Sun and moon may help trigger slow earthquakes through resonance

The gravitational pull of the sun and moon may play a more important role in slow earthquakes than previously understood, with new modelling suggesting that even very small tidal stresses can influence how certain faults slip. Slow earthquakes differ from conventional earthquakes because they release accumulated strain gradually over hours, days or longer periods, often without producing strong shaking at the surface. Although links between tidal cycles and slow seismic activity have been observed before, the mechanism controlling that relationship has remained difficult to explain.

A research team led by Yishuo Zhou at PSL University in Paris investigated the problem using a simplified computer representation of a fault. The model used a spring-block system governed by rate-and-state friction, a framework that describes how resistance along a fault changes depending on its sliding velocity and the evolving condition of the fault surface. The researchers then applied small periodic stresses designed to reproduce the tidal forces generated by the gravitational influence of the sun and moon.

These tidal stresses are extremely small, typically only a few kilopascals, but the simulations showed that their effect can become much larger when the timing is right. If the period of the tidal loading matches the natural response timescale of the fault, the repeated forcing can produce resonance. The process is similar to pushing a swing: each individual push may be small, but when the timing matches the motion of the swing, the effect builds progressively.

Under certain combinations of friction, stress amplitude and timing, a fault that would otherwise slide steadily could shift into a slow earthquake. Stronger tidal forcing generally produced faster slip events, while weaker forcing could allow the fault to continue creeping more gradually. The response was not always predictable, however. Even regular, smooth tidal forces sometimes generated complex and irregular movement, demonstrating how a relatively simple external influence can interact with nonlinear fault behaviour.

The findings may help explain observations from tectonically active regions where slow earthquakes and tremors appear to vary with tidal cycles. Activity in areas such as southwest Japan and the Cascadia subduction zone has been observed to peak around 12-hour and 24-hour intervals, broadly corresponding with tidal periods. The modelling suggests that some faults may respond most strongly near the maximum tidal stress, while others may be more sensitive to the rate at which that stress is changing.

This relationship could eventually become useful for studying fault properties that are difficult to measure directly. By comparing observed slow-earthquake activity with well-known tidal cycles, researchers may be able to infer characteristics such as frictional behaviour, sensitivity to stress changes and the amount of slip required before weakening begins. Slow earthquakes are particularly important in subduction zones because they provide information about how stress is accumulated and released along plate boundaries that can also produce much larger megathrust earthquakes.

The study remains a simplified representation of a single fault patch, and natural fault systems are far more complicated. Real faults contain variable geology, fluids, interconnected structures and changing stress conditions that cannot be fully reproduced in a basic model. The results therefore do not suggest that the sun or moon directly cause ordinary earthquakes. Instead, they indicate that on faults already close to slipping, repeated tidal stresses may provide a small but well-timed trigger for slow seismic movement.