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Japan Meteorological Agency seismic intensity scale

Not to be confused with Japan Meteorological Agency magnitude scale.

The Japan Meteorological Agency (JMA) Seismic Intensity Scale[1] (known in Japan as the Shindo seismic scale)[2] is a seismic intensity scale used in Japan to categorize the intensity of local ground shaking caused by earthquakes.

Japan Meteorological Agency seismic intensity scale
Map of Japan showing the distribution of maximum JMA Seismic Intensities by prefecture for the 2011 Tōhoku Earthquake

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.[3][4] The agency provides the public with real-time reports through the media and Internet[5] giving event time, epicenter (location), magnitude, and depth followed by intensity readings at affected localities.

History

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The Tokyo Meteorological Observatory, which in 1887 became the Central Meteorological Observatory[6] first defined a four-increment intensity scale in 1884 with the levels bi (微, faint), jaku (弱, weak), kyō (強, strong), and retsu (烈, violent). In 1898 the scale was changed to a numerical scheme, assigning earthquakes levels 0–7.[7]

In 1908, descriptive parameters were defined for each level on the scale, and the intensities at particular locales 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.[7]

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, 5-lower (5–), 5-upper (5+), 6-lower (6–), 6-upper (6+), and 7. This scale has been in use since 1996.[7]

Scale overview

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The JMA scale is expressed in levels of seismic intensity from 0 to 7 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[8][9][10]), the ground-acceleration values in the following table are approximations.[better source needed]

JMA Seismic Intensity Scale[11][12][13]
Intensity Instrumental Intensity Effects on:
People 		Indoors Outdoors Residential buildings Other structures Utilities Ground and slopes Peak ground acceleration[14] Mercalli equivalent (appr.)
0 ≤0.5 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. Awakens light sleepers. Hanging objects sway. Shaking without damage. No damage 0.025–0.08 m/s2 II–III
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 Most people are frightened by the shaking. Some seek escape. Most sleepers are awoken. Hanging objects swing and dishes inside cupboards rattle. Unsecured objects topple over. Moving objects produce loud noises. Power lines sway. Tremors are 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) are possible. No landslides or ground cracking 0.25–0.80 m/s2 IV–VI
5− (5弱) 4.5–4.9 Most people are frightened, and feel the need to hold on to something stable to support themselves. Some may try to escape from danger by running outside. Some people find it difficult to move. Hanging objects swing. Most unsecured objects topple. Dishes fall from cupboards and books on shelves fall to the ground. Unsecured furniture will move. 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–VII
5+ (5強) 5.0–5.4 Many people are considerably frightened and find it difficult to move. Most road users will stop their vehicles, as the shaking makes it extremely difficult to drive. Most dishes in a cupboard and most books on a bookshelf fall. Occasionally, a TV set on a rack falls down, heavy furniture such as drawers fall over, and sliding doors slip out of their grooves. Due to earthquake-induced deformation of doorframes, it may become impossible to open or close interior doors after the shaking stops. Unreinforced concrete-block walls can collapse and tombstones overturn. Poorly installed vending machines can fall over. 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–VIII
6− (6弱) 5.5–5.9 Difficult to keep standing. A lot of heavy and unanchored furniture moves or falls. Due to earthquake-induced deformation of doorframes, it is impossible to open interior doors 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 walls and pillars of earthquake-resistant buildings homes are damaged. Apartment buildings can collapse from their 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 VII–IX
6+ (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 VIII–X
7 ≥6.5 It is impossible to move at will due to the intense shaking, which can throw those who do not secure themselves around. Most heavy and unanchored furniture moves 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. >4 m/s2 IX–XII

Intensity 7

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Intensity 7 (震度7, Shindo-nana) is the maximum intensity in the Japan Meteorological Agency seismic intensity scale, covering earthquakes with an instrumental intensity (計測震度) of 6.5 and up.[15] At Intensity 7, it becomes impossible to move at will.[13] The intensity was created following the 1948 Fukui earthquake. It was observed for the first time in the 1995 Great Hanshin earthquake and categorized as "brutal earthquakes".

Shindo 7 earthquakes
Earthquake[16] Date Magnitude Area of Intensity 7
1995 Great Hanshin earthquake January 17, 1995 6.9 Mw[17] Kobe, Nishinomiya, Ashiya, Takarazuka, Tsuna, Hokudan, Ichinomiya (Hyogo)
2004 Chūetsu earthquake October 23, 2004 6.6 Mw Kawaguchi (Niigata)
2011 Tōhoku earthquake March 11, 2011 9.0 Mw Kurihara (Miyagi)[18]
2016 Kumamoto earthquakes April 14, 2016 6.2 Mw Mashiki (Kumamoto)
April 16, 2016 7.0 Mw Nishihara, Mashiki (Kumamoto)
2018 Hokkaido Eastern Iburi earthquake September 6, 2018 6.6 Mw Atsuma (Hokkaido)
2024 Noto earthquake January 1, 2024 7.5 Mw Shika, Wajima (Ishikawa)

Seismic intensity measurement

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Observation system

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Since April 1997, Japan has been using automated devices known as "seismic intensity meters" to measure and report the strength of earthquakes based on the JMA scale. This replaced the old system that relied on human observation and damage assessment.

