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

The JMA intensity scale differs from magnitude measurements like the moment magnitude () and the earlier Richter scales, which represent how much energy an earthquake releases. Similar to the Mercalli scale, the JMA scale measures the intensities of ground shaking at various observation points within the affected area. Intensities are expressed as numerical values called ; the higher the value, the more intense the shaking. Values are derived from 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.

Intensity data is collected from 4,400 observation stations equipped with "Model 95 seismic intensity meters" that measure strong ground motion. The agency provides authorities and the general 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.

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. The JMA uses seismic intensity meters to automatically calculate ground acceleration in real-time, reporting intensities based on measurements from observation points.

+JMA Seismic Intensity Scale Intensity Category Effects onpeople Indoors Outdoors Wooden houses Reinforced concrete buildings Utilities and infrastructure Ground and slopes Mercalli equivalent (appr.) 0 N/AUndetectable by humans but recorded by instruments.No significant effect.I 1 Mild to moderateSlightly noticeable to some people who are quiet indoors.No significant effect.I–II 2Noticeable to many quiet people indoors; may wake a few.Hanging objects (e.g., lamps) sway slightly.No significant effect.II–IV 3Felt by most indoors; some walking people notice; many awake.Dishes may rattle in cupboards.Power lines sway slightly.No significant effect.IV–V 4Startles most; felt by nearly everyone walking; awakens most people.Lamps swing noticeably, dishes rattle, and unstable items may fall.Power lines sway visibly; drivers may sense shaking.No significant effect.Rail and road services may halt for safety checks; speed limits may be enforced.No significant effect.V–VI 5− (5弱) 5 Lower StrongFrightens many; they may grab onto stable objects.Lamps swing violently; items on shelves fall; some furniture may shift or topple.Windows might crack; utility poles move; minor road damage possible.Small cracks may appear in walls of low earthquake resistance houses.No significant effect.Automatic safety valves cut off residential gas. Disruptions to water and electricity possible.Minor cracks; some liquefaction possible. Rockfalls and small landslides may occur.VI–VII 5+ (5強) 5 UpperMovement is difficult without support.Items fall more often; TVs may topple; unsecured furniture likely to tip.Walls may collapse; poorly installed vending machines may fall over; driving becomes hazardous.Walls may crack in low earthquake resistance houses.Cracks may form in walls, beams, and pillars of low earthquake resistance buildings.Gas pipes and/or water mains may be damaged. Gas and/or water service are disrupted in some regions. Blackouts possible.VII 6− (6弱) 6 LowerStanding is difficult.Most furniture moves or tips; doors may jam.Wall tiles and windows sustain damage and may fall.For less resistant houses, walls crack more often; houses may lean or collapse. Small cracks may form in walls of more resistant houses.For less resistant buildings, cracks occur more often; X-shaped cracks may develop. Cracks may appear in structural elements of more resistant buildings.Gas pipes and/or water mains are damaged. Gas, water and electricity services are disrupted.Cracks appear. Landslides and terrain shifts are likely.VIII 6+ (6強) 6 Upper BrutalStanding is impossible; moving is only possible by crawling. People may be thrown through the air.Most unsecured furniture shifts or topples.Wall tiles and windows are highly likely to fall; unreinforced concrete-block walls collapse.Low earthquake resistance houses are likely to lean or collapse. Larger cracks are possible in more resistant houses.Ground-level pillars may break; collapse is possible for less resistant structures. For more resistant structures, cracks become more widespread; walls may slip.Gas pipes and water mains are damaged. Gas, water and electricity services are disrupted.Larger cracks develop. Landslides occur.IX 7 Most unsecured furniture is thrown or topples violently.Wall tiles and windows are almost certain to fall; reinforced concrete-block walls may collapse.Less resistant houses are highly likely to collapse. For more resistant houses, cracks are more frequent and leaning may occur.Most or all buildings (even earthquake-resistant ones) suffer severe damage.Gas pipes and water mains are damaged. Gas, water and electricity services are disrupted.The ground is considerably distorted by extensive cracks; massive landslides and debris flows happen, which can change topographic features.X+

History Establishment and revision Seismic observations in Japan began in 1872. In 1884, Sekiya Seikei, Director of the Earthquake Division under the Home Ministry, compiled the 18-article "Earthquake Report Guidelines" and initiated data collection from 600 county offices nationwide. This was Japan's first unified seismic intensity scale. At that time, the scale had four levels: , , , and . For example, a faint tremor event was described with a brief explanation, such as "Slightly felt by those who have experience of earthquakes".Reference Materials on the Use of Seismic Intensity and the Transition of Seismic Intensity Classes, p. 30 (II-8)Ishibashi, pp. 352–353

