JMA Seismic Intensity Scale: How Japan Measures Ground Shaking
When an earthquake strikes, the first question many ask is not how large the earthquake was, but how much the ground actually shook beneath their feet. In Japan, this measurement is provided by the Japan Meteorological Agency (JMA) using a specialized system known as the Shindo (震度) seismic intensity scale. Unlike magnitude, which measures the total energy released at the source, the Shindo scale categorizes the intensity of local ground shaking at specific observation points.
Because factors such as the distance from the hypocenter (the focus of the earthquake), the depth of the quake, local soil conditions, and regional geology all play a role, a single earthquake will result in many different intensity readings across a country. This allows for a highly localized understanding of how much impact a specific area is experiencing.
Key Facts
- Shindo vs. Magnitude: Magnitude measures energy release; Shindo measures local ground shaking intensity.
- Observation Network: Approximately 4,400 seismic intensity meters collect data across Japan.
- Highest Level: Intensity 7 is the maximum level, described as "brutally devastating."
- Real-time Reporting: The JMA provides immediate updates on epicenter, magnitude, depth, and local intensity via the internet and media.
- Early Warning: The Earthquake Early Warning system triggers when estimated maximum intensity is 5− or greater.
Understanding the Shindo Scale
The JMA scale is similar in concept to the Modified Mercalli scale used internationally. It translates physical measurements—specifically ground acceleration and the duration of shaking—into numerical values. These values help authorities and the public understand the potential for damage to buildings, infrastructure, and human safety.
| Intensity | Category | Effects on People & Infrastructure | Mercalli Equivalent (Approx.) |
|---|---|---|---|
| 0 | N/A | Not felt | I |
| 1 | Mild to moderate | Felt by some indoors/outdoors | II–IV |
| 2 | Moderate | Felt by many indoors | IV–V |
| 3 | Strong | Felt by most; objects may rattle | V–VI |
| 4 | Strong | Noticeable shaking; dishes may fall | V–VI |
| 5− (Lower) | Strong | Difficult to stand; some damage | VI–VII |
| 5+ (Upper) | Strong | Hard to stay upright; significant damage | VII |
| 6− (Lower) | Brutal | Movement at will is difficult | VIII |
| 6+ (Upper) | Brutal | Extreme difficulty moving; heavy damage | IX+ |
| 7 | Brutal | Movement almost impossible; people thrown | X+ |
The Evolution of Seismic Observation in Japan
Japan's history of seismic observation is extensive, beginning as early as 1872. In 1884, Sekiya Seikei established the first unified scale, which originally featured four levels: bishin (faint tremor), jakushin (weak tremor), kyōshin (strong tremor), and retsushin (violent tremor).
From Manual Reports to Instrumental Precision
For much of the early 20th century, seismic intensity was determined through human observation and field surveys. However, the devastating 1948 Fukui earthquake—where house collapse rates exceeded 90% in some areas—revealed that the existing scale could not adequately capture such extreme destruction. This led to the establishment of Intensity 7 in 1949.
The transition to modern, automated measurement began in earnest in the late 1980s. By March 1994, seismometers were installed at all observation points, replacing subjective human judgment with precise instrumental data. Today, the network relies on Model 95 seismic intensity meters, which feature high sampling rates and the ability to record data on flash memory cards.
Notable Intensity 7 Events
Intensity 7 represents the most extreme level of shaking. While it was created following the Fukui earthquake, it was first officially observed during the 1995 Great Hanshin earthquake. Below are notable historical instances where Intensity 7 was recorded:
- 1995 Great Hanshin Earthquake: Magnitude 6.9 (Hyogo Prefecture)
- 2011 Tōhoku Earthquake: Magnitude 9.0 (Miyagi Prefecture)
- 2016 Kumamoto Earthquakes: Magnitude 6.2 and 7.0 (Kumamoto Prefecture)
- 2024 Noto Earthquake: Magnitude 7.5 (Ishikawa Prefecture)
Modern Monitoring and Disaster Response
The JMA manages a sophisticated observation system. As of January 2023, approximately 4,400 meters are in use, with about 700 managed directly by the JMA and 3,700 by the National Research Institute for Earth Science and Disaster Resilience (NIED) and local governments. These meters are equipped with satellite communication to ensure data transmission even if landlines fail during a disaster.
To improve accessibility, the JMA uses a color-coded system for earthquake information on the internet. This helps visually impaired and elderly users quickly identify the severity of an event:
- Intensity 7: Dark Purple
- Intensity 6+: Dark Red
- Intensity 6−: Red
- Intensity 5+: Orange
- Intensity 5−: Yellow
- Intensity 4: Cream
- Intensity 3: Blue
- Intensity 2: Light Blue
- Intensity 1: White
Furthermore, the Earthquake Early Warning system provides critical seconds of notice. It issues warnings to the public when an estimated maximum intensity of 5− or greater is expected, targeting regions likely to feel an intensity of 4 or more.
Frequently Asked Questions
What is the difference between magnitude and seismic intensity?
Magnitude measures the total energy released by an earthquake at its source, whereas seismic intensity (Shindo) measures the strength of the ground shaking at a specific location.
Why are there two types of Intensity 5 and 6?
The scale was subdivided into "lower" (5−, 6−) and "upper" (5+, 6+) categories to provide more granular detail regarding the level of shaking and potential damage.
How does the JMA collect intensity data?
Data is collected from a network of approximately 4,400 seismic intensity meters equipped with high-resolution sensors that measure ground acceleration and duration.
What happens during an Intensity 7 earthquake?
Intensity 7 is the highest level on the scale. During such an event, movement at will becomes almost impossible, and people may be thrown through the air due to the extreme force of the shaking.
Does the Shindo scale apply outside of Japan?
While the JMA scale is specific to Japan, other countries use similar systems. For example, Taiwan uses a 10-point system via its Central Weather Administration, which was previously modeled after Japan's pre-1996 scale.