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QuakeFYI

Calculateur d'Énergie Sismique

Convert earthquake magnitude to energy equivalent in joules, TNT tons, and atomic bombs.

Calculation

Comprendre l'énergie et la magnitude des séismes

La magnitude des séismes est mesurée sur une échelle logarithmique, ce qui signifie que chaque augmentation d'un point entier représente une augmentation de dix fois l'amplitude mesurée et environ 31,6 fois plus d'énergie libérée. Cette relation exponentielle est décrite par la formule énergie-magnitude de Gutenberg-Richter : log₁₀(E) = 1,5M + 4,8, où E est l'énergie en joules et M la magnitude de moment. Un séisme de magnitude 7,0 libère environ 1 000 fois plus d'énergie qu'un séisme de magnitude 5,0 — à peu près l'équivalent de 500 000 tonnes de TNT.

L'échelle de magnitude de moment (Mw), qui a remplacé l'échelle de Richter d'origine pour la plupart des usages, est basée sur le moment sismique — une mesure de l'énergie totale libérée par le glissement de la faille. Contrairement à la magnitude locale de Richter (ML), la magnitude de moment ne sature pas aux valeurs élevées, ce qui en fait l'échelle préférée pour les grands séismes. Le moment sismique dépend de trois facteurs : la rigidité de la roche, la surface de la faille rompue et le déplacement moyen le long de la faille.

La science derrière le calcul

  • L'échelle de Richter a été développée en 1935 par Charles Richter pour les séismes du sud de la Californie ; l'échelle de magnitude de moment l'a remplacée dans les années 1970 pour une utilisation mondiale.
  • Seulement environ 1 à 10 % de l'énergie totale d'un séisme est rayonnée sous forme d'ondes sismiques ; le reste est consommé par la fracturation de la roche et la génération de chaleur le long de la faille.
  • L'équivalence TNT fournit une comparaison intuitive : le séisme de Tōhoku de 2011 (M9,1) a libéré une énergie équivalente à environ 600 millions de tonnes de TNT.
  • Les petits séismes (M2-3) libèrent une énergie comparable à quelques kilogrammes d'explosifs, tandis que les grands séismes (M8+) rivalisent avec les arsenaux nucléaires.

Utilisations courantes

  • Comparer la puissance relative des séismes historiques pour comprendre leur potentiel destructeur.
  • Enseigner aux étudiants les échelles logarithmiques et les relations exponentielles d'énergie dans les cours de sciences de la Terre.
  • Mettre les magnitudes des séismes en perspective en utilisant des équivalents énergétiques du quotidien comme le TNT ou la foudre.

How to Use

  1. 1
    Enter the Earthquake Magnitude

    Input the moment magnitude (Mw) of the earthquake. Mw is the standard scale used by seismological agencies since the 1970s and is the most accurate measure across all magnitude ranges.

  2. 2
    Select Your Energy Units

    Choose whether to see energy equivalents in joules, kilotons of TNT, or Hiroshima atomic bomb equivalents. The calculator applies the USGS energy-magnitude relation: log E = 5.24 + 1.44 Mw.

  3. 3
    Compare Across Magnitudes

    Add a second magnitude to see the energy ratio between the two events. Because the scale is logarithmic, each unit increase in Mw corresponds to about 31.6 times more released energy.

About

Earthquake energy and magnitude are connected through one of science's most consequential logarithmic scales. Charles Richter introduced the local magnitude (ML) scale in 1935, calibrated to a specific seismograph at a specific distance in Southern California. While the name 'Richter scale' persists in popular usage, seismologists now use moment magnitude (Mw), developed by Hiroo Kanamori and Thomas Hanks in 1979, which remains consistent across the full spectrum from microearthquakes to the largest megathrust events and does not saturate at high magnitudes as earlier scales did.

The physical quantity underlying Mw is the seismic moment (M0), calculated as M0 = μ × A × d, where μ is the shear modulus of the rock (typically 3 × 10^10 Pa for the crust), A is the ruptured fault area, and d is the average displacement across the fault. Mw is then derived as Mw = (2/3) × log10(M0) − 6.07. This formulation means that fault geometry directly determines magnitude: a rupture covering a 200 × 100 km fault plane with 5 m of average slip yields a specific, calculable M0 and hence a well-defined Mw.

