Japan measures how hard the ground shook on two separate scales, and they do not have the same number of steps. The seismic intensity scale runs from 0 to 7 but has ten steps, because 5 and 6 are each split into a weaker and a stronger level. Beside it sits a second scale for long-period ground motion in tall buildings, and that one has four. Both feed the same alert, and since 1 February 2023 either of them can set it off.
The intensity scale has ten steps, and the long-period scale beside it has four
The Meteorological Agency states its intensity scale as ten levels: 0, 1, 2, 3, 4, 5 weaker, 5 stronger, 6 weaker, 6 stronger and 7. The numbers stop at 7, so the ten steps come from splitting two of the numbers rather than from adding three more.
The figures are not judgements any more. Until April 1996 intensity was estimated from what people felt and from the state of the surroundings; from April 1996 onward it has been measured automatically by intensity meters and reported without a human step. The agency also states that the intensities it publishes come from observation points installed by three different bodies: the agency itself, local authorities, and the National Research Institute for Earth Science and Disaster Resilience. The explanatory table that sets out what phenomena and damage occur in the surroundings when a given intensity is observed was revised on 31 March 2009, following a review panel the agency set up jointly with the Fire and Disaster Management Agency in December 2008.
The second scale exists because intensity does not describe tall buildings well. The agency states that shaking from long-period ground motion in a high-rise cannot be adequately expressed by intensity, and so it uses a separate index divided into four classes. Forecasting of that index began on 1 February 2023, and on the same date it was added to the criteria for the earthquake early warning.
The warning goes out at intensity 5 weaker, and it names areas down to intensity 4
The agency began issuing the warning grade of the earthquake early warning on 1 October 2007. Two conditions have to be met together: seismic waves observed at two or more observation points, and a maximum intensity forecast of 5 weaker or above, or a maximum long-period class of 3 or above.
The agency gives its reasons for both halves. The two-station rule is there to avoid a false report caused by something such as a lightning strike immediately next to a seismometer. The 5 weaker threshold is set there because that is where notable damage begins.
| Grade | Trigger as published | Long-period trigger |
|---|---|---|
| Forecast grade | P or S wave amplitude of 100 gal or more at any one station, or an analysis giving magnitude 3.5 or more, or maximum intensity 3 or more | Maximum long-period class 1 or more |
| Warning grade | Waves at two or more stations and maximum intensity 5 weaker or more | Maximum long-period class 3 or more, added 1 February 2023 |
| Special warning | Maximum intensity 6 weaker or more | Long-period class 4, added 1 February 2023 |
What the warning contains is fixed. It gives the origin time, the estimated position of the hypocentre and the name of the epicentral region, then the names of the areas where strong shaking is forecast and, in addition, the names of the areas forecast to reach intensity 4. The country is divided into about 200 areas for this purpose. Areas at 4 are included for two stated reasons: the error in forecasting intensity means the real figure there may be 5 weaker, and the progress of the fault rupture may bring a place to 5 weaker a little later.
The warning never tells you how many seconds are left, and the agency says why
Two things a reader might expect are deliberately left out. The first is the predicted intensity itself. The agency states that it does not publish the specific forecast figures, because they carry an error of about plus or minus 1 and because it wants to avoid follow-up reports where it can; the wording used instead is strong shaking.
The second is the time remaining. The agency states that it does not publish the margin before the shaking arrives, and the reason it gives is the size of the unit it works in: the smallest area the agency announces spans roughly a prefecture divided into three or four parts, and the margin differs considerably from place to place inside one of those. A follow-up warning is issued when later analysis puts a new area at 5 weaker or above, or long-period class 3 or above, and the agency states that it cancels only false reports caused by something other than an earthquake. If an area forecast at 5 weaker turns out to be 3 or below, that is not cancelled.
A special warning starts at intensity 6 weaker, and the alert does not mark it as one
Special warnings came into operation on 30 August 2013 under an amendment to the Meteorological Service Act and the Ministry of Land, Infrastructure, Transport and Tourism Establishment Act, Act No. 23 of 2013. For ground motion, the criterion the agency has fixed is shaking forecast at intensity 6 weaker or above, or long-period class 4, and the agency states that criterion is set by its own notification No. 3 of 2023.
The reader will not be told which one they are hearing. The agency states that within the earthquake early warning it does not distinguish a special warning from an ordinary warning, and it gives two reasons: the technique for predicting which areas will reach 6 weaker or above still has room for improvement in immediacy and accuracy, and telling the two apart for the general public in the very short time available is difficult. The instruction on the page is the same either way.
