The Sun and Moon don’t just raise the oceans: Tidal forces resonate with Earth’s faults, triggering “slow earthquakes” that release energy without making any noise.
It’s not a fatal attraction, but almost. Every day the Sun and Moon raise and lower the oceans. Less known is that they also pull up and down the solid rock we walk on and subject it to very small tensions. Now a group of scientists has better understood what happens when these tensions encounter a fault, that is, a fracture in the earth’s crust that is already ready to move.
Earthquakes that don’t make noise
They are called “slow earthquakes”: sliding along faults that release their energy gradually, over times much longer than those of ordinary earthquakes to the point that we often don’t notice them. They don’t collapse anything and are usually not heard, but the instruments record them. They occur in some of the planet’s major tectonic zones, including subduction zones, where one plate slides under another.
In places like southwestern Japan and the Cascadia Subduction Zone in North America, scientists have already observed that these events follow the rhythm of the tides. However, a puzzle remained: how could such a weak force have such a noticeable effect?
A fault inside the computer
To respond, a team led by Yishuo Zhou, from PSL University in Paris, built a virtual fault line. The study, with Ankit Gupta, Hideo Aochi, Alexandre Schubnel, Satoshi Ide and Harsha Bhat among the authors, was released on September 20, 2026 on Joeurnal of Geophysical Research: Solid Earth.
The model is deliberately simple: a block that slides along a surface while being pulled by a spring. To make it realistic, the researchers incorporated well-established friction rules, which describe how the rock’s resistance changes when it is stationary or moving, and also considered the behavior of different types of rocks. Then they subjected the system to short pushes and repeated wave forces, designed to mimic the gravitational pull of the Moon and Sun.
The secret is the rhythm
The result is that three things count together: timing, intensity and friction. Tidal forces “alter the rhythm” of faults that would otherwise flow constantly and the trigger is triggered when the rhythm of the thrusts coincides with the speed with which the fault naturally responds to the stresses, i.e. its movement linked solely to the geology of the place. That reaction time depends on the internal friction of the rock.
The mechanism is called “resonance”. If the thrust amplitude is low, the fault continues to slip silently.
Beyond a certain threshold and with the right period, it slides stronger. The authors use a simple image to give an idea of what happens: pushing a swing at the right pace makes it rise more.
But let’s try to understand this concept better: imagine a fault that moves very slightly every day, constantly and without shocks. It is “stable”: the rock around it is rigid enough to prevent it from creating a snap on its own (it would be a strong earthquake). Zhou’s model starts here, from a fault that wouldn’t do anything spectacular if we left it alone. Now the three things that matter come into play: the “intensity”, that is, how strong the push of the tide is. We’re talking about a few kilopascals, much less than what would be needed to rupture a fault. Then there is the “timing”: that is, how often the push arrives. Tides are regular cycles, coming and going, and their period matters. And finally “friction”: it decides the “natural time” of the fault’s reaction, that is, how long it takes to respond to a stress.
The seesaw effect: the secret of resonance
Each fault, due to its friction, has its own internal rhythm. Coming to the example with the swing, it must be said that once you give a push the swing will have its own rhythm: if you push it every time it returns towards the person pushing it, it rises more and more even with light pushes. If you push it haphazardly, or at the wrong pace, it won’t go anywhere. This is how the fault works. It has a natural rhythm, given by its friction. The tide pushes it periodically. When the tidal period coincides with the internal rhythm of the fault, the thrusts add and the effect amplifies. Here’s the resonance: a weak stimulus produces a much larger response. However, not everything is predictable: even regular and uniform tidal forces sometimes produce disordered and irregular movements. Stronger pushes give faster events, and under certain conditions the model shows that the same mechanism can also trigger fast, not just slow, slides.
