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Scientists may have identified how the gravitational influence of the Sun and Moon can trigger certain types of earthquakes, according to a new modeling study reported by ScienceAlert.
The Sun and Moon exert a weak but constant tidal force on Earth’s crust. Researchers have long known that these forces can contribute to so-called slow earthquakes, or creep events, in which tectonic stress is released much more gradually than during conventional earthquakes. Because the movement occurs over a longer period, these events can go unnoticed by people at the surface.
The mechanism behind these events, however, has remained unclear. The new research suggests that resonance may play a key role — similar to the phenomenon that makes a glass ring when a wet finger moves around its rim, but occurring on a planetary scale.
Small forces can trigger movement
Researchers modeled tidal stresses and the processes of slip, displacement and friction along tectonic faults using a spring-block model and a rate-and-state friction law.
“Although these stresses are very small — typically a few kilopascals, comparable to the pressure of a light touch of the hand — they can trigger slow earthquakes on some faults,” the researchers said.
The model focused on faults that normally move at a relatively steady rate but can be pushed out of equilibrium by external forces. As the rate of movement increases, friction along the fault decreases, allowing the fault to slip more easily.
Under certain conditions, the model produced both rapid and slow slips.
Resonance may determine the type of earthquake
According to the researchers, the resulting seismic activity depends largely on two characteristics of the tidal force: its amplitude and its period.
When the tidal force is relatively weak, a fault may continue slipping slowly without producing noticeable seismic activity. But if the force exceeds a certain threshold and its period falls within a specific range, it can trigger more significant movement along the fault.
The key factor is whether the timing of the tidal stress matches the fault’s “characteristic response time” — the time required for the fault zone to respond to changes in external stress and friction.
The researchers compared the process to pushing a swing at exactly the right rhythm. Even a small force can produce a much larger response when applied at the appropriate frequency.
Observations from Japan and Cascadia
The model’s predictions were consistent with observations from several regions where slow seismic activity follows tidal cycles.
Researchers pointed to observations in southwestern Japan and the Cascadia subduction zone in the Pacific Northwest, where slow earthquake activity can peak at intervals of roughly 12 and 24 hours.
The findings suggest that tidal forces do not simply act as a constant trigger. Instead, the timing and strength of the forces may determine whether a fault continues creeping slowly or undergoes a more noticeable seismic event.
The study could help scientists better understand why some faults respond strongly to tiny external stresses while others remain stable.
Recent research has also identified hundreds of previously unknown deep earthquakes beneath Antarctica, highlighting how much seismic activity may remain hidden from conventional observation.
