地震がより強くなっているというのは本当か?

The Myth: Earthquakes Are Getting Stronger

Related to but distinct from the "earthquakes are becoming more frequent" myth is the claim that individual earthquakes are becoming more powerful — that climate change, nuclear testing, or some other human or environmental force is intensifying the maximum magnitude of earthquakes. This myth surfaces particularly after major events, when commentators sometimes suggest the extreme magnitude reflects an escalating planetary crisis. The physics of モーメントマグニチュード断層面積・平均すべり量・岩石の剛性の積である地震モーメントに基づく、地震規模を測定する現代の標準的な尺度（Mw）。あらゆる規模の地震に対して精度が高い。 and the geological record provide a clear answer, though some nuances deserve careful treatment.

How Earthquake Magnitude Is Measured

Modern earthquake magnitude is measured primarily through the モーメントマグニチュード断層面積・平均すべり量・岩石の剛性の積である地震モーメントに基づく、地震規模を測定する現代の標準的な尺度（Mw）。あらゆる規模の地震に対して精度が高い。 scale (Mw), which was developed in the 1970s to overcome limitations of Charles Richter's original リヒタースケール1935年にチャールズ・リヒターが考案した、局地地震のマグニチュードを測定するための最初の対数マグニチュード尺度。現在はモーメントマグニチュードに大きく置き換えられているが、報道では今も広く使われている。 and is now the standard for large earthquakes. The moment magnitude is derived from the 地震モーメント断層面積・平均変位量・岩石のせん断弾性率の積として算出される、地震が放出した総エネルギーの尺度。モーメントマグニチュードの算出基盤となる。 — the product of the shear modulus of the rock, the area of the fault that ruptured, and the average slip across that area. It is a measure of the total energy radiated as seismic waves.

The moment magnitude scale is logarithmic: each unit increase represents about 32 times more energy. A M9.0 earthquake like the 2011 Tohoku event releases approximately 32 times more energy than a M8.0 event, and about 1,000 times more energy than a M7.0 event. The largest earthquakes ever recorded (estimated M9.4-9.6 for the 1960 Valdivia, Chile, earthquake) are bounded by the maximum fault dimensions physically possible on Earth.

What Determines Maximum Earthquake Magnitude

The maximum possible earthquake magnitude on any given fault system is determined by the physical dimensions of the fault. Fault rupture cannot exceed the length of the fault segment, the width (depth extent) of the seismogenic zone, and the amount of accumulated slip. For the largest subduction zones — like the Cascadia Subduction Zone, the Japan Trench, or the South American subduction zone — rupture lengths of 1,000+ km are possible, enabling M9+ events. For strike-slip faults like the San Andreas, the geometry limits maximum magnitude to approximately M8.0-8.5. These limits are set by geology and physics, not by time period.

The グーテンベルク・リヒター則地震の頻度とマグニチュードの関係を示す統計法則で、マグニチュードが1単位増えるごとに、地震の発生頻度はおよそ10分の1になる。 relationship describes the frequency-magnitude distribution of earthquakes in any region: for each unit of magnitude increase, frequency decreases by roughly a factor of 10. The slope of this relationship (the b-value) and the maximum magnitude are properties of the fault system determined by geology. They do not change on human timescales in response to climate, nuclear testing, or any other human activity.

The Historical Record vs. Instrumental Period

The 地震観測網地震活動を継続的に監視する、連携した地震観測点の集合。世界地震観測網（GSN）は150以上の観測点から構成され、世界規模の観測範囲を提供する。 that provides consistent global monitoring of earthquakes only achieved modern global coverage in the 1960s-1970s. Before this period, earthquake catalogs become increasingly incomplete as you go further back in time. This creates an observational artifact: the largest recorded earthquakes in the modern era (1960 Chile M9.5, 1964 Alaska M9.2, 2004 Indian Ocean M9.2, 2011 Japan M9.0) could create the impression that the most extreme events are recent phenomena.

