有感半径計算機

Calculate how far an earthquake can be felt and what intensity you would experience.

Calculation

地震の有感半径はどのように決まるか

地震の有感半径とは、震央から人が揺れを知覚できる最大距離のことです。主にマグニチュードと深さに依存しますが、地域の地質、時間帯（静止している時の方が揺れに気づきやすい）、建物の種類にも影響されます。地震学モデルは、改正メルカリ震度（MMI）が距離とともにどのように減少するかを記述する震度減衰関係を用いて有感半径を推定します。人間の知覚閾値はおおよそMMI II（建物の上階にいる人が気づく可能性のある微かな振動）です。

深さは有感半径の決定において重要な役割を果たします。深さ5 kmの浅い地震は地表付近にエネルギーを集中させ、強烈だが局所的な揺れを生み出します。同じマグニチュードの地震でも深さ100 kmではエネルギーがはるかに大きな体積に分散され、より広い有感域となりますがピーク強度は低くなります。深い地震がしばしば非常に広い範囲で感じられるのはこのためです。2013年のM8.3オホーツク海地震（深さ609 km）はロシア全土で感じられましたが、長い伝播経路でのエネルギー減衰により被害は最小限でした。

改正メルカリ震度階級

MMI I〜II：無感またはほとんど感じない。地震計によってのみ検出されるか、上階で静止している少数の人のみが感知。
MMI III〜IV：屋内で多くの人が感知。吊り下げ物が揺れ、トラックが通過したような感覚。被害なし。
MMI V〜VI：ほぼ全員が感知。食器が割れ、本が棚から落ち、壁にわずかなひび。軽微な構造被害の可能性。
MMI VII以上：耐震性の低い構造物に被害。立っていることが困難。適切に設計された構造物でもMMI VIII以上では中程度の被害の可能性。

主な用途

緊急計画と市民への広報のために地震がどこまで感じられるかの推定。
地震の深さと知覚可能な揺れの範囲との関係の理解。
リスク評価のための異なる地震シナリオの潜在的影響範囲の比較。

How to Use

1
Enter Magnitude and Depth
Input the earthquake's moment magnitude (Mw) and focal depth in kilometers. Depth is the most important modifier of felt radius after magnitude—shallow events (< 15 km) are felt over smaller but more intensely shaken areas.
2
Select Regional Attenuation
Choose your tectonic region. Eastern continental regions (eastern US, stable cratonic areas) have lower attenuation and transmit seismic waves farther than western active tectonic regions (western US, Japan) where the crust attenuates energy more rapidly.
3
Read the Intensity Map
View the estimated MMI contours at distances ranging from the epicenter to several hundred kilometers. Each contour corresponds to a descriptor from 'not felt' to 'violent shaking,' based on USGS ShakeMap attenuation relations.

About

The geographical extent of earthquake felt shaking depends on a cascade of physical processes that attenuate seismic energy from the source to distant sites. Ground Motion Prediction Equations (GMPEs) are the mathematical backbone of felt-radius estimation, empirically derived from hundreds of thousands of ground motion recordings worldwide. Modern GMPEs are region-specific: the eastern US crustal model (Atkinson and Boore 2006) predicts felt radii roughly twice those of equivalent-magnitude western US events because the cold, rigid eastern craton transmits seismic waves with lower attenuation (higher Q values) than the warm, fractured western crust.

The USGS 'Did You Feel It?' citizen science platform has transformed intensity estimation. Since its launch in 1997, it has collected over 100 million intensity reports from millions of contributors worldwide, providing dense spatial coverage of felt shaking that instrumental networks alone cannot match. Statistical algorithms convert individual reports—rating shaking intensity on a structured questionnaire covering felt motion, sounds, object movement, and structural effects—into aggregate community intensity values that correlate strongly with instrumental measurements. This crowdsourced intensity data is now incorporated into ShakeMap products alongside seismograph recordings.

