改訂メルカリ震度スケール: あなたが感じることを測定する

From Giuseppe Mercalli to the Modern MMI Scale

The story of earthquake intensity measurement begins in 19th-century Italy, where volcanic and seismic activity provided frequent natural experiments. Italian priest and geologist Giuseppe Mercalli developed an early intensity scale in 1883, later revised in 1902, that described the observable effects of earthquakes on people, objects, and structures. Unlike magnitude — a property of the earthquake itself — intensity describes what happened at a particular place.

Mercalli's scale was modified and expanded over the following decades by American seismologists Harry Wood and Frank Neumann, who published the Modified Mercalli Intensity (MMI) scale in 1931. This version, with further refinements in 1956, remains the standard intensity scale used in the United States today. Europe uses a similar but distinct scale called the European Macroseismic Scale (EMS-98). Both trace their lineage to Mercalli's original observational framework.

The 12 Levels: I Through XII Explained

The 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 runs from Roman numeral I to XII. At the low end, MMI I means the earthquake was not felt at all — only 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 recorded it. MMI II is felt only by people who are resting, particularly on upper floors of tall buildings. MMI III is felt noticeably indoors, especially on upper floors, and may be mistaken for the vibration of a passing truck.

MMI IV–V marks the range where a quake is felt by most people and begins to cause minor disturbances: hanging objects swing, dishes rattle, liquids slosh in containers. MMI VI–VII is where light structural damage begins — plaster cracks, chimneys break, poorly constructed buildings sustain damage. MMI VIII–IX represents severe shaking that damages even well-built structures, causes partial collapses of weak buildings, and can produce 地表地震断層（地表断層）地震時に断層に沿って地表面に生じる目に見える変位。地表断層の破壊帯を横切って建設された構造物は、構造強度にかかわらず引き裂かれることがある。 and 地震誘発地すべり地震の揺れによって引き起こされる、土砂や岩石の斜面下方への移動。地すべりは地域全体を埋没させることがあり、揺れそのものより多くの犠牲者を出すこともある。s. MMI X–XII describes catastrophic damage: well-built wood-frame structures thrown off foundations, bridges destroyed, and at XII, near-total destruction of all structures.

How Intensity Differs from Magnitude

The distinction between 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。 and 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。 cannot be overstated, because the two concepts are frequently confused in media reporting. 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。 is a single number assigned to the earthquake at its source — it does not change based on where you stand. 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。 varies across the affected region and is assigned to specific locations, not to the earthquake as a whole.

A single earthquake will produce many different intensity values across its affected area. A magnitude 6.5 earthquake occurring directly beneath a densely populated urban area might produce MMI VIII or IX at the epicentre, while a town 150 kilometres away experiences only MMI III. Another earthquake of identical magnitude occurring in a remote desert at depth might never produce MMI values above IV at any populated location. This is why earthquake fatality and damage statistics are driven far more by intensity than by magnitude.

Did You Feel It: Crowdsourcing Intensity Data

Traditional intensity mapping required weeks or months of field surveys by scientists interviewing survivors and inspecting damage. The 「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, developed by 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。 in 1999, revolutionised intensity mapping by crowdsourcing observations from the public. Within minutes of a significant earthquake, thousands of people visit the website to report what they experienced — whether they felt shaking, how strong it was, what objects moved or fell, and whether any damage occurred.

These self-reported data are aggregated using statistical algorithms that correct for reporting biases and extrapolate spatially, producing colour-coded intensity maps with unprecedented speed and geographic detail. By the time field geologists can mobilise, the 「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。 database may already contain 100,000 or more observations. This data also feeds directly into 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, which agencies use for emergency response planning.

Why Intensity Varies: Distance, Soil, and Building Type

Three factors dominate intensity variation. The most intuitive is distance: shaking energy spreads out as waves travel through the Earth, and intensity generally decreases with distance from the epicentre. The rate of decrease depends on regional geology — in the eastern United States, shaking propagates remarkably efficiently, and a moderate earthquake can be felt across ten times the area it would be in California.

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。 is often the most dramatic local factor. Soft, water-saturated sediments — river deltas, bay muds, reclaimed land, and thick alluvial deposits — can amplify shaking by factors of 10 or more compared to adjacent hard rock. This effect explains why certain neighbourhoods consistently suffer more damage than others in the same city. Building type and quality are the third critical factor. An unreinforced masonry building may collapse at MMI VII while a modern moment-frame structure nearby sustains no significant damage. The 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。 of vulnerable buildings is therefore one of the most effective strategies for reducing intensity-driven casualties.

Using Intensity Maps for Emergency Response

Within minutes of a significant earthquake, 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。 publishes 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 that display estimated shaking intensity across the affected region. Emergency managers use these maps to rapidly identify which areas likely experienced the heaviest shaking and therefore require the most urgent response. The maps help prioritise deployment of 捜索救助（SAR）地震後、倒壊した構造物に閉じ込められた生存者を発見・救出するための組織的な活動。発生から最初の72時間が、生存者を発見できる重要な時間帯とされる。 teams, guide media communications about affected areas, and trigger automated notifications to utilities and infrastructure operators.

Intensity maps also have long-term applications. Historical intensity data, compiled from old newspaper accounts, personal diaries, and church records, allow seismologists to reconstruct the shaking patterns of earthquakes that occurred long before seismographs existed. This historical record is an essential input into 地震ハザードマップ特定の期間内に地震の揺れが指定の水準を超える確率を示した地図。技術者・都市計画者・保険会社が地震リスクを評価するために用いる。s and probabilistic seismic hazard analyses that guide building codes and land use planning. Knowing that a particular valley experienced MMI VIII in an 1850 earthquake tells engineers something important about the ground conditions there, even without a single instrumental recording.