But this apparent concentration of extreme events in the modern era largely reflects improved detection and recording. [[Paleoseismology]] — the study of prehistoric earthquakes from geological evidence — reveals that M8-9+ events have occurred repeatedly throughout geological history on the same fault systems we observe today. Japanese historical records document major tsunamis and earthquakes consistent with M9-class Tohoku-type events going back many centuries. The sediment record of the Cascadia Subduction Zone shows complete M9 ruptures occurring roughly every 200-500 years for thousands of years.

Why the Modern Instrumental Period Looks Extreme

The four M9.0+ events since 1960 (Chile 1960, Alaska 1964, Indian Ocean 2004, Tohoku 2011) might suggest a modern clustering of extreme events. Seismologists have examined this question carefully. The global rate of M9+ events in the instrumental era (approximately 1 per 20 years) is broadly consistent with geological estimates of recurrence intervals, though the clustering of large events in a 60-year window is on the high side of expected variability. This is within the expected natural randomness of a Poisson process with rare events — four events in 60 years is unusual but not statistically implausible given a 20-year mean recurrence.

Climate Change and Earthquake Magnitude

A common contemporary version of the myth is that climate change is making earthquakes stronger. There is no credible mechanism by which atmospheric warming could increase the maximum magnitude of tectonic earthquakes, and no evidence that it does. Tectonic earthquake magnitude is determined by fault geometry and accumulated strain, not by surface conditions.

However, there are legitimate research questions about climate-earthquake interactions at much smaller scales and longer timescales. Deglaciation — the melting of major ice sheets — changes the load on the crust and can affect seismicity rates over centuries to millennia as the crust rebounds (isostatic rebound). Some researchers have hypothesized that accelerated glacial melting could affect seismicity in formerly glaciated regions (Scandinavia, Iceland, Alaska) over coming decades to centuries, though the magnitudes involved are far smaller than the scale of effects implied by popular claims.

The Role of 地震観測網地震活動を継続的に監視する、連携した地震観測点の集合。世界地震観測網（GSN）は150以上の観測点から構成され、世界規模の観測範囲を提供する。 Improvements

The same monitoring improvements that make it appear more earthquakes are occurring (as discussed in the "earthquakes more frequent" guide) also affect the apparent record for large events. Better 地震観測網地震活動を継続的に監視する、連携した地震観測点の集合。世界地震観測網（GSN）は150以上の観測点から構成され、世界規模の観測範囲を提供する。 coverage means that earthquakes that would have been assigned lower magnitudes due to incomplete waveform data in the 1960s might be assessed at higher magnitudes today. Some upward revisions of historical earthquake magnitudes are the result of reanalysis using modern methods, not actual stronger shaking.

The Sumatra 2004 earthquake was initially reported as M9.0, then revised upward to M9.2-9.3 as more data became available and more complete waveform analysis was applied. This reflects improved methodology, not escalating seismicity.

What Is Genuinely Changing

While tectonic earthquake maximum magnitude is not changing, the consequences of earthquakes of a given magnitude are changing — upward. Growing population in coastal and seismically active zones, increasing urbanization in regions with inadequate 耐震設計地震力に耐えられるよう構造物を設計する手法。現代の耐震設計は、大地震における構造被害はある程度許容しつつ、倒壊の防止と人命の保護を目指す。 standards, aging infrastructure, and global supply chain interdependencies mean that a M7.8 earthquake in 2026 affects far more people and causes far greater economic disruption than a comparable event in 1926. This is a genuine and serious trend, but it reflects human vulnerability, not geological escalation.

The appropriate response is investing in 耐震補強既存の建物の耐震性を向上させるための強化工事。鋼製ブレースの追加、基礎の補強、構造物と基礎のボルト固定などが一般的な手法である。 programs, updating building codes in rapidly urbanizing developing nations, improving 緊急地震速報（EEW）地震を検知し、強い揺れが到達する前に人々やシステムに警報を送るシステム。数秒から数十秒の猶予を提供し、身を守る行動をとるのに十分な時間となる。 infrastructure, and building institutional capacity for post-earthquake response — not alarm about inherently increasing earthquake power. The earth's geology is not conspiring against us; we are making choices about where and how we build that determine how consequential each earthquake becomes.