The installation of these meters began in 1991 with the "Model 90 seismic intensity meter," which didn't have the capability to record waveforms. In 1994, an upgraded version, the "Model 93 seismic intensity meter," was introduced. This model could record digital waveforms on memory cards. Later, the "Model 95 seismic intensity meter" was introduced, which had several improvements including the ability to observe double the acceleration limit and a higher sampling rate. Today, all of JMA's seismic intensity meters are of this "Model 95" type.[19][20]

Specifications of the Model 95 Seismic Intensity Meter[21]

Observation components: NS (North-South), EW (East-West), UD (Up-Down) – three components (seismic intensity is a composite of the three components)
Measurement range: 2048 gal to -2048 gal
Sampling: 100Hz rate, 24-bit
Recording standard: Seismic intensity of 0.5 or higher (collected in one-minute intervals)
Recording medium: IC memory card

By the end of 2009, about 4,200 of these meters were in use for JMA's "seismic intensity information," and by August 2011, this number had grown to 4,313. This was a significant increase from the roughly 600 units in use when the switch to measured seismic intensity was made. This shows that Japan's network for observing seismic activity is one of the most comprehensive in the world. Of these meters, around 600 are managed by the JMA, about 780 by the National Research Institute for Earth Science and Disaster Resilience (NIED), and roughly 2,900 by local government bodies.[22][23]

The network was designed with the aim of having one seismometer in each municipality before the major municipal mergers of the Heisei era. Additional units were installed in remote islands and areas with low populations to ensure complete coverage.

Besides the seismic intensity meters used for JMA's information, many other meters have been installed by local government bodies that are not used for JMA's information. Public institutions and public transportation organizations have also independently installed meters to ensure the safety of infrastructure like dams, rivers, and railways.

Observation instrument installation

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To ensure the accuracy of earthquake intensity measurements, there are specific guidelines for setting up seismic intensity meters. The JMA doesn't use data from meters that are set up in unsuitable locations for their earthquake intensity information.

Firstly, these meters should be placed on a sturdy stand designed for them. Because the ground can shake more on embankments or cliffs, the meters should be set up outside on flat, stable ground with no steps nearby, and at least two-thirds of the stand should be buried in the ground. There are also rules about nearby structures. The meters should be far enough away from trees or fences that could fall over and hit the meter. If the meters are set up inside, they should be placed near the pillars on the ground floor, and they can be set up anywhere from the basement to the second floor. Meters aren't set up in buildings that have earthquake isolation or control construction.

Seismic intensity meters should be securely attached to the stand or, if they're inside, to the floor. It's recommended to follow the setup instructions provided for each type of meter and, if possible, to secure them with anchor bolts.

The JMA rates the setup location of seismic intensity meters used for earthquake intensity information on a scale from A to E. Grades A to C are acceptable, D is generally not used but may be used after careful consideration, and E is not acceptable.

However, there have been cases where earthquake intensity information was used even though the meters were set up in unsuitable locations, and later the accuracy of the information was questioned and corrected. For instance, during the July 2008 Iwate earthquake, an earthquake intensity of 6+ (later changed to 6-) was recorded in Ono, Hirono Town, Iwate Prefecture. This intensity was much higher than in nearby municipalities, which led to an investigation. On October 29 of the same year, the JMA announced that the meter in Ono was in an unsuitable location for earthquake observation and removed it from the earthquake intensity data, correcting the maximum intensity from 6+ to 6-.[24] Since the meter in Ono was originally rated as acceptable, it's been suggested that other meters could also be in deteriorating setup locations.

Density of station placement and maximum seismic intensity

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The number of seismic monitoring stations significantly grew in 1996, thanks to the JMA increasing the number of seismic observation points. This growth has made it easier to detect strong earthquakes near their origin point. For example, the 1984 Nagano earthquake, which caused a lot of damage but was only rated as a 4 in terms of seismic intensity, and the 1946 Nankai earthquake, a huge earthquake that was rated as a 5, would have been given lower ratings if there weren't any monitoring stations near their origin points before 1995. After the increase in monitoring stations, even if an earthquake is the same size as before, it's likely to be given a higher seismic intensity rating, and high intensity ratings like 6- are reported more often.[25][26] The increase in seismic observation points has made it possible to detect earthquake intensities closer to their origin point, and the JMA is studying the differences between the highest earthquake intensities detected at all monitoring stations and the intensities measured at JMA offices,[4][27] to understand how the increase in monitoring stations has changed the maximum seismic intensities. Here are a few examples:

Comparison between maximum intensities recorded at observation stations and maximum intensities recorded at JMA offices[4][27]
Event name Max. intensity observed by observation station Station location Max. intensity observed by JMA office Office location
2004 Chūetsu earthquake 7 (6.5) Kawaguchi, Kawaguchi Town 5- (4.5) Otemachi, Joetsu City (Takada)
2005 Fukuoka earthquake 6- (5.7) Maizuru, Chuo-ku, Fukuoka 5+ (5.1) Ohori, Chuo-ku, Fukuoka
2007 Noto earthquake 6+ (6.4) Hashide, Monzen-cho, Wajima City 6+ (6.1) Hoshi-cho, Wajima City
2007 Chūetsu offshore earthquake 6+ (6.3) Chuo-cho, Kashiwazaki City 5+ (5.3) Otemachi, Joetsu City (Takada)
2008 Iwate–Miyagi Nairiku earthquake 6+ (6.2) Ichihasama, Kurihara City 5- (4.6) Sendai Miyagino-ku Gorin
July 2008 Iwate earthquake 6- (5.8) Furudate, Ito Town 5+ (5.4) Ofunato, Ofunato City
2011 Tōhoku earthquake and tsunami 7 (6.6) Tsukidate, Kurihara City 6- (5.8) Kanamachi, Mito City[28]
2016 Kumamoto earthquakes (April 16 mainshock) 7 (6.7) Miyazono, Mashiki Town 6+ (6.0) Kumamoto Nishi-ku Kasuga[29]
2018 Hokkaido Eastern Iburi earthquake 7 (6.5) Kanuma, Atsuma Town[30] 4 (4.4) Katsuno-cho, Otaru City[a][31]

In earthquakes with smaller magnitudes, the range of seismic intensity 6- becomes narrower. Even so, if there are many observation points, some will fall within the range of seismic intensity 6-. However, if there are fewer observation points, there is a high possibility that the maximum seismic intensity will be lower because it will not be captured by the observation points. Before 1995, an earthquake with a maximum seismic intensity of 6 was certainly a "major earthquake" in terms of magnitude. However, since 1996, even very shallow minor earthquakes are more likely to report seismic intensities of 5 or 6, so it is not appropriate to treat "earthquakes with a maximum seismic intensity of 6" on par with those before 1995.[25] It may seem as if there have been more earthquakes since the Great Hanshin-Awaji Earthquake, but this is not because there have been more earthquakes, but because there have been more reports of seismic intensity.[25]

Furthermore, seismic intensity observation points are not uniformly distributed by area. They are often installed in regions with high population density, especially in urban areas. This tendency is particularly strong for observation points set up by local public entities. In these high population density areas, there tends to be a higher amplification rate of seismic intensity in the surface soil layer.[26]

 
 
Distribution of seismic intensity observation points for the offshore Miyagi earthquakes in 1978 and 2005. The former had a magnitude of 7.4 with a maximum seismic intensity of 5, while the latter had a magnitude of 7.2 with a maximum seismic intensity of 6-. The density of observation points was higher in 2005.

Seismic intensity calculation

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The seismometers used by the JMA and others observe shaking through accelerometers. They first measure the three components of motion – vertical, north–south, and east–west – as time-domain signals of acceleration. The instrumental seismic intensity (decimal value) is then calculated through the following process:[32]

  1. The time-domain signals of vertical, north–south, and east–west motion are converted into frequency-domain signals through Fourier transform.
  2. To correct for the effects of the earthquake wave period, filtering is applied to each of the frequency-domain signals of vertical, north–south, and east–west motion. The filter used here is a product of several filters, each of which is a function of frequency ( ).
    • Low-cut (low frequency elimination) filter:  
    • High-cut (high frequency elimination) filter:   (where  )
    • Periodic effect filter:  
  3. Convert the frequency-domain signals of the vertical, north–south, and east–west movements that have been filtered back into time-domain (acceleration) signals by inverse Fourier transform.
  4. Combine the three components of vertical, north–south, and east–west movements to create a single composite acceleration.
  5. Find a threshold value   such that the total time when the absolute value of the composite acceleration is   or more is exactly 0.3 seconds. In other words, let   be the composite acceleration signal as a function of time  . We need to find a threshold value   such that  , where   is the Heaviside step function, and  ,   are the bounds of the time interval being considered. The aim is to standardize the magnitude  , which is the basis for calculating the seismic intensity, to a shaking that lasts for 0.3 seconds, in order to bring the seismic intensity calculated closer to the actual damage caused by the shaking.
  6. Calculate  .
  7. Round the third decimal place of   and truncate the second decimal place to determine the instrumental seismic intensity.