Felt radius data over decades reveals systematic regional patterns. In the stable cratonic interior of the eastern US, M5.0 earthquakes have been felt at distances exceeding 1,000 km—the 2011 Virginia M5.8 was felt from Georgia to Nova Scotia. In California's tectonically active crust, a M5.0 is typically felt within 200 km. The New Madrid Seismic Zone, straddling Missouri, Arkansas, and Tennessee, presents a particular challenge: the region has produced M7.0–M8.0 events (estimated, 1811–1812) and sits atop thick, seismically efficient sedimentary sequences; a repeat sequence today would impact a population orders of magnitude larger than the sparse frontier settlements of the early 19th century.

FAQ

改正メルカリ震度階級とは何ですか？

The Modified Mercalli Intensity (MMI) scale, developed by Giuseppe Mercalli in 1902 and revised by Harry Wood and Frank Neumann in 1931, describes the intensity of ground shaking at a specific location on a Roman numeral scale from I to XII. MMI I is 'not felt except by a very few under especially favorable conditions.' MMI IV is 'felt indoors by many'; dishes rattle. MMI VI is 'felt by all'; heavy furniture moves, some plaster falls. MMI VIII causes 'considerable damage in ordinary buildings'; partial collapse of poorly built structures. MMI X describes 'most masonry and frame structures destroyed.' MMI XII represents 'total destruction'; objects thrown into the air. Reports from residents after earthquakes are systematically collected by USGS 'Did You Feel It?' and used to map intensity distributions.

Why do some earthquakes feel different even at the same distance?

Several factors cause shaking to feel different at identical distances from the same earthquake. Soil amplification is the dominant local effect: soft sediments amplify ground motion and extend its duration compared to hard rock. Local geology can amplify shaking by factors of 2–10 in peak ground velocity. Building resonance also plays a role: tall buildings sway more during long-period waves that travel farther from the source, so a high-rise in a distant city may sway noticeably while a single-story home nearby is unaffected. The radiation pattern of the earthquake source—controlled by fault geometry—creates directional variations in amplitude. Finally, topographic effects (ridge amplification, valley trapping) can locally increase or decrease shaking by factors of 1.5–2.

How many people typically feel a large earthquake?

USGS PAGER (Prompt Assessment of Global Earthquakes for Response) estimates population exposure to shaking levels in near-real-time after earthquakes. A M6.5 occurring in a populated region may be felt by 10–50 million people at intensities ranging from barely felt (MMI II–III) to damaging (MMI VII+). The 2011 Tohoku M9.1 earthquake was felt across the entire Japanese archipelago (population ~127 million) and recorded instrumentally in North America, Europe, and Australia. USGS 'Did You Feel It?' (DYFI) reports provide crowd-sourced intensity data; major California earthquakes regularly generate hundreds of thousands of DYFI reports within 30 minutes, demonstrating the extraordinary geographic reach of felt shaking.

Can animals detect earthquakes before humans feel them?

Anecdotal reports of unusual animal behavior before earthquakes have a multi-century history and have been studied scientifically with mixed results. Some well-documented mechanisms could theoretically allow animal pre-earthquake detection: radon gas emanating from stressed rock before rupture, electromagnetic signals from piezoelectric effects in quartz-bearing rock, and changes in well water levels. The 2011 Peru study (Nature, Bleier et al.) documented behavioral changes in dogs correlating with seismic events in the preceding days. However, controlled prospective studies—designed to distinguish genuine precursory behavior from random behavioral variation—have not produced reliable results. The scientific consensus is that while physical precursors sometimes precede earthquakes, no animal behavior pattern has demonstrated predictive skill sufficient for operational early warning.

What is seismic early warning and how does it differ from prediction?

Earthquake early warning (EEW) and earthquake prediction are fundamentally different concepts. EEW detects the initial, less-damaging P-waves from a seismic event seconds after rupture begins and transmits automated alerts before the slower, more-destructive S-waves and surface waves arrive at distant locations. ShakeAlert, deployed by USGS across the western US, provides 2–90 seconds of warning depending on distance from the epicenter—enough time to stop trains, open fire station doors, or adopt protective posture. Japan's J-ALERT system provides up to 60–90 seconds warning for distant events. Earthquake prediction, by contrast, attempts to specify the time, location, and magnitude of a future earthquake before it occurs—a goal that remains scientifically unachieved for deterministic forecasts on timescales of days to years.