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| Seismic intensity scale (Japan) | |
|---|---|
| Name | Seismic intensity scale (Japan) |
| Jurisdiction | Japan |
| Administered by | Japan Meteorological Agency |
| Introduced | 1949 |
Seismic intensity scale (Japan) is the Japan Meteorological Agency's system for quantifying the local effects of seismic shaking on people, structures, and natural features. It complements global measures such as the Richter magnitude scale and the Moment magnitude scale by describing ground motion at specific locations across Honshu, Hokkaido, Kyushu, and Shikoku. The scale informs emergency response by linking observed shaking to actionable thresholds used by the Japan Meteorological Agency, Central Disaster Management Council, and local prefectural governments.
The scale classifies shaking into discrete categories from 0 to 7 to communicate intensity for cities like Tokyo, Osaka, Sendai, and Fukuoka and for facilities such as Narita International Airport, Kansai International Airport, and nuclear sites including Fukushima Daiichi Nuclear Power Plant. It is used alongside seismic networks operated by institutions such as the National Research Institute for Earth Science and Disaster Resilience, the University of Tokyo, and the Geological Survey of Japan. The scale's outputs influence decisions by agencies like the Japan Self-Defense Forces and the Japan Coast Guard during events including the 2011 Tōhoku earthquake and tsunami, the 1995 Great Hanshin earthquake, and the 1923 Great Kantō earthquake.
Development began in the mid-20th century when postwar reconstruction and lessons from events affecting Kobe and Tokyo Bay motivated standardisation. Early precursors included intensity descriptions used after the Meiji period seismic events and the investigations by figures associated with the Imperial University of Tokyo. The present format was formalised by the Japan Meteorological Agency through revisions in 1949, 1996, and later updates to align with advances from projects such as the Hi-net high-sensitivity seismograph network and collaborations with the United States Geological Survey and international bodies in the aftermath of disasters like the 1964 Niigata earthquake.
Categories range 0–7 with intermediate distinctions at 5-lower, 5-upper, 6-lower, and 6-upper; category 7 represents the most violent shaking observed on land in Japan. Definitions combine qualitative human and structural effects—used in urban centres such as Nagoya, Yokohama, and Sapporo—with quantitative acceleration thresholds derived from peak ground acceleration and velocity metrics influenced by research from the Public Works Research Institute and academic groups at Kyoto University and Tohoku University. Each step correlates with expected outcomes for wooden houses typical in Japanese architecture, reinforced concrete buildings studied in structural engineering programs, and lifeline systems including rail lines like Tōkaidō Shinkansen and utilities managed by companies such as Tokyo Electric Power Company.
Instrumentation includes accelerometers at dense arrays like the K-NET and KiK-net networks maintained by the National Research Institute for Earth Science and Disaster Resilience, as well as municipal stations in cities such as Shizuoka and Akita. Data processing employs real-time telemetry used by the Japan Meteorological Agency and emergency centres in Nagasaki and Miyagi Prefecture to produce intensity maps. Calibration standards reference work from the International Association of Seismology and Physics of the Earth's Interior and laboratory testing conducted at university facilities including the Disaster Prevention Research Institute at Kyoto University.
The JMA issues seismic intensity information via announcements to broadcasters such as NHK and agencies including the Cabinet Office (Japan) and municipal disaster management offices in Kobe and Ishinomaki. The information triggers automated actions for operators at rail companies like JR East and JR West, ports such as Port of Yokohama, and energy operators including Chubu Electric Power. The system supports the national earthquake early warning framework, coordinating with mobile notifications, television crawlers, and the Bulletin Board System used by emergency planners.
Intensity is location-specific and distinct from earthquake magnitude measures such as the Moment magnitude scale and historical magnitude estimates for events like the Ansei Edo earthquake. Engineering practices in seismic design—reflected in building codes administered by the Ministry of Land, Infrastructure, Transport and Tourism—use intensity data to assess seismic performance of structures across seismic zones including the Nankai Trough and the Japan Trench. Post-event forensic studies after incidents at infrastructure such as the Akashi Kaikyō Bridge and urban recovery efforts in Kobe integrate intensity maps with magnitude, fault rupture models, and soil amplification analyses led by researchers at institutions like Hokkaido University.
Major applications include emergency response to the 2011 Tōhoku earthquake and tsunami where intensities recorded in coastal cities informed evacuation and utility restoration; intensity gradients across the Tohoku region were central to tsunami modelling used by the Japan Meteorological Agency and coastal authorities. The intensity distribution in the 1995 Great Hanshin earthquake drove revisions to seismic retrofitting programmes in Kobe and nationwide regulatory updates overseen by the Building Center of Japan. Case studies involving earthquakes near Nagasaki, Oita, and Miyako Island illustrate use of the system for rural and island communities, while research collaborations with the United States Geological Survey and the National Oceanic and Atmospheric Administration have advanced integration of intensity data into international hazard frameworks.