Round the instrumental seismic intensity (if negative, it is 0, if 8 or more, it is 7) to determine the seismic intensity level from 0 to 7. In the case of seismic intensity 5 and 6, it is further divided into lower and upper depending on whether it is rounded up or down (refer to the Scale overview section).

Comparison with other seismic scales

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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.[33]

See also

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Notes

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  1. ^ The Tomakomai Shirakaba (Tomakomai Observation Station), which was close to the epicenter, ceased operations in 2004.

References

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  1. ^ 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.
  2. ^ ""A closer look at the shindo seismic scale" (in Japanese)". 2018-06-27. Retrieved 2020-03-25.
  3. ^ ""About The JMA's strong ground motion monitoring" (in Japanese)". Retrieved 2019-01-22.
  4. ^ a b c "List of current and past JMA seismic intensity observation points (in Japanese)". Retrieved 2019-01-22.
  5. ^ "Japan Meteorological Agency – Earthquake Information".
  6. ^ "History". Japan Meteorological Agency. Retrieved 2021-10-13.
  7. ^ a b c Ishibashi, Katsuhiko (April–June 2004). "Status of historical seismology in Japan". Annals of Geophysics. 47 (2/3): 352.
  8. ^ How seismic intensity is calculated (Japanese) Archived 2008-09-17 at the Wayback Machine
  9. ^ "Seismic intensity and acceleration (Japanese)". Archived from the original on 2008-07-05.
  10. ^ Agency, 気象庁 Japan Meteorological. "気象庁 – 計測震度の算出方法".
  11. ^ "JMA seismic intensity scale".
  12. ^ "気象庁 | 震度について". Japan Meteorological Agency. Retrieved 2021-07-23.
  13. ^ a b "気象庁 | 気象庁震度階級関連解説表". Japan Meteorological Agency. Retrieved 2021-07-23.
  14. ^ "The Great Hanshin Earthquake Disaster". 2006-09-09. Archived from the original on 2006-09-09.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
  15. ^ "気象庁 | 計測震度の算出方法". Japan Meteorological Agency. Retrieved 2021-07-23.
  16. ^ "【図解】最大震度7を観測した地震（Yahoo!ニュース オリジナル THE PAGE）" (in Japanese). Yahooニュース. Retrieved 2022-04-04.
  17. ^ ISC (2015), ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009), Version 2.0, International Seismological Centre
  18. ^ 日本放送協会. "3.11東日本大震災 最大震度7と大津波 巨大地震の衝撃 – NHK". www3.nhk.or.jp. Retrieved 2022-04-04.
  19. ^ "気象庁｜強震観測について" (in Japanese). Japan Meteorological Agency. Retrieved 2024-01-17.
  20. ^ "3 観測と地震予知". Institute for Fire Safety & Disaster Preparedness (in Japanese). Archived from the original on 2016-03-06.
  21. ^ "気象庁における強震波形観測・収録と提供". mmjp.or.jp (in Japanese). Archived from the original on 2016-04-24.
  22. ^ 震度に関する検討会報告書 (PDF) (Report) (in Japanese). March 2009. Retrieved 2024-01-17.
  23. ^ "Monitoring of Earthquakes, Tsunamis and Volcanic Activity". Japan Meteorological Agency. Retrieved 2024-01-17.
  24. ^ "岩手県洋野町大野の震度データについて－ 本年７月の岩手県沿岸北部の地震の最大震度を６強から６弱に修正 －" (Press release) (in Japanese). Japan Meteorological Agency. 2008-10-29. Retrieved 2024-01-17.
  25. ^ a b c "第1部：地震の基礎知識、1章：大きな地震と小さな地震". hinet.bosai.go.jp (in Japanese). Retrieved 2024-01-17.
  26. ^ a b 観測点配置に着目した震度観測の変遷と最大震度に関する研究 (PDF) (in Japanese), retrieved 2024-01-17
  27. ^ a b "Seismic Station List". Japan Meteorological Agency. Retrieved 2024-01-17.
  28. ^ 平成 23 年３月 地震・火山月報（防災編） [Monthly Report on Earthquakes and Volcanoes in Japan] (PDF) (in Japanese), retrieved 2024-01-17
  29. ^ 平成28年４月 地震・火山月報（防災編） [Monthly Report on Earthquakes and Volcanoes in Japan – April 2016] (in Japanese), retrieved 2024-01-17
  30. ^ 平成30年９月 地震・火山月報（防災編） (PDF) (in Japanese), retrieved 2024-01-17
  31. ^ 平成 30 年９月 地震・火山月報（防災編） (PDF) (in Japanese), retrieved 2024-01-17
  32. ^ "計測震度の算出方法" (in Japanese). Japan Meteorological Agency. Retrieved 2024-01-17.
  33. ^ 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). 22. 気象庁気象研究所: 177–193. Archived from the original on 2013-03-20.
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