Outil de Comparaison de Séismes

Comparez deux séismes côte à côte sur l'énergie, la profondeur, la magnitude, le rayon de perception et l'impact.

Analysis

Pourquoi comparer les magnitudes des séismes est important

Parce que la magnitude des séismes utilise une échelle logarithmique, la différence entre deux magnitudes est bien plus grande qu'il n'y paraît. Un séisme de magnitude 7,0 n'est pas simplement « deux unités plus grand » qu'un séisme de magnitude 5,0 — il libère environ 1 000 fois plus d'énergie. Cette mise à l'échelle exponentielle rend les comparaisons côte à côte essentielles pour comprendre la véritable puissance relative des différents séismes. Le séisme d'Haïti de 2010 (M7,0) et le séisme du Japon de 2011 (M9,1) ne diffèrent que de 2,1 unités de magnitude, mais l'événement japonais a libéré plus de 350 fois plus d'énergie.

La profondeur ajoute une dimension critique supplémentaire à la comparaison des séismes. Un séisme superficiel de M6,5 à 10 km de profondeur peut causer bien plus de dommages en surface qu'un séisme profond de M7,0 à 500 km de profondeur, car les événements superficiels concentrent l'énergie sismique près des zones peuplées tandis que les événements profonds la distribuent sur un volume de roche beaucoup plus grand. La combinaison de la magnitude, de la profondeur et de la distance aux zones peuplées détermine en fin de compte l'impact destructeur d'un séisme — c'est pourquoi deux séismes de magnitude identique peuvent avoir des conséquences très différentes.

Comprendre l'échelle exponentielle

Chaque augmentation de 1,0 de la magnitude représente 31,6 fois plus d'énergie ; chaque augmentation de 2,0 représente environ 1 000 fois plus d'énergie.
Le rayon de perception augmente approximativement avec la magnitude — un séisme de M7 peut être ressenti 10 fois plus loin qu'un séisme de M5.
Classification de la profondeur : les séismes superficiels (0-70 km), intermédiaires (70-300 km) et profonds (300-700 km) se comportent très différemment en termes d'impact en surface.
La comparaison de l'énergie en équivalent TNT aide à combler l'écart entre les nombres abstraits de magnitude et la puissance destructrice réelle.

Utilisations courantes

Comparer un séisme récent à un événement historique bien connu pour contextualiser sa gravité.
Enseigner aux étudiants les échelles logarithmiques en montrant la différence d'énergie entre deux magnitudes.
Comprendre comment la profondeur affecte le potentiel destructeur relatif des séismes de magnitudes similaires.

How to Use

1
Select Two Earthquakes
Search the database for two earthquakes by name, date, location, or USGS event ID. Both recent and historical events going back to the 1900 USGS catalog are available.
2
Choose Comparison Metrics
Select which parameters to compare: magnitude, energy release, depth, felt radius, fatalities, economic losses, tectonic setting, and maximum recorded intensity (MMI).
3
Review Side-by-Side Analysis
Examine the comparison table and energy ratio chart. The tool calculates the factor-difference in energy release and annotates each metric with contextual notes from the seismological record.

About

Comparing earthquakes reveals the enormous range of Earth's seismic output and the complex interplay between source parameters and surface impacts. The global seismic record contains millions of cataloged events: roughly 500,000 detectable earthquakes occur each year, of which about 100,000 can be felt and approximately 100 cause damage. This frequency-magnitude distribution follows the Gutenberg-Richter relation, a remarkably consistent power law observed across tectonic environments: for every unit increase in magnitude, there are roughly 10 times fewer events. This means that while M3.0 earthquakes occur hundreds of times daily worldwide, M8.0 events occur about once per year.

The tectonic setting fundamentally shapes earthquake character. Subduction zone megathrust earthquakes—like the 1960 Chile M9.5 and 2011 Tohoku M9.1—produce extremely long rupture durations (200–500 seconds), generate transoceanic tsunamis, and have predominantly low-angle reverse focal mechanisms. Transform fault earthquakes like those on the San Andreas system produce strike-slip motion, shorter ruptures, and generally lower tsunami potential. Intracontinental thrust belt earthquakes (Himalaya, Zagros, Andes) are associated with crustal thickening and can be devastating due to their proximity to densely populated mountain valleys.

