地震の測定方法: マグニチュード vs 震度

Magnitude: Measuring Energy at the Source

When scientists report that an earthquake had 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。 of 6.5, they are describing a single property of the earthquake itself — the total energy released at the source. Magnitude is calculated from measurements made by 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。 instruments around the world, and it does not change depending on where you are. A magnitude 6.5 event in California is the same magnitude whether measured in Sacramento or Stockholm.

The most widely used magnitude scale today is 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 (abbreviated Mw), which measures the total "seismic moment" of an earthquake: the product of the fault area that ruptured, the average distance the fault slipped, and the rigidity of the rock. Scientists prefer it because it does not saturate at high values the way earlier scales did, making it reliable for earthquakes of all sizes. Use the Earthquake Energy Calculator to explore how different magnitudes translate to energy released and shaking radius.

Intensity: Measuring Effects at Your Location

震度人・構造物・自然環境への観測された影響から判定される、特定地点における揺れの強さの尺度。震央からの距離が離れるほど小さくなる。 answers a completely different question: how hard did the ground shake at this particular spot? The same earthquake can produce very different intensities at different locations. At the 震央地下で地震が発生した震源の真上にあたる地表の地点。ニュース報道では地震の発生場所としてよく報じられる。, intensity may reach the highest levels on the scale. A hundred kilometres away, it might be much gentler. And in some distant city, sensitive people might barely feel it — or not feel it at all.

The standard measure of intensity in the United States is the Modified 修正メルカリ震度階級感知できない揺れ（I）から壊滅的な被害（XII）まで、特定の地点で観測された地震の影響を測定する12段階の階級。マグニチュードと異なり、震央からの距離によって値が変化する。 scale (MMI), which uses Roman numerals from I to XII. MMI I means the quake was not felt at all; MMI V is felt by nearly everyone and may rattle windows; MMI VII causes significant damage to poorly built structures; MMI XII represents total destruction of virtually all buildings. Intensity data is collected by the USGS 「揺れを感じましたか？」（DYFI）地震後に一般市民から震度に関する報告を収集し、市民参加型の震度マップを作成するUSGSのプログラム。地震を感じた誰もが報告を投稿できる。 system, which aggregates thousands of reports from members of the public within minutes of any significant quake.

Why the Same Earthquake Has Different Intensities

Three main factors explain why intensity varies so widely. First, distance from the epicenter: shaking energy spreads out as waves travel, so intensity naturally decreases with distance. Second, 地盤増幅（サイト効果）軟弱な土壌や堆積層が地震波を増幅させることによって生じる、揺れの強さの増大。軟弱地盤上の構造物は、基盤岩上の構造物に比べて2〜10倍強い揺れを経験することがある。: soft sediments like bay mud, alluvial fans, and reclaimed land amplify shaking far more than hard bedrock. Certain areas of San Francisco built on former bay fill experienced intensities far higher than nearby bedrock neighbourhoods during the 1989 Loma Prieta earthquake, even though they were about the same distance from the epicenter. Third, building quality and local construction practices profoundly affect how much damage occurs at any intensity level.

Depth also matters. A shallow earthquake 10 kilometres deep will produce much more intense shaking at the surface directly above it than an identical-magnitude earthquake 100 kilometres down, because the energy has less distance to travel and spread out before reaching the surface.

The Logarithmic Nature of Earthquake Scales

One of the most commonly misunderstood aspects of earthquake measurement is that magnitude scales are logarithmic. Each whole-number step on the moment magnitude scale represents approximately 31.6 times more energy released. A magnitude 7.0 earthquake releases about 31.6 times more energy than a magnitude 6.0, and about 1,000 times more energy than a magnitude 5.0.

This means that while a magnitude 5.0 might feel significant to someone in its path, a magnitude 8.0 in the same region would release roughly one million times more energy. The 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。, which most people have heard of, was also logarithmic in this way, and its legacy continues to shape how the public thinks about earthquake sizes even though scientists now 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。 for nearly all reporting.

How Scientists Calculate Magnitude and Intensity

Magnitude is calculated by analysing the 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。 — the record of ground motion recorded by 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。. Different magnitude formulas use different parts of the seismogram. Local magnitude (ML, the original Richter scale) uses the amplitude of the largest wave on a standard instrument. Surface wave magnitude uses the amplitude of surface waves at a period of about 20 seconds. Moment magnitude is derived from the long-period part of the seismogram and is increasingly calculated in near-real-time through automated systems.

Intensity is determined either by field surveys in the aftermath of a quake — where engineers and geologists inspect damage and interview residents — or increasingly through crowdsourced data from websites like USGS Did You Feel It?, which aggregates self-reported observations from tens of thousands of people. Modern シェイクマップ（ShakeMap）地震発生後の揺れの強さの分布を示すUSGSの成果物。地震計データ、地震動モデル、「揺れを感じましたか？」の報告を組み合わせて作成される。 products combine instrumental data with these reports and soil models to produce colour-coded maps of intensity within minutes.

Reading Earthquake Reports: A Practical Guide

When you see an earthquake reported in the news, here is how to interpret the key numbers. The magnitude tells you how large the event was at the source. A magnitude below 3.0 is usually imperceptible to humans. Magnitude 3–4 is felt by some people near the epicenter. Magnitude 5 can cause minor damage. Magnitude 6 is capable of significant damage. Magnitude 7 is a major earthquake. Magnitude 8 and above is a great earthquake that can affect entire countries.

The depth tells you how far below the surface the rupture began. Shallow earthquakes (less than 70 km) generally cause more surface shaking per unit of magnitude than deep ones. The intensity at your location depends on all these factors combined. If you felt an earthquake, reporting your experience through 「揺れを感じましたか？」（DYFI）地震後に一般市民から震度に関する報告を収集し、市民参加型の震度マップを作成するUSGSのプログラム。地震を感じた誰もが報告を投稿できる。 helps scientists refine intensity maps and better understand ground shaking in your area.