Energy equivalents help communicate earthquake power to non-specialist audiences. The most commonly cited comparison is the atomic bomb: the Hiroshima bomb released approximately 63 terajoules. A magnitude 6.0 earthquake releases energy comparable to about 1 Hiroshima bomb, while a magnitude 8.0 releases energy comparable to about 1,000. These comparisons, while vivid, can mislead: earthquake energy is released over a fault plane tens to hundreds of kilometers long over tens of seconds, and only a fraction couples into the seismic waves that cause damage at the surface. The depth, focal mechanism, and local site response all shape the destruction as much as the raw energy figure.

FAQ

Quelle est la différence entre magnitude et intensité ?
Magnitude is an objective, instrumentally measured quantity describing the total energy released at the earthquake source, reported as a single number regardless of where it is measured. The moment magnitude scale (Mw) is now universal and calculated from the seismic moment—the product of the fault area, average slip, and rock rigidity. Intensity, by contrast, is a subjective measure of ground shaking severity at a specific location, described by the Modified Mercalli Intensity (MMI) scale from I (not felt) to XII (total destruction). Intensity decreases with distance from the epicenter and varies with local geology, so the same earthquake can produce MMI V in one city and MMI VIII in another.
How much energy does a magnitude 7 earthquake release?
Using the USGS energy-magnitude relation (log E = 5.24 + 1.44 Mw), a magnitude 7.0 earthquake releases approximately 2 × 10^15 joules, equivalent to roughly 475 kilotons of TNT or about 32 Hiroshima-sized atomic bombs. For comparison, a magnitude 8.0 releases about 31.6 times more energy than a 7.0, and a magnitude 9.0 releases about 1,000 times more. The 2011 Tohoku M9.1 earthquake released energy equivalent to approximately 600 million tons of TNT, or about 40,000 Hiroshima bombs. It is worth noting that seismic energy represents only a fraction (roughly 5–10%) of the total strain energy released; the rest is converted to heat at the fault surface.
Why does each magnitude unit feel so much stronger?
The moment magnitude scale is logarithmic in seismic moment but the energy-magnitude relationship has a steeper exponent. A one-unit increase in Mw corresponds to a factor of 10^1.5 ≈ 31.6 in energy release. Peak ground acceleration (the shaking you actually feel) scales differently: a one-unit increase in Mw roughly doubles the felt shaking amplitude as measured by instruments, though local site conditions, depth, and distance complicate this relationship. This is why the jump from M6.0 to M7.0 is so consequential for structural damage—the energy released increases by a factor of roughly 32, but the duration of strong shaking also increases substantially, compounding structural fatigue.
What is the largest earthquake ever recorded?
The 1960 Valdivia earthquake in southern Chile holds the record at M9.5, occurring along the Nazca–South American subduction zone. It ruptured approximately 1,000 km of fault surface, generated a transoceanic tsunami that killed people as far away as Hawaii and Japan, and triggered volcanic activity in the Andes. The seismic moment released was approximately 1.8 × 10^23 newton-meters. In comparison, the 2004 Sumatra–Andaman earthquake (M9.1–9.3) and the 2011 Tohoku earthquake (M9.1) are the next largest recorded events. All occurred at subduction zone megathrusts, the only tectonic setting capable of producing such extreme events.
Is a magnitude 10 earthquake possible?
A magnitude 10.0 earthquake is considered physically implausible given the geometry of Earth's plate boundaries. The magnitude is determined by fault dimensions and average slip: a M10.0 would require a fault rupture of roughly 4,000–5,000 km in length, more than the entire length of the longest subduction zone on Earth (the Chile-Peru trench). While cascading multi-segment ruptures are possible—the 1964 Alaska earthquake ruptured about 800 km—no tectonic configuration exists that could sustain a single coherent rupture at M10 scale. The theoretical upper bound for subduction zone earthquakes is generally placed around M9.5–9.6.