In 1898, the scale was expanded to include "faint tremor (no sensation)" and intermediate levels such as "weak tremor (slightly weaker intensity)" and "strong tremor (slightly weaker intensity)." The scale expanded to 7 levels, numbered from 0 to 6, but at this point, explanatory text was omitted. In 1908, explanatory text was reinstated for each level. In 1936, the "Earthquake Observation Law," which is the current guideline for seismic observation, was established, and the terms for faint tremor (no sensation), weak tremor (slightly weaker intensity), and strong tremor (slightly weaker intensity) were renamed to "no feeling," "light tremor," and "moderate tremor".Reference Materials on the Use of Seismic Intensity and the Transition of Seismic Intensity Classes, pp. 37–38 (II-15 – II-16) During this time, the number of observation points further increased. According to materials from the JMA, in 1904, there were 1,437 observation points including both official stations and private contracted stations (e.g., local observation posts), and this number remained stable until the 1950s (around 1955-1964).

In January 1949, the "Earthquake Observation Law" was revised to establish intensity 7, and the scale was expanded to 8 levels, from 0 to 7; this was because concern arose that damage caused by the 1948 Fukui earthquake, which saw over 90% house collapse in some areas, could not be accurately expressed with intensity 6.Reference Materials on the Use of Seismic Intensity and the Transition of Seismic Intensity Classes, p. 39 (II-17)Ishibashi, p. 353 Furthermore, the judgment for intensity 7 was based on a field survey conducted later by the JMA's mobile observation team, with specific criteria like "house collapse rate of 30% or more."Fukui Earthquake 50th Anniversary Special, p. 2 During this revision, the terms "no feeling," "slight," "weak," "rather strong," "strong," "very strong," "disastrous," and "very disastrous" were assigned to each intensity level. Seismic intensity was also made a factor in tsunami forecasting, and descriptions of the sensation of intensity 4 and 6 were added to the explanatory text for quicker judgment. Later, in 1978, the sensation of all intensity levels was added.Reference Materials on the Use of Seismic Intensity and the Transition of Seismic Intensity Classes, pp. 13–14 (I-10 – I-11)

Transition to instrumental measurements Previously, JMA staff determined seismic intensity by observing ground shaking and building damage, matching their observations to a guideline chart. Although guidelines existed, intensity assessments were subjective and lacked consistency. In the early years of the Heisei era, it took around 10 minutes or longer for each meteorological station to collect seismic information and issue a report along with the estimated scale.Reference Materials on the Use of Seismic Intensity and the Transition of Seismic Intensity Classes, pp. 13 (I-10), 23 (II-1), 26 (II-4), 29 (II-8), 31 (II-10), 32 (II-11), 51 (II-29)

Between 1958 and 1969, the number of seismic observation points dropped from over 1,000 to about 150 due to station consolidations and closures. As a result, issues such as a lack of seismic observation points, subjective judgments by observers, variability in damage from intensities above level 5, and delays in issuing intensity reports emerged. These challenges led to the consideration of using automatic instruments for seismic intensity measurement, and in 1985, a committee was established within the JMA to explore the use of instruments. In 1988, based on the committee's report, experimental measurements using seismometers began, and by March 1994, seismometers were installed at all observation points. During this period, observation points increased to 300 in 1993 and 600 in 1996.

Meanwhile, major earthquakes such as the 1994 offshore Sanriku earthquake and the 1995 Great Hanshin Earthquake revealed issues like wide variability in damage in areas with intensities 5 and 6, as well as delays in determining intensity 7 (which required field surveys by the JMA's mobile observation team). These issues highlighted the need for quicker and more detailed damage assessment.Fukui Earthquake 50th Anniversary Special, p. 4

On April 1, 1996, the scale was revised, eliminating sensory-based observations and fully transitioning to instrument-based measurements. This placed the JMA scale among non-Cancani macroseismic scales, distinguishing it from those belonging to the 12-degree Cancani family, such as the Modified Mercalli intensity scale or the European macroseismic scale.Musson et al., p. 417 Levels 5 and 6 were subdivided into "lower" and "upper," creating a 10-level scale. As a result, terms like "faint tremor" and "light tremor" were discontinued, and a new "related explanatory table" was created to provide explanations previously contained in the old descriptions. Additionally, seismic intensity level 7, which had been determined by damage rates, was standardized with instrumental observations, with a level of 6.5 or higher on the instrumental scale being classified as intensity 7 on the 10-level scale.Reference Materials on the Use of Seismic Intensity and the Transition of Seismic Intensity Classes, pp. 13–18 (I-10 – I-15) Furthermore, in addition to the approximately 600 JMA observation points, data from around 800 sites operated by the National Research Institute for Earth Science and Disaster Resilience (NIED) and about 2,800 local government sites were also used for JMA reports, increasing the total number of observation points to about 4,200, a significant increase from previous levels.Reference Materials on the Use of Seismic Intensity and the Transition of Seismic Intensity Classes, p. 14 (I-10)

Intensity 7