月の満ち欠けと地震: 関連があるか?

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 クーロン応力伝達地震が周辺の断層の応力状態を変化させ、将来の地震を誘発または遅延させうる過程。どの断層が破壊に近づいたかを予測するために用いられる。 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 地震クラスタリング地震がランダムに発生するのではなく、本震・余震系列や群発地震のようにクラスター（集団）として発生する傾向。地震が独立してランダムに発生するという一般的な仮定に反する。 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 地震予知と地震予測の違い予知は将来の地震の正確な時刻・場所・規模を特定することを指すが、これは現在のところ不可能である。予測は、ある期間における地震発生の可能性を確率的に見積もるものである。 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 クーロン応力伝達地震が周辺の断層の応力状態を変化させ、将来の地震を誘発または遅延させうる過程。どの断層が破壊に近づいたかを予測するために用いられる。 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 大森公式（オモリ則）時間経過に伴う余震発生頻度の減衰を示す経験則で、余震の発生率は本震からの経過時間にほぼ反比例して減少する。 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 火山性地震マグマの移動、ガス圧、または火山付近の岩石の破砕によって引き起こされる、火山活動に関連する地震。群発することが多く、噴火の前兆となる場合がある。 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 群発地震明確に卓越した本震を持たず、数日から数か月にわたって局地的な領域で発生する一連の地震。火山活動や流体の注入と関連することが多い。 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 地震防災キット（非常持ち出し袋）地震後の状況を生き延びるために事前にまとめておく物資一式で、通常は水（1人1日1ガロンを3日分）、食料、応急処置用品、懐中電灯、ラジオなどを含む。, a practiced 「まず低く、頭を守り、動かない」（Drop, Cover, and Hold On）地震の揺れの最中に国際的に推奨される防護行動。両手と両膝をつき、頑丈な家具の下に隠れ、揺れが収まるまでその姿勢を保つ。 response, and a clear 地震への備え家具の固定、連絡計画の作成、非常用物資の維持、訓練の実施など、地震の被害を最小限に抑えるための継続的な計画・準備の過程。 plan — regardless of what phase the moon is in.