Japan Meteorological Agency seismic intensity scale

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. == 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". 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". 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). 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." During this revision, the terms "no feeling," "slight," "weak," "rather strong," "strong," "very strong," "disastrous," and "very disastrous" were assigned to each intensity level. 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. Between 1958 and 1969, the number of seismic observation points dropped from over 1,000 to about 150 due to station consolidations and closures. for the 2011 Tōhoku Earthquake 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. 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. 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. == Intensity 7 ==

Intensity 7 (, Shindo-nana) is the highest level on the JMA seismic intensity scale, earthquakes of this intensity are referred to as "brutally devastating earthquakes". At Intensity 7, movement at will is almost impossible, and people may be thrown through the air. The intensity was created following the 1948 Fukui earthquake. It was observed for the first time in the 1995 Great Hanshin earthquake. == Seismic intensity measurement ==

Observation system Since April 1997, Japan has been using specialized and automated strong ground motion accelerometers known as the "seismic intensity meter" 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 1988 with the "Model 87 seismic intensity meter," which were not capable of storing data and could only observe a moderate acceleration range. Later, the "Model 95 seismic intensity meter" was introduced, which had several improvements including the ability to observe acceleration higher than previous limits, a higher sampling rate and better resolution, along with the capability to record observed data in flash memory cards. As of March 1997, all of JMA's seismic intensity meters were replaced by this "Model 95" type. By the end of 2009, about 4,200 of these meters were in use for JMA's "seismic intensity information," and by January 2023, this number had grown to around 4,400. 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. The JMA's meters are equipped with satellite communication mechanisms that allow them to transmit data in case of landline malfunctions; in the event of a major earthquake, the JMA assesses the integrity of its meters and/or set up temporary observation stations where necessary. 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. Instrument installation 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. Intensity calculation 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: • Apply the Fourier transform on time-domain signals to convert them into frequency-domain signals. • Apply a filter \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 F_{a1}(f) = \sqrt{\frac{1}{f}} is the period effect filter; 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 \alpha = 0.694, \beta = 0.241, \gamma = 0.0557, \delta = 0.009664, \epsilon = 0.00134, \zeta = 0.000155, x = f/10; F_{a3}(f) = \sqrt{1 - \exp(-(f/0.5)^3)} is the low-cut filter. • 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. • Find a value a such that for exactly 0.3 seconds, the norm of the acceleration vector is no less than a. • Calculate I = 2\log_{10} a + 0.94. • Round 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). == Information dissemination ==

Earthquake Information bulletins 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. • 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 1 km square grid before January 31, 2023, and on a 250m square grid since February 1, 2023. 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. 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. Internet The JMA distributes earthquake information on its website, including information on time of occurrence, hypocenter location, depth, and seismic intensities across different regions. 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. 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 (). Other major platforms also disseminate seismic intensity information, either through web, mobile apps or both, such as Yahoo Japan, Weathernews, and the NERV Disaster Prevention mobile app. NIED strong-motion monitor 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 (0.125 Hz − 4.0 Hz) data across stations of its network through a web service called the Strong-motion Monitor (強震モニタ). 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 5 km square grid. and Gehirn Inc through their "NERV Disaster Prevention" mobile app. == Disaster response based on seismic intensity ==

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 Intensity 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. 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. 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. 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. == Use outside Japan ==

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. 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. 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. == See also ==