Forces from the Moon and Sun that are too weak for humans to feel may still influence hidden movements beneath Earth’s surface, according to new research that explains how tiny tidal stresses can affect certain faults under the right conditions.
A team of geoscientists led by Yishuo Zhou of Université PSL in Paris found that repeated gravitational forces from the Moon and Sun may contribute to fault movement through a resonance-like process, where the timing of small stresses matches the natural response of a fault. The study was published in the Journal of Geophysical Research: Solid Earth.
Researchers explained that the mechanism is similar to pushing a swing at the right rhythm. While tidal forces are extremely small compared with the forces generated by shifting tectonic plates, repeated stresses at the right timing can make some faults more likely to slip.
The study focused on slow earthquakes, which are gradual fault movements that release energy over longer periods instead of producing the sudden shaking associated with major earthquakes. Using numerical models, the researchers examined how fault friction, timing, and stress cycles affect whether small tidal forces can influence movement.
“Even very weak tidal stresses can induce faults to slip through a process called resonance,” the researchers wrote, according to Phys.org’s report on the findings.
The research adds to decades of studies exploring whether Earth’s tides can influence earthquake activity. A 2004 study published in Science by Elizabeth Cochran, John Vidale, and Sachiko Tanaka found that tidal stresses were linked to changes in the occurrence of shallow thrust earthquakes, with the strongest relationship observed in faults where one section of Earth’s crust moves over another.
Scientists emphasize that tidal forces are not the main cause of earthquakes. The movement of tectonic plates remains the primary driver of major earthquakes, while tidal stresses may act as a small additional influence on faults that are already close to failure.
The findings are especially relevant to earthquake-prone regions such as the Philippines, which lies along the Pacific Ring of Fire and is home to numerous active fault systems. Among the country’s most monitored faults are the Philippine Fault Zone and the Marikina Valley Fault System, which includes the West Valley Fault beneath parts of Metro Manila.
The Philippine Institute of Volcanology and Seismology has warned that a magnitude 7.2 earthquake scenario along the West Valley Fault could severely affect Metro Manila and surrounding provinces. The agency estimates that such an event could result in more than 33,000 deaths and heavily damage about 168,000 buildings.
However, scientists said the new study does not predict when an earthquake will occur and does not show that tidal forces from the Moon or Sun can trigger the “Big One” in the Philippines. Evidence of tidal influence remains strongest in certain thrust faults, while strike-slip faults like the West Valley Fault have shown weaker connections.
Instead, researchers say the study provides a clearer understanding of how small external forces can interact with sensitive faults. By learning more about these subtle processes, scientists hope to improve knowledge of fault behavior in regions where earthquake preparedness remains a major concern.
