Phases Lunaires et Tremblements de Terre: Y a-t-il une Connexion?

The Myth: Moon Phases Affect Earthquake Frequency

The moon's gravitational pull is undeniably real — it drives ocean tides, and at sufficient sensitivity, it deforms the solid earth as well. This genuine physical effect has fed a persistent popular belief: that the lunar cycle influences earthquake frequency, with some people claiming that more earthquakes occur during full moons or new moons, or during lunar perigee when the moon is closest to Earth. The hypothesis has intuitive appeal because it invokes a real physical mechanism. But does the data support it?

The Real Physics of Earth Tides

The moon (and to a lesser extent, the sun) does exert tidal forces on the solid Earth. As the Earth rotates and the moon revolves, tidal bulges sweep around the planet's surface. The resulting "Earth tides" — periodic deformation of the solid crust — are measurable with sensitive instruments and amount to vertical displacements of up to 30 cm at the equator. The stress changes associated with Earth tides on fault systems can be calculated precisely using established celestial mechanics.

Here is the key quantitative point: Earth tidal stress changes on fault planes are on the order of 0.001-0.01 megapascals (1-10 kilopascals). This is at the very low end of what might influence fault systems that are already critically stressed — tectonic stresses on active faults are on the order of 10-100 megapascals, and earthquake-triggering stress changes from Coulomb Stress TransferThe process by which an earthquake changes stress on nearby faults, potentially triggering or delaying future earthquakes. Used to forecast which faults are brought closer to failure. transfer are typically 0.01-1 megapascal. Earth tidal stresses are therefore very small relative to the stress levels relevant to earthquake nucleation.

What Studies Have Found

Several careful statistical studies have searched for lunar correlations in earthquake catalogs, yielding nuanced results. A 2016 study in Nature Geoscience found a statistically significant correlation between Earth tides and large (M8+) earthquakes at subduction zones — specifically, that the largest earthquakes preferentially occur during periods of higher tidal stress on the fault plane. The correlation was modest and only clearly apparent for the very largest events.

This is scientifically interesting, but it is not what the popular moon phases earthquakes myth claims. The folk claim is usually that the full or new moon causes more earthquakes — a claim about the phase of the moon as visible from Earth rather than the precise tidal stress state on a specific fault. These are different things. Tidal stress on a fault depends on the geometry of the fault, its location on Earth, and the precise positions of the moon and sun — not simply on whether the moon appears full.

Studies specifically testing the "full moon = more earthquakes" hypothesis by comparing earthquake catalogs to lunar phase calendars have generally found no significant correlation, or correlations too small to have practical predictive value. The Earthquake ClusteringThe tendency for earthquakes to occur in clusters (mainshock-aftershock sequences or swarms) rather than randomly in time. Violates the common assumption of independent, random occurrence. patterns in real earthquake catalogs reflect tectonic processes (aftershock sequences, regional stress transfer) that are many orders of magnitude larger in effect than any lunar modulation.

Confirming Bias in Lunar Earthquake Claims

The cognitive pitfalls here are substantial. People who believe in the lunar earthquake connection will notice and remember earthquakes that occur near the full moon and will not notice or will discount the many earthquakes that occur at other phases. Earthquake occurrence is continuous and global — with thousands of magnitude 2+ events daily — so any observer can easily find recent earthquakes near whatever lunar phase they are searching for. This is pure confirmation bias, not evidence.

Prediction apps and websites that offer "elevated earthquake probability" windows based on lunar perigee or full moons exploit this cognitive tendency. A correct prediction is memorable; the many incorrect high-probability windows that see no major earthquake are forgotten. The Earthquake Prediction vs ForecastingPrediction claims to specify exact time, place, and magnitude of a future earthquake — currently impossible. Forecasting provides probabilistic estimates of earthquake likelihood over time periods. distinction matters enormously here: even if tidal stress does marginally modulate earthquake probability on already-failing faults, this effect is far too small and non-specific to use for operational earthquake forecasting.

Comparing Signal Magnitudes

To put the lunar effect in perspective: the Coulomb Stress TransferThe process by which an earthquake changes stress on nearby faults, potentially triggering or delaying future earthquakes. Used to forecast which faults are brought closer to failure. change from a M6.0 earthquake can increase the probability of nearby fault rupture by orders of magnitude in the months following the event. Aftershock probabilities based on Omori's LawAn empirical law describing the decay rate of aftershock frequency over time: the rate of aftershocks decreases roughly as the inverse of time since the mainshock. and regional seismicity statistics far outperform any lunar-based forecasting in peer-reviewed prospective tests. Even barometric pressure changes, though unable to trigger earthquakes, cause larger absolute stress changes in the crust than Earth tides do in many situations.

The fact that scientists spend effort studying tidal correlations reflects the completeness of earthquake science — researchers investigate every plausible physical mechanism, no matter how small. Finding a small, statistically marginal effect for the very largest earthquakes is scientifically interesting as evidence that fault systems are exquisitely sensitive when near critical stress. But "sensitive near criticality" is very different from "the full moon causes earthquakes."

Tidal Triggering and Volcanic Earthquake Swarms

An interesting genuine case of tidal influence on seismicity involves Volcanic EarthquakeAn earthquake associated with volcanic activity, caused by magma movement, gas pressure, or rock fracturing near a volcano. Often occurs in swarms and can signal an impending eruption. swarms on mid-ocean ridge systems, where the crust is already hot, thin, and near the solidus. Some studies have found that earthquake swarms on segments of the East Pacific Rise and other spreading centers are modulated by tidal stress, with swarms preferentially occurring during the extensional phase of the tidal cycle. These are Earthquake SwarmA sequence of earthquakes occurring in a localized area over days to months with no clearly dominant mainshock. Often associated with volcanic activity or fluid injection. events on oceanic faults under very different conditions from typical continental seismicity, and they do not generalize to supporting the idea that California has more earthquakes during full moons.

The Practical Upshot

The lunar earthquake connection is primarily a myth, with one small scientifically interesting kernel: tidal forces do exert real if tiny stresses on faults, and for the very largest earthquakes on the most sensitive fault systems, a marginal correlation with tidal loading has been detected. This does not translate into any practically useful forecast capability. Earthquake preparedness should be continuous and not scheduled around lunar calendars. Every day requires a functioning Earthquake Emergency KitA pre-assembled collection of supplies for surviving the aftermath of an earthquake, typically including water (1 gallon/person/day for 3 days), food, first aid, flashlight, and radio., a practiced Drop, Cover, and Hold OnThe internationally recommended protective action during earthquake shaking. Drop to your hands and knees, take cover under sturdy furniture, and hold on until shaking stops. response, and a clear Earthquake PreparednessThe ongoing process of planning and preparation to minimize earthquake impact, including securing furniture, creating communication plans, maintaining emergency supplies, and practicing drills. plan — regardless of what phase the moon is in.