Japan Meteorological Agency seismic intensity scale

Not to be confused with Japan Meteorological Agency magnitude scale.

Japan Meteorological Agency seismic intensity scale

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

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. 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.

History

The Tokyo Meteorological Observatory, which in 1887 became the Central Meteorological Observatory 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.

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.

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.

Scale overview

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), the ground-acceleration values in the following table are approximations.

JMA Seismic Intensity Scale

Intensity	Instrumental Intensity	Effects on: People	Indoors	Outdoors	Residential buildings	Other structures	Utilities	Ground and slopes	Peak ground acceleration	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

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. At Intensity 7, it becomes impossible to move at will. 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	Date	Magnitude	Area of Intensity 7
1995 Great Hanshin earthquake	January 17, 1995	6.9 Mw	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)
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

Observation system

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 replaces 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 acceleration double the previous limit and a higher sampling rate. Today, all of JMA's seismic intensity meters are of this "Model 95" type.

Specifications of the Model 95 Seismic Intensity Meter

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.

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 by JMA, many other meters have been installed by local government bodies that are not used by JMA. 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

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-rated meters are 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-. 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

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. 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, 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

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
2016 Kumamoto earthquakes (April 16 mainshock)	7 (6.7)	Miyazono, Mashiki Town	6+ (6.0)	Kumamoto Nishi-ku Kasuga
2018 Hokkaido Eastern Iburi earthquake	7 (6.5)	Kanuma, Atsuma Town	4 (4.4)	Katsuno-cho, Otaru City

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. 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.

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.

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

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:

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 ($f$).
   • Low-cut (low frequency elimination) filter: ${\sqrt {1-\exp \left(-\left({\frac {f}{0.5}}\right)^{3}\right)}}$
   • High-cut (high frequency elimination) filter: ${\frac {1}{\sqrt {1+0.694x^{2}+0.241x^{4}+0.0557x^{6}+0.009664x^{8}+0.00134x^{10}+0.000155x^{12}}}}$ (where $x={\frac {f}{10}}$)
   • Periodic effect filter: ${\sqrt {\frac {1}{f}}}$
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. Sum the three components of vertical, north–south, and east–west movements to create a single composite acceleration.
5. Find a threshold value $a$ such that the total time when the absolute value of the composite acceleration is $a$ or more is exactly 0.3 seconds. In other words, let $A(t)$ be the composite acceleration signal as a function of time $t$. We need to find a threshold value $a$ such that $\int _{T_{1}}^{T_{2}}H(|A(t)|-a)\,\mathrm {d} t=0.3$, where $H(x)$ is the Heaviside step function, and $T_{1}$, $T_{2}$ are the bounds of the time interval being considered. The aim is to standardize the magnitude $a$, 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 $I=2\log _{10}a+0.94$.
7. Round $I$ to two decimal places, then truncate the second decimal place to determine the instrumental seismic intensity.

Round the instrumental seismic intensity to the nearest integer to determine the seismic intensity level from 0 to 7. If the instrumental seismic intensity is negative, it is considered 0; if ≥8, it is considered 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).

See also

• Earthquake engineering
• Japanese Coordinating Committee for Earthquake Prediction
• List of earthquakes in Japan
• Nuclear power in Japan (seismicity section)
• Seismic intensity scales
• Seismic magnitude scales

External links

• 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).