A forecast-grade alert can start from one station and be cancelled within seconds
The forecast grade is the one that drives machinery and household receivers, and its conditions are looser. It is issued when the P or S wave amplitude at any single station reaches 100 gal or more, or when the analysis of the observed waves yields a hypocentre, a magnitude, forecast intensities and forecast long-period classes, and that magnitude is 3.5 or more, or the maximum intensity is 3 or more, or the maximum long-period class is 1 or more. The agency notes that this criterion may change.
Because one station is enough, the agency has a withdrawal procedure. If it has issued a forecast on a single station’s processing and a second station’s processing has not happened within the set time, it treats the movement as noise such as lightning and issues a cancel report within a few seconds to about ten-odd seconds. In island areas and other places where observation points are thin on the ground, a cancel report may be issued for a real earthquake, and in that case the cancel report itself may take about 30 seconds to appear. The agency issues the forecast grade roughly five to ten times for one earthquake, in the few seconds to one minute after detection, with the first report favouring speed and accuracy improving through the sequence until a final report ends it.
The law names one broadcaster, and bars everyone but the agency from issuing the warning
The legal frame came in with the partial amendment of the Meteorological Service Act, Act No. 115 of 2007, in force on 1 December 2007. Four things follow from it, and they are unusually specific for a public-facing page.
- The agency must issue forecasts and warnings of ground motion suited to general use. The agency adds that a ground-motion forecast means forecasting the shaking at each place from the first faint tremor, and that it does not include predicting that an earthquake will occur.
- Anyone other than the agency who wants to run a ground-motion forecasting business must be licensed by the Director-General of the agency.
- No one other than the agency may issue a ground-motion warning.
- When the agency issues a ground-motion warning it must immediately notify the organisation designated by cabinet order, and the agency states that this is at present the Japan Broadcasting Corporation, which must then broadcast the warning immediately. Licensed forecasting businesses must endeavour to pass the agency’s warning on swiftly.
So the statutory chain has exactly one named broadcaster in it. Everything else that carries the warning to a phone or a loudspeaker sits outside that clause, in the licensed-forecaster provision or in the general obligation to pass it on quickly.
One row in the agency’s damage table carries a different kind of magnitude
The agency publishes a table of the main damaging earthquakes near Japan from 1996 onwards. Its third column is headed simply M, and the note under the heading says only that M is the scale of the earthquake, meaning magnitude. One row breaks that pattern. The entry for 11 March 2011, off Sanriku, gives 9.0 with a footnote, and the footnote reads: moment magnitude. No other row in the table carries that footnote, so in that table the largest figure is also the one measured on a different basis from the rest of the column.
| Field in the 11 March 2011 row | Figure as printed |
|---|---|
| Magnitude | 9.0, footnoted as a moment magnitude |
| Maximum intensity | 7 |
| Tsunami | 9.3 m or more |
| Dead | 19,787 |
| Missing | 2,549 |
| Injured | 6,254 |
| Houses completely destroyed | 122,204 |
| Houses half destroyed | 287,896 |
| Houses partly damaged | 749,016 |
Each of those figures is dated and qualified on the page. The tsunami height carries a footnote saying that observation facilities were damaged by the tsunami, that data could not be obtained for some periods, and that later waves may have been higher still. The casualty and building figures carry a footnote saying they include damage from aftershocks and from earthquakes outside the aftershock area after 11 March where the damage cannot be separated out, and the row is marked as the position at 1 March 2026. The table states that, except where a note says otherwise, the human and physical damage figures come from the Fire and Disaster Management Agency rather than from the Meteorological Agency itself.
The agency says a Nankai Trough earthquake can arrive with no advance information at all
The agency describes the Nankai Trough earthquake as a large earthquake that has recurred at intervals of roughly 100 to 150 years on the plate boundary running from Suruga Bay to the sea off Hyuganada. For a largest-class event it states that intensity 7 is possible in parts of the stretch from Shizuoka to Miyazaki, and that a tsunami of over 10 m is assumed across a wide band of Pacific coast from the Kanto region to Kyushu.
The history it gives is of a sequence rather than a single shock. The whole assumed source region has ruptured at once in the past, and there has also been a large earthquake in the eastern half followed, after an interval, by a large earthquake in the western half. The most recent pair were the 1944 Showa Tonankai earthquake and the Showa Nankai earthquake of 21 December 1946, about two years apart, and the agency notes that the western half could go first instead. It also notes that 2026 marks 80 years since the 1946 earthquake.
The sentence that matters for planning is the flat one. The agency states that a Nankai Trough earthquake can occur suddenly, with no unusual phenomena observed along the trough beforehand and no extra information issued at all. Where that information is issued, it carries a keyword, and the agency’s instruction is that the disaster response follow the keyword. The agency names three of them as examples and does not present the list as closed. The forecast side of the system, as the same agency’s legal page puts it, does not include predicting that an earthquake will occur.
Sources Checked
Japan Meteorological Agency — about seismic intensity
Confirmed the ten levels of 0, 1, 2, 3, 4, 5 weaker, 5 stronger, 6 weaker, 6 stronger and 7, the change from estimation by feel and surroundings to automatic measurement by intensity meters from April 1996, that observation points are installed by the agency, local authorities and the National Research Institute for Earth Science and Disaster Resilience, and that the explanatory table was revised on 31 March 2009 after a review panel set up jointly with the Fire and Disaster Management Agency in December 2008.
https://www.jma.go.jp/jma/kishou/know/shindo/index.html
Japan Meteorological Agency — about long-period ground motion
Confirmed that shaking from long-period ground motion in high-rise buildings cannot be adequately expressed by intensity and is expressed instead by a separate index of four classes, and that forecasting of that index began on 1 February 2023 and was added to the criteria for the earthquake early warning on the same date.
https://www.data.jma.go.jp/eqev/data/choshuki/kaisetsu.html
Japan Meteorological Agency — the warning and forecast grades of the earthquake early warning
Confirmed that the warning grade began on 1 October 2007, that it requires waves at two or more observation points and a maximum intensity forecast of 5 weaker or above or a maximum long-period class of 3 or above, the stated reasons for the two-station rule and the 5 weaker threshold, the content of the warning including origin time, hypocentre estimate, epicentral region name, areas of strong shaking and areas forecast at intensity 4, the division of the country into about 200 areas, the stated reasons for withholding the specific forecast intensity and the margin of time, the prefecture-divided-into-three-or-four size of the announcement unit, the follow-up and cancellation rules, the start of special warnings on 30 August 2013 under Act No. 23 of 2013, the special warning criterion of intensity 6 weaker or above or long-period class 4 set by the agency’s notification No. 3 of 2023, the stated reasons for not distinguishing special warnings inside the alert, the forecast-grade conditions of 100 gal at any station or magnitude 3.5 or more or maximum intensity 3 or more or maximum long-period class 1 or more, the five to ten reports within a few seconds to about a minute, the cancel report within a few seconds to about ten-odd seconds with about 30 seconds possible in thin-network island areas, and the provisions of Act No. 115 of 2007 in force 1 December 2007 covering the agency’s duty, the licence for other forecasters, the bar on anyone else issuing a warning, and the notification to the Japan Broadcasting Corporation as the organisation designated by cabinet order together with its duty to broadcast immediately.
https://www.data.jma.go.jp/eew/data/nc/shikumi/shousai.html
Japan Meteorological Agency — main damaging earthquakes near Japan from 1996
Confirmed that the magnitude column is headed M and described only as the scale of the earthquake, that the 11 March 2011 entry off Sanriku gives 9.0 with a footnote reading moment magnitude, the maximum intensity of 7, the tsunami of 9.3 m or more with its footnote about damaged observation facilities and possibly higher later waves, the 19,787 dead, 2,549 missing and 6,254 injured, the 122,204 houses completely destroyed, 287,896 half destroyed and 749,016 partly damaged, the footnote that these include aftershock and unseparable damage, the stated position date of 1 March 2026, and the note that human and physical damage figures come from the Fire and Disaster Management Agency except where noted.
https://www.data.jma.go.jp/eqev/data/higai/higai1996-new.html
Japan Meteorological Agency — about the Nankai Trough earthquake
Confirmed the description of recurrence at intervals of roughly 100 to 150 years on the plate boundary from Suruga Bay to off Hyuganada, intensity 7 possible in parts from Shizuoka to Miyazaki and an assumed tsunami of over 10 m along a wide band of Pacific coast from Kanto to Kyushu, the record of both whole-region rupture and an eastern-half event followed later by a western-half event, the 1944 Showa Tonankai and 21 December 1946 Showa Nankai pair about two years apart with the western half able to go first, the note that 2026 is 80 years after 1946, the statement that the earthquake can occur suddenly with no unusual phenomena observed and no extra information issued, and the three keywords named as examples for that information, with the list not presented as closed.
https://www.jma.go.jp/jma/kishou/know/jishin/nteq/index.html
Last updated: September 2026
