地震比較ツール

2つの地震をエネルギー、深さ、マグニチュード、有感半径、影響で並べて比較します。

Analysis

地震のマグニチュード比較が重要な理由

地震のマグニチュードは対数スケールを使用しているため、2つのマグニチュードの差は見た目よりもはるかに大きいです。M7.0の地震はM5.0より単純に「2単位大きい」のではなく、約1,000倍のエネルギーを放出します。この指数関数的なスケーリングにより、異なる地震の真の相対的な威力を理解するためには並列比較が不可欠です。2010年のハイチ地震（M7.0）と2011年の日本の地震（M9.1）はわずか2.1マグニチュード単位の差ですが、日本の地震は350倍以上のエネルギーを放出しました。

深さは地震比較にもう一つの重要な次元を加えます。深さ10 kmのM6.5の浅い地震は、深さ500 kmのM7.0の深い地震よりもはるかに大きな地表被害をもたらす可能性があります。浅い地震は人口密集地域付近に地震エネルギーを集中させ、深い地震はより大きな岩石の体積に分散させるからです。マグニチュード、深さ、人口密集地域からの距離の組み合わせが最終的に地震の破壊的影響を決定します。同じマグニチュードの2つの地震がまったく異なる結果をもたらす可能性があるのはこのためです。

指数スケールの理解

マグニチュードが1.0上がるごとにエネルギーは31.6倍、2.0上がるごとに約1,000倍になります。
有感半径はおおよそマグニチュードに比例します。M7の地震はM5の地震の約10倍遠くで感じられます。
深さの分類：浅い（0〜70 km）、中間（70〜300 km）、深い（300〜700 km）地震は、地表への影響が大きく異なります。
TNT換算でのエネルギー比較は、抽象的なマグニチュードの数値と現実世界の破壊力との間のギャップを埋めるのに役立ちます。

主な用途

最近の地震をよく知られた歴史的な地震と比較して、その深刻さを理解すること。
2つのマグニチュード間のエネルギー差を示すことで、対数スケールについて学生に教えること。
深さが類似マグニチュードの地震の相対的な破壊力にどのように影響するかを理解すること。

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.