Japan Meteorological Agency seismic intensity scale
The JMA intensity scale should not be confused or conflated with magnitude measurements like the moment magnitude (Mw) and the earlier Richter scales, which represent how much energy an earthquake releases. Much like the Mercalli scale, the JMA scheme quantifies how much ground-surface shaking takes place at measurement sites distributed throughout an affected area. Intensities are expressed as numerical values called shindo (震度, "seismic intensity"); the higher the value, the more intense the shaking. Values are derived from peak ground acceleration and duration of the shaking, which are themselves influenced by factors such as distance to and depth of the hypocenter (focus), local soil conditions, and nature of the geology in between, as well as the event's magnitude; every quake thus entails numerous intensities.
The data needed for calculating intensity are obtained from a network of 670 observation stations using "Model 95" strong ground motion accelerometers. The agency provides the public with real-time reports through the media and Internet giving event time, epicenter (location), magnitude, and depth followed by intensity readings at affected localities.
The JMA[who?] first defined a four-increment intensity scale in 1884 with the levels bi (微, faint), jaku (弱, weak), kyo (強, strong), and retsu (烈, violent). In 1898 the scale was changed to a numerical scheme, assigning earthquakes levels 0–7.
In 1908, descriptive parameters were defined for each level on the scale, and the intensities accompanying an earthquake were assigned a level according to perceived effect on people at each observation site. This was widely used during the Meiji period and revised during the Shōwa period with the descriptions seeing an overhaul.
Following the Great Hanshin Earthquake of 1995, the first quake to generate shaking of the scale's strongest intensity (7), intensities 5 and 6 were each redefined into two new levels, reconfiguring the scale into one of 10 increments: 0–4, lower/upper 5 (５弱/５強, "weak/strong" 5), lower/upper 6 (６弱/６強 "weak/strong" 6), and 7. This scale has been in use since 1996.
The JMA scale is expressed in levels of seismic intensity from 0 (weakest) to 7 (strongest) in a manner similar to that of the Mercalli intensity scale, which is not commonly used in Japan. Real-time earthquake reports are calculated automatically from seismic-intensity-meter measurements of peak ground acceleration throughout an affected area, and the JMA reports the intensities for a given quake according to the ground acceleration at measurement points. Since there is no simple, linear correlation between ground acceleration and intensity (it also depends on the duration of shaking), the ground-acceleration values in the following table are approximations.[better source needed]
|Intensity/Meter reading||Effects on:
|Indoors||Outdoors||Residential buildings||Other structures||Utilities||Ground and slopes||Peak ground acceleration||Mercalli equivalent (appr.)|
|0 / 0–0.4||Imperceptible to most people.||Indoor objects will not shake.||No damage||<0.008 m/s²||I|
|1 / 0.5–1.4||Perceptible to some people in the upper stories of multi-story buildings||Objects may sway or rattle.||No damage||0.008–0.025 m/s²||I–II|
|2 / 1.5–2.4||Perceptible to most people indoors. Wakens light sleepers.||Hanging objects sway.||Shaking without damage.||No damage||0.025–0.08 m/s²||II–IV|
|3 / 2.5–3.4||Perceptible to everyone indoors. Frightens some people.||Objects inside rattle noticeably and can fall from raised surfaces.||Overhead power lines sway. Perceptible to people outdoors.||Houses may shake intensely. Light damage possible to homes with low earthquake resistance.||Light damage to older buildings with low earthquake resistance. Light damage possible to earthquake-resistant buildings.||Unaffected||0.08–0.25 m/s²||III–IV|
|4 / 3.5–4.4||Frightens most people. Some seek escape. Wakens most sleepers.||Hanging objects swing and cupboard dishes rattle. Unsecured objects topple. Loud noises.||Power lines sway. Tremor perceptible to people outside.||Light damage to less earthquake-resistant homes. Most homes shake intensely and walls may crack. Apartment buildings will shake.||Light damage to non-residential buildings. Little damage to earthquake-resistant structures.||Interruptions (esp. electricity)||No landslides or ground cracking||0.25–0.80 m/s²||V–VII|
|5-lower (5弱) / 4.5–4.9||Most people try to escape from danger by running outside. Some people find it difficult to move.||Hanging objects swing. Most unsecured objects topple. Cupboarded dishes and shelved books fall; furniture moves.||Utility poles swagger. Windows may break or fall, unreinforced cinderblock walls topple, some road damage||Wall and column damage to low earthquake-resistant residential structures||Wall cracks in low earthquake-resistant buildings. Light damage to regular and earthquake-resistant structures||Automatic valves cut residential gas. Some water supply interruptions. Blackouts.||Soft ground may crack. Rockfalls and small slope failures possible||0.80–1.40 m/s²||V–VIII|
