How the Sun and Moon Quietly Shake the Earth Beneath Our Feet

A Cosmic Tug With Earthly Consequences

The Sun and Moon exert small but constant tidal stresses on Earth's crust, and scientists have observed that these forces can trigger creeping "slow earthquakes" — events that release their energy far more slowly than ordinary quakes and are completely imperceptible to those of us living on the surface. It's a mind-bending idea: the same celestial forces that pull our oceans into tides are also, quietly and persistently, nudging the ground beneath our feet. But exactly how they accomplish this has remained relatively mysterious — until now. New modeling suggests the mechanism could be resonance, similar to running a wet finger around the rim of a wine glass to produce a ringing tone, only playing out at planetary scales.

Geo-planetary scientists publishing in the journal *JGR Solid Earth* modeled these tidal perturbations using a spring-block system and rate-and-state friction, revealing how minute pressures of just a few kilopascals can destabilize tectonic faults. To put that in perspective, those stresses are "comparable to the pressure from a gentle hand press," as the researchers themselves describe it. That something so delicate could move mountains — literally — is one of the more remarkable findings in recent seismology.

The Physics of Resonance, at a Planetary Scale

Scientists employed computer modeling to show that these forces operate according to the physics of resonance — meaning that a push, despite being gentle, can create a significant change under the right rhythm. When the cyclic pull of the Sun and Moon matches the natural internal rhythm of a fault line, slow earthquakes are triggered.

The model suggests that the timing of triggering depends on both the tidal cycle and the fault line's own properties. In other words, not every fault responds the same way. Some may be far more sensitive to tidal nudges than others, and the new framework helps scientists begin to sort out which is which. Scientists have previously observed strong tidal modulation of slow earthquakes and tremor in places including southwest Japan and the Cascadia subduction zone of North America. These regions have now become critical testing grounds for the new theory.

A Surprisingly Useful Tool for Forecasting

The Sun and Moon also offer scientists something unusually useful: predictable forcing. Their positions can be calculated with extreme accuracy, allowing researchers to determine when particular tidal stresses should act on faults. Unlike many geological forces, which can change in complicated and poorly observed ways underground, astronomical tides repeatedly apply small stresses at known times.

By modeling the influence of tidal perturbations on Earth's crust, this work provides a framework for interpreting tidal patterns in slow earthquakes, which could eventually feed into earthquake forecasting efforts. For example, seismologists could "reverse engineer" detected earthquakes to ascertain the properties of faults, such as their frictional strength, by matching measured seismic activity to tidal activity. Understanding resonance will also allow scientists to measure specific days of the month when gravitational pull is at its peak — data that could be used by disaster response teams to narrow down "high-risk" windows for potential tectonic shifts.

Silent Earthquakes With Loud Implications

The slow earthquakes caused by the Sun and Moon happen silently for years underground, until one day they contribute to a destructive earthquake. That long, quiet buildup is precisely what makes them so dangerous — and so important to understand. Understanding these plate interfaces aids researchers in assessing the rupture potential of future megathrust earthquakes, which rank as the world's most powerful seismic events and carry the potential to generate tsunamis and awaken volcanoes.

For seismologists, this could turn one of the smallest known influences on earthquakes into a useful way of understanding much larger forces hidden underground. The idea that the Sun and Moon — objects hundreds of thousands to millions of miles away — could serve as a key to unlocking the secrets of fault lines deep within our planet is as humbling as it is promising. Science has long known these celestial bodies shape our tides. Now, they may also help us predict our next big quake.