Japan Meteorological Agency seismic intensity scale

The Japan Meteorological Agency (JMA) Seismic Intensity Scale^[a] (known in Japan as the Shindo seismic scale)^[1][2] 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 (Mw) 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 shindo (震度, "seismic intensity"); the higher the value, the more intense the shaking^[1][2]. 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^[3] equipped with "Model 95 seismic intensity meters"^[4] that measure strong ground motion. The agency provides authorities and the general 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.

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.

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: bishin (微震, faint tremor), jakushin (弱震, weak tremor), kyōshin (強震, strong tremor), and retsushin (烈震, violent tremor). For example, a faint tremor event was described with a brief explanation, such as "Slightly felt by those who have experience of earthquakes"^[8][9][10].

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"^[11]. 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)^[9].

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 arised 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^[12][13][14]. 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."^[15] During this revision, the terms "no feeling," "slight," "weak," "rather strong," "strong," "very strong," "disastrous," and "very disastrous" were assigned to each intensity level^[13][14]. 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^[16].

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^[17].

Between 1958 and 1969, the number of seismic observation points dropped from over 1,000 to about 150 due to station consolidations and closures^[17]. 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^[17].

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^[18].

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^[19]. 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^[20][21]. 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^[22].

Intensity 7 (震度7, Shindo-nana) is the highest level on the JMA seismic intensity scale. At Intensity 7, movement at will is almost impossible, and people may be thrown through the air^[6]. 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".

Since April 1997, Japan has been using specialized and automated strong ground motion accelerometers known as the "seismic intensity meter" (計測震度計) 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^[4][26].

Specifications of the Model 95 Seismic Intensity Meter^[27]

Observation components: NS (North-South), EW (East-West), UD (Up-Down) – three orthogonal 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 (recorded 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 March 2023, this number had grown to around 4,400^[3]. 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 700 are managed by the JMA, and roughly 3,700 by the National Research Institute for Earth Science and Disaster Resilience (NIED) and local government bodies^[3].

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 transportation agencies have installed their own meters to monitor critical infrastructure such as dams, rivers, and railways^[28].

To ensure accurate earthquake intensity measurements, specific guidelines govern the proper installation of seismic intensity meters. The JMA excludes data from meters placed in unsuitable locations when assessing earthquake intensity.

Meters must be installed on robust, specially designed stands. Because embankments and cliffs can amplify ground shaking, it is crucial to place meters on flat, stable ground away from steps. At least two-thirds of the stand must be securely buried in the ground. Additionally, meters should be positioned far enough from nearby structures, such as trees or fences, to avoid potential impact from falling objects^[29].

For indoor installations, meters should be placed on the ground floor near pillars, with acceptable locations ranging from the basement to the second floor. Buildings with earthquake isolation or vibration control systems are unsuitable for meter placement^[29].

The JMA evaluates the installation quality of seismic intensity meters used for official earthquake intensity information. Each installation is scored based on its environmental conditions, with total scores assigned a grade from A to E. Installations graded A to C are deemed acceptable. D-rated meters may be used after thorough evaluation, while E-rated meters are excluded from use.

There have been instances where earthquake intensity data was used despite meters being installed in unsuitable locations, leading to questions about the accuracy of the information and subsequent corrections. One notable example occurred during the July 2008 Iwate earthquake, when a seismic intensity of 6+ was recorded in Ono, Hirono Town, Iwate Prefecture. This value was significantly higher than those reported in neighboring municipalities, prompting an investigation. On October 29 of the same year, the Japan Meteorological Agency (JMA) concluded that the meter in Ono was improperly installed for earthquake observation. As a result, it was removed from the earthquake intensity data, and the recorded maximum intensity was revised from 6+ to 6−^[30]. This incident raised concerns that other meters, even those initially rated as acceptable, might also be installed in deteriorating or unsuitable environments over time.

The seismic intensity meters used by the JMA and others observe shaking through accelerometers. They first measure three components of acceleration – vertical, north–south, and east–west – as time-domain signals. These are then converted into frequency-domain signals by the Fourier transform, filtered, then converted back into time-domain signals for final calculations. The specific process involved is as follows^[21][31]:

1. Apply the Fourier transform on time-domain signals to convert them into frequency-domain signals.
2. Apply a filter ${\textstyle \lambda _{a}(f)=F_{a1}(f)F_{a2}(f)F_{a3}(f)}$, which is a product of several filters, to the frequency-domain signals, where:

${\displaystyle F_{a1}(f)={\sqrt {\frac {1}{f}}}}$ is the period effect filter. ${\displaystyle F_{a2}(f)=(1+\alpha x^{2}+\beta x^{4}+\gamma x^{6}+\delta x^{8}+\epsilon x^{10}+\zeta x^{12})^{-1/2}}$ is the high-cut filter, where ${\displaystyle \alpha =0.694}$, ${\displaystyle \beta =0.241}$, ${\displaystyle \gamma =0.0557}$, ${\displaystyle \delta =0.009664}$, ${\displaystyle \epsilon =0.00134}$, ${\displaystyle \zeta =0.000155}$, ${\displaystyle x=f/10}$.