|5-upper (5強) / 5.0–5.4||Many people are considerably frightened and find it difficult to move.||Most dishes in a cupboard and most books on a bookshelf fall. Occasionally, a TV set on a rack falls, heavy furniture such as a chest of drawers fall, sliding doors slip out of their groove and the deformation of door frames makes it impossible to open doors.||Unreinforced concrete-block walls can collapse and tombstones overturn. Many automobiles stop because it becomes difficult to drive from the shaking. Poorly installed vending machines can fall.||Less earthquake-resistant homes and apartments suffer heavy/significant damage to walls and pillars and can lean.||Medium to large cracks are formed in walls. Crossbeams and pillars of less earthquake-resistant buildings and even highly earthquake-resistant buildings also have cracks.||Gas pipes and water mains are damaged. (Gas service and/or water service are interrupted in some regions.)||Cracks may appear in soft ground. Rockfalls and small slope failures would take place.||1.40–2.50 m/s²||VI–IX|
|6-lower (6弱) / 5.5–5.9||Difficult to keep standing.||A lot of heavy and unanchored furniture moves or falls. It is impossible to open the door in many cases. All objects will shake violently.||Strongly and severely felt outside. Light posts swing, and electric poles can fall down, causing fires.||Less earthquake-resistant houses collapse and even walls and pillars of other homes are damaged. Apartment buildings can collapse by floors falling down onto each other.||Less earthquake-resistant buildings easily receive heavy damage and may be destroyed. Even highly earthquake-resistant buildings have large cracks in walls and will likely be moderately damaged, at the very least. In some buildings, wall tiles and windowpanes are damaged and fall.||Gas pipes and/or water mains will be damaged. Gas, water and electricity are interrupted.||Small to medium cracks appear in the ground, and larger landslides take place.||2.50–3.15 m/s²||VIII–X|
|6-upper (6強) / 6.0–6.4||Impossible to stand; cannot move without crawling.||Most heavy and unanchored furniture moves or becomes displaced.||Trees can fall down due to violent shaking. Bridges and roads suffer moderate to severe damage.||Less earthquake-resistant houses will collapse or be severely damaged. In some cases, highly earthquake-resistant residences are heavily damaged. Multi-story apartment buildings will fall down partially or completely.||Many walls collapse, or at least are severely damaged. Some less earthquake-resistant buildings collapse. Even highly earthquake-resistant buildings suffer severe damage.||Occasionally, gas and water mains are damaged. (Electrical service is interrupted. Occasionally, gas and water service are interrupted over a large area.)||Cracks can appear in the ground, and landslides take place.||3.15–4.00 m/s²||IX–X|
|7 / 6.5 and up||Thrown off by the shaking and impossible to move at will.||Most heavy and unanchored furnitures move or becomes displaced.||In most buildings, wall tiles and windowpanes are damaged and fall. In some cases, reinforced concrete-block walls collapse.||Most or all residences collapse or receive severe damage, no matter how earthquake-resistant they are.||Most or all buildings (even earthquake-resistant ones) suffer severe damage.||Electrical, gas and water service are interrupted.||The ground is considerably distorted by large cracks and fissures, and slope failures and landslides take place, which can change topographic features.||Greater than 4 m/s²||X–XII|
Comparison with other seismic scalesEdit
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A 1971 study that collected and compared intensities according to the JMA and the Medvedev–Sponheuer–Karnik (MSK) scales showed that the JMA scale was more suited to smaller earthquakes whereas the MSK scale was more suited to larger earthquakes. The research also suggested that for small earthquakes up to JMA intensity 3, a correlation between the MSK and JMA values could be calculated with the formula MSK = JMA1.5 + 1.5, whereas for larger earthquakes the correlation was MSK = JMA1.5 + 0.75.
- This is the official name; see http://www.jma.go.jp/jma/en/Activities/earthquake.html and http://www.jma.go.jp/jma/en/Activities/inttable.html, both of which treat it as a proper noun.
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- 気象庁震度階級（明治17年～昭和23年） Archived 2009-04-22 at the Wayback Machine in Japanese
- 震度 Archived 2008-06-18 at the Wayback Machine in Japanese
- How seismic intensity is calculated (Japanese) Archived 2008-09-17 at the Wayback Machine
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- Agency, 気象庁 Japan Meteorological. "気象庁 – 計測震度の算出方法".
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- "The Great Hanshin Earthquake Disaster". 9 September 2006. Archived from the original on 9 September 2006.CS1 maint: BOT: original-url status unknown (link)
- 広野卓蔵; 佐藤馨 (1971). "MSK震度と気象庁震度の比較". 気象研究所研究報告 (in Japanese). 気象庁気象研究所. 22: 177–193. Archived from the original (PDF) on 2013-03-20.
- Recent earthquakes in Japan listed by time of occurrence with localities, magnitude, and maximum intensity. Click on the time of occurrence to see a map showing affected areas; click an affected area on the map to see a more localized shake map showing distribution of intensities (in English).
- The JMA Seismic Intensity Scale with detailed descriptions (in English).