The Japan Meteorological Agency (JMA) Seismic Intensity Scale (known in Japan as the Shindo seismic scale) is a seismic intensity scale used in

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...

to categorize the intensity of local ground shaking caused by

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

The JMA intensity scale should not be confused or conflated with magnitude measurements like the moment magnitude (Mw) and the earlier Richter scales, which represent how much energy an earthquake releases. Much like the Mercalli scale, the JMA scheme quantifies how much ground-surface shaking takes place ''at measurement sites distributed throughout an affected area''. Intensities are expressed as numerical values called ; the higher the value, the more intense the shaking. Values are derived from

peak ground acceleration Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a par ...

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. The data needed for calculating intensity are obtained from a network of 670 observation stations using "Model 95" strong ground motion

accelerometers An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration (the rate of change of velocity) of a body in its own instantaneous rest frame; this is different from coordinate acceleration, which is acce ...

. The agency provides the public with real-time reports through the media and Internet giving event time, epicenter (location), magnitude, and depth followed by intensity readings at affected localities.

History

The Tokyo Meteorological Observatory, which in 1887 became the Central Meteorological Observatory first defined a four-increment intensity scale in 1884 with the levels , , , and . In 1898 the scale was changed to a numerical scheme, assigning earthquakes levels 0–7. In 1908, descriptive parameters were defined for each level on the scale, and the intensities at particular locales accompanying an earthquake were assigned a level according to perceived effect on people at each observation site. This was widely used during the

Meiji period The is an era of Japanese history that extended from October 23, 1868 to July 30, 1912. The Meiji era was the first half of the Empire of Japan, when the Japanese people moved from being an isolated feudal society at risk of colonization ...

and revised during the

Shōwa period Shōwa may refer to: * Hirohito (1901–1989), the 124th Emperor of Japan, known posthumously as Emperor Shōwa * Showa Corporation, a Japanese suspension and shock manufacturer, affiliated with the Honda keiretsu Japanