リヒタースケール解説: 歴史、公式、そして限界

Charles Richter and the Birth of Earthquake Measurement

In 1935, a young seismologist at the California Institute of Technology named Charles Francis Richter had a practical problem. The seismological laboratory received reports of dozens of earthquakes each week from stations across Southern California, but there was no standard way to compare their sizes. Some quakes were clearly larger than others, but no one had a number for "how much larger."

Richter, working with his colleague Beno Gutenberg, devised an elegant solution. He would use the maximum amplitude recorded on a 地震計地震波によって生じる地面の揺れを検知し記録する装置。現代のデジタル地震計は、ナノメートル未満の変位も検出できる。 of a standard type — the Wood-Anderson torsion seismometer — and adjust for the distance between the station and the earthquake. The result was a dimensionless number he initially called the "magnitude" scale but which the world came to call the Richter scale. His 1935 paper established the framework that all subsequent magnitude scales would build upon, and 地震記象（地震波形記録）地震計が記録した出力で、地面の揺れを時間の関数として示したもの。地震学者はこの記録を解析し、地震の規模・深さ・位置を求める。 analysis became the foundation of earthquake measurement worldwide.

How the Richter Scale Works: The Logarithmic Formula

The 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。, technically known as local magnitude (ML), is defined by a deceptively simple formula. Richter took the logarithm (base 10) of the maximum wave amplitude measured in micrometres on a standard seismograph at 100 kilometres distance and called that the magnitude. For stations at different distances, he applied correction factors derived from tables of how amplitude decreases with distance in Southern California.

The logarithmic nature has a critical practical implication: each whole number increase in magnitude corresponds to a tenfold increase in the amplitude of shaking recorded on the seismograph. In terms of actual energy released, the relationship is steeper — each magnitude unit represents approximately 31.6 times more energy. A マグニチュード地震が放出した総エネルギー量を表す単一の数値。整数値が1増えるごとに、放出エネルギーはおよそ31.6倍になる。 7.0 earthquake therefore releases about 31.6 times more energy than a 6.0, and about 1,000 times more than a 5.0.

Limitations of the Richter Scale for Large Earthquakes

The original Richter scale worked well for moderate earthquakes in Southern California, the region for which it was calibrated. When seismologists tried to apply it to large earthquakes elsewhere, problems emerged. The most serious was "saturation" — for very large earthquakes, different magnitude scales give inconsistent readings that fail to grow appropriately with earthquake size.

The 局地マグニチュード（ML）震央から600km以内のウッド・アンダーソン式地震計で記録された最大振幅から算出される、本来のリヒターマグニチュード。局地・浅発地震にのみ有効。 scale saturates because it is based on high-frequency waves recorded on a particular type of instrument. For great earthquakes with magnitude above about 6.5 to 7.0, the energy is mostly carried at very long periods — slow, sweeping motions of the ground that the original Wood-Anderson seismometer could not capture well. This is why scientists noticed that the Richter scale gave similar values for the 1960 Chile earthquake and the 1964 Alaska earthquake, even though both events were clearly "off the charts" in terms of destruction. Those two events are now known to have had moments equivalent to approximately magnitude 9.5 and 9.2, respectively.

Why Scientists Switched to Moment Magnitude

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), developed by Thomas Hanks and Hiroo Kanamori in 1979, solved the saturation problem by measuring something physically meaningful: the seismic moment of the earthquake. Seismic moment is the product of three quantities — the rigidity of the rock, the area of the fault that ruptured, and the average amount of slip. This can be calculated from 地震記象（地震波形記録）地震計が記録した出力で、地面の揺れを時間の関数として示したもの。地震学者はこの記録を解析し、地震の規模・深さ・位置を求める。 analysis at very long periods, using 広帯域地震計0.001〜50Hzという広い周波数帯域にわたって地震波を記録できる地震計。現代の世界的な地震観測網における主要な観測機器。s that record the full spectrum of ground motion.

Because seismic moment scales with the true physical size of the earthquake without saturating, Mw gives consistent and physically meaningful values for all earthquake sizes. The formula relating Mw to seismic moment is logarithmic, and Richter himself calibrated the original scale so that for moderate earthquakes the values agree closely. This means you can often see ML and Mw values that are nearly identical for moderate quakes, which eases the transition.

Common Misconceptions About the Richter Scale

Several persistent misconceptions surround the Richter scale. The most common is that it is the scale scientists use today — it is not. The アメリカ地質調査所（USGS）地震の監視、National Earthquake Information Centerの運用、世界の地震データのリアルタイム公開を担う、アメリカ政府の主要機関。 and virtually all major seismological agencies worldwide now report moment magnitude (Mw), yet news media continue to say "on the Richter scale" as if that were the current standard. The mistake is understandable because for most newsworthy earthquakes in the magnitude 5–7 range, Mw and ML give very similar numbers.

Another misconception is that there is a maximum value for the scale. There is no theoretical ceiling. Richter himself noted that the scale was open-ended at both ends. Similarly, there is no minimum — seismologists routinely detect and catalogue micro-earthquakes at negative magnitudes. The practical limits are set by what instruments can detect and what physics can produce.

A third misconception is that each magnitude step is "10 times more destructive." This confuses amplitude (which does increase tenfold per step) with energy (which increases 31.6-fold per step) with actual damage (which depends on depth, distance, soil, and construction quality in complex ways).

Richter Scale in Media: Why It Persists

Despite being scientifically superseded, the "Richter scale" name continues to dominate media coverage of earthquakes for several reasons. It is short, simple, and familiar — a single word that any headline can carry. The phrase entered popular culture deeply during the mid-20th century when Richter himself was a prominent public scientist who gave numerous interviews and was telegenic in explaining earthquakes to the public.

The persistence also reflects the close numerical agreement between scales for the moderate earthquakes that most news stories cover. When a reporter writes "magnitude 5.8 on the Richter scale," the actual moment magnitude is likely 5.8 or very close to it. The scientific inaccuracy rarely produces a misleading number in those cases. Seismologists have largely accepted that the battle to retire the term from popular usage is unwinnable, even as they continue to use 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。 exclusively in research and official communications.