Historical earthquake comparisons must account for detection capability changes over time. Before the establishment of the World-Wide Standardized Seismograph Network (WWSSN) in the 1960s, the catalog is incomplete for smaller magnitudes and location accuracies are far lower. Modern moment tensor catalogs (CMT, maintained since 1976) provide standardized source parameters for systematic comparison. Digital broadband networks since the 1980s enable waveform-based analyses that extract fault geometry, stress drop, and directivity effects—parameters inaccessible from earlier analog records.

FAQ

How do scientists compare earthquakes scientifically?

Seismologists compare earthquakes across several independent dimensions. Magnitude (Mw) characterizes source size. Focal mechanism (strike-slip, normal, reverse/thrust) describes the geometry of fault motion and influences the radiation pattern of seismic energy. Focal depth stratifies events into crustal (< 70 km), intermediate (70–300 km), and deep (> 300 km). Stress drop—the change in shear stress across the fault during rupture—influences the high-frequency content of shaking and explains why some moderate earthquakes feel 'sharp' while larger ones feel 'rolling.' Comparing earthquakes rigorously requires examining all these parameters simultaneously, not magnitude alone.

Why do smaller earthquakes sometimes cause more damage?

Several factors cause smaller earthquakes to sometimes be more destructive than larger ones. Depth is paramount: a M6.0 at 5 km depth can cause far more surface damage than a M6.5 at 80 km. The resonance of seismic waves with building natural periods matters enormously—if dominant wave periods match building heights (typically 0.1 s per floor), resonance amplifies structural response. Local soil conditions can amplify ground motion by a factor of 5–10 at soft sediment sites. Building stock vulnerability is equally critical: a M5.6 in rural Afghanistan or Haiti, where building quality is poor, can kill thousands, while the same event in Japan with modern construction codes may cause no fatalities.

What makes a 'great' earthquake different from a major earthquake?

The Gutenberg-Richter magnitude classification defines great earthquakes as Mw ≥ 8.0 and major earthquakes as Mw 7.0–7.9. The distinction is not merely semantic: great earthquakes rupture fault lengths of 200+ km and durations exceeding 60 seconds, exciting long-period surface waves that circumnavigate the globe multiple times. Great earthquakes can generate tsunamis capable of crossing ocean basins, trigger volcanic unrest, and temporarily alter Earth's rotation rate and axial tilt by measurable amounts. The 2011 Tohoku earthquake shortened Earth's day by approximately 1.8 microseconds. Globally, about 13–17 major earthquakes (M7.0–7.9) and 1–2 great earthquakes (M ≥ 8.0) occur per year on average.

How do shallow and deep earthquakes compare in terms of damage?

Shallow earthquakes (< 20 km depth) typically cause far greater localized damage than deep events of the same magnitude because the energy has less rock to travel through before reaching the surface. The short travel path means less geometric spreading and less attenuation, resulting in higher peak ground accelerations. Deep earthquakes (> 300 km) rarely cause surface damage because by the time the waves reach the crust, they have spread over a much larger volume. A notable exception is the scenario of deep earthquakes beneath populated areas with resonant sediment basins: the 1994 Northridge earthquake (19 km) caused US$20 billion in losses while a similar-magnitude deep earthquake in the Tonga trench the same year caused none.

Are earthquake comparisons useful for predicting future events?

Comparing past earthquakes provides important inputs for probabilistic seismic hazard analysis (PSHA) but does not enable deterministic prediction of future events. The Gutenberg-Richter relation—an empirical power law relating earthquake frequency to magnitude—describes the statistical distribution of seismicity on a fault system. Characteristic earthquake models propose that specific fault segments repeatedly rupture in events of similar magnitude and recurrence interval, based on paleoseismic evidence from trenching. While these models constrain long-term hazard forecasts with 30–50 year horizons, the precise timing of individual earthquakes remains unpredictable beyond short-term windows (days to weeks) following large foreshock sequences.