger slip on some faults.
Besides creating tides in the oceans, the gravitational force of the Sun and Moon also slightly stretches and compresses the Earth's solid crust. These changes, called “tidal stress,” are very small-on the order of a few kilopascals and comparable to a gentle hand squeeze. Nonetheless, it has been observed that they can trigger slow earthquakes on some faults.
To investigate how this effect works, researchers used a computer model of a fault. The model results show that if the repetition period of tidal stress matches the natural response timescale of the fault, the fault may become more sensitive to it. The authors liken this to resonance: just as gently pushing a swing at the right rhythm can increase its amplitude, weak but repeated stress variations can increase the slip rate of a sensitive fault.
Thus the Sun and Moon do not create the tectonic stress that accumulates on faults, but tidal forces can, under suitable conditions, change the timing or magnitude of slip on a fault already primed to fail. The study's model shows these slip events can be slow or rapid depending on fault properties, and the amplitude and period of the stress variations.
These changes are not noticeable on a day-to-day basis, but they can matter for faults under high stress. Faults are fractures in the Earth's crust where the two sides can move relative to each other. In some of these faults, the movement of tectonic plates gradually builds large stress, bringing rocks close to the point where slip becomes possible.
Earthquakes no one may feel
The focus of this research is a specific type of tectonic activity called a “slow earthquake.” Unlike ordinary earthquakes, where fault slip and energy release occur quickly and can violently shake the ground, in slow earthquakes the movement can continue for days or weeks. In such cases, parts of the fault slowly slide past each other and little or no strong shaking may be felt at the surface. This characteristic makes studying this phenomenon important for understanding fault behavior, because some deep Earth changes can continue for long periods without obvious surface signs.
How does this finding help earthquake detection?
Understanding the resonance phenomenon allows scientists to identify particular days of the month when tidal gravitational forces peak. Relief and emergency management teams can use these data to narrow down “high-risk” time windows for possible tectonic slip. These findings also allow scientists to determine how sensitive some faults are to tidal forces so they can monitor their condition from the surface and gain necessary preparedness.
These findings may explain why some faults are sensitive to very weak tidal forces while others are not. Comparing the timing of slow earthquakes with the tidal stress cycle may also help researchers better estimate fault properties such as frictional resistance.
However, the results of this study do not mean we can now predict the timing of a major earthquake from the positions of the Moon and the Sun. What this research provides is a way to examine more precisely one of the small forces that may affect the timing of slip on faults already under stress, whose energy has been accumulating in the crust for years or even centuries.