${\displaystyle F_{a3}(f)={\sqrt {1-\exp(-(f/0.5)^{3})}}}$ is the low-cut filter.

1. Convert the filtered frequency-domain signals back into time-domain signals by inverse Fourier transform. Sum the acceleration components into a single composite acceleration by vector summation.
2. Find a value ${\displaystyle a}$ such that for exactly 0.3 seconds, the norm of the acceleration vector is no less than ${\displaystyle a}$.
3. Calculate ${\displaystyle I=2\log _{10}a+0.94}$.
4. Round ${\displaystyle I}$ to two decimal places, then truncate the second decimal place to determine the instrumental seismic intensity. Then, round the instrumental seismic intensity to the nearest integer to determine the seismic intensity. If the instrumental seismic intensity is negative, the seismic intensity is considered 0; if ≥8, the seismic intensity is considered 7. In case of Intensities 5 and 6, it is further divided into lower and upper depending on whether it is rounded up or down (refer to the table on the right).

When an earthquake occurs, the JMA announces the observed seismic intensity, the epicenter of the earthquake, and the presence or absence of a tsunami as “Earthquake Information" bulletins. Among them, those related to seismic intensity are listed below^[3].

• Seismic Intensity Information: About a minute and a half after the earthquake, the JMA announces names of sub-prefectural regions observing Intensity 3 or higher.
• Earthquake and Seismic Intensity Information: If Intensity 1 or higher is observed, the JMA announces the hypocenter and magnitude of the event, seismic intensity observation points observing Intensity 1 or higher, and localities observing Intensity 3 or higher.
• Estimated Seismic Intensity Distribution Map: If Intensity 5− or higher is observed, the JMA issues an Estimated Seismic Intensity Distribution Map showing expected seismic intensity based on observational data, taking into account site amplification effects. The map contains seismic intensity distribution of areas observing Intensity 4 or higher. The seismic intensity distribution was estimated on a 1km square grid before January 31, 2023, and on a 250m square grid after February 1, 2023^[32].

When the initial seismic waves are observed at multiple locations and the maximum intensity is estimated to be at least 5−, an Earthquake Early Warning is issued for areas with an estimated intensity of 4 or higher. This is an alert to warn of strong earthquake tremors, not the observed seismic intensity^[3].

Seismic intensity information is distributed to numerous parties, including mobile network operators, businesses licensed for earthquake forecasting or information dissemination, media outlets, Fire and Disaster Management Agency, prefectural governments, police, Japan Coast Guard and other designated public authorities, who then disseminate the information to the general public^[3].

The JMA distributes earthquake information on its website, including information on time of occurrence, hypocenter location, depth, and seismic intensities across different regions^[5].

On March 7, 2013, the JMA updated its website’s color scheme for earthquake information to unify weather displays and improve accessibility for visually impaired and elderly users^[33][34]. All seismic intensity indicators are now displayed in different colors. Intensity 7 is indicated in dark purple (⬤), 6+ indicated in dark red (⬤), 6− indicated in red (⬤), 5+ indicated in orange (⬤), 5− indicated in yellow (⬤), 4 indicated in cream (⬤), 3 indicated in blue (⬤), 2 indicated in light blue (⬤) and 1 indicated in white (⬤)^[33][5].

The display for the epicenter was also modified. Previously, a red “×” mark (×) was used; after the update, a red “×” mark with a yellow border is now used^[5].

Many Japanese news outlets also have dedicated pages on their websites distributing earthquake information, including seismic intensity information, from the JMA^{[35][36][37][38]}. Other major platforms also disseminate seismic intensity information, either through web, mobile apps or both, such as Yahoo Japan^[39], Weathernews^[40], and the NERV Disaster Prevention mobile app^[41].

The National Research Institute for Earth Science and Disaster Resilience distributes realtime instrumental seismic intensity, peak ground acceleration, velocity, displacement, and velocity response of different frequencies (0.125Hz − 4.0Hz) data across stations of its network through a web service called the Strong-motion Monitor (強震モニタ)^[42][43].

The web service displays information on ongoing Earthquake Early Warning bulletins, realtime intensities at all observation points as an image, estimated epicenter location and expected propagation of P waves and S waves, along with predicted seismic intensity distribution represented as a 5km square grid^[42][43].

Various other businesses also provide access to the Strong-motion Monitor through their own platforms, such as Yahoo Japan through a web app^[44] and Gehirn Inc through their "NERV Disaster Prevention" mobile app^[45].

Administrative agencies obtain seismic intensity information from the JMA and other sources and use this information as a criterion for deciding the initial actions to be taken immediately after an earthquake. Generally, at a seismic intensity of 4 to 5− or higher, the National Police Agency and Fire and Disaster Management Agency (through a line of prefectural police headquarters to police stations, and prefectural fire and disaster management divisions to fire headquarters) begin investigations. If the intensity reaches 5− or higher, the Japan Coast Guard and Ministry of Defense carry out damage assessments^[46]. Specifically, helicopters from the regional Coast Guard offices that recorded the maximum intensity, fighter jets scrambled by Air Self-Defense Force squadrons^[b], and maritime patrol aircraft deployed by the Maritime Self-Defense Force are dispatched, and the crews conduct visual inspections. Additionally, if the intensity reaches 4 or higher, the Cabinet Office estimates earthquake damage. When an intensity of 5+ is recorded in Tokyo's 23 wards or 6− or higher elsewhere, the Emergency Response Team of the Prime Minister's Office calls an emergency meeting^[46].

Since October 2007, the JMA has implemented the Earthquake Early Warning system for the general public. This system issues warnings when the estimated maximum intensity is 5− or greater, targeting regions expected to feel an intensity of 4 or more. For advanced users, the criteria include observations of ground accelerations over 100 gal, an estimated magnitude of 3.5 or higher, and an estimated maximum intensity of 3 or greater^[3][48].

While the current intensity scale is emphasized for very short periods (0.1 to 1 second) that match human perception, damage to buildings is often associated with periods of 1 to 2 seconds. It has been proposed that for higher intensity levels, calculating intensity based on the elastic velocity response at 1 to 2 seconds correlates more closely with building damage and maintains continuity with the pre-1996 seismic intensity scale derived from observed damage^[49].

Additionally, unlike traditional macroseismic scales, the modern JMA scale determines intensity using instrumental ground motion data, rather than observed effects. A paper by Musson et al. argues that this reliance has distanced the scale from macroseismology's original purpose: to describe human and structural impacts of earthquakes^[19]. The JMA scale’s reliance on Japanese ground motion parameters and its unique subdivisions also make it less suited for international comparisons with other scales, posing challenges when used outside Japan^[50].

In Taiwan, the seismic intensity scale used is a 10-point system similar to Japan’s, known as the Central Weather Administration seismic intensity scale^[51]. Prior to this, Taiwan had adopted a scale identical to Japan's pre-September 1996 system, which had been established on August 1, 2000. However, this earlier scale did not include the subdivisions of intensity levels 5 and 6 into "upper" and "lower" categories, which had been introduced later in Japan^[52].

In South Korea, a seismic intensity scale modeled after Japan’s was used in the past, but since 2001, the country has switched to the Modified Mercalli intensity scale^[53].

• 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

• 震度の活用と震度階級の変遷等に関する参考資料 [Reference Materials on the Use of Seismic Intensity and the Transition of Seismic Intensity Classes] (PDF) (in Japanese), Japan Meteorological Agency, 2009, archived (PDF) from the original on 2024-12-17, retrieved 2024-12-20
• Musson, Roger M. W.; Grünthal, Gottfried; Stucchi, Max (2010-04-01). "The comparison of macroseismic intensity scales". Journal of Seismology. 14 (2): 413–428. doi:10.1007/s10950-009-9172-0. ISSN 1573-157X. Retrieved 2024-12-21 – via HAL.
• Ishibashi, K. (2004-12-25). "Status of historical seismology in Japan". Annals of Geophysics. 47 (2–3). doi:10.4401/ag-3305. ISSN 2037-416X.
• 福井地震50周年特集 震度の歴史と福井地震 [Fukui Earthquake 50th Anniversary Special: The History of Seismic Intensity Scales and the Fukui Earthquake] (PDF) (in Japanese), Seismological Society of Japan, pp. 4–5, retrieved 2024-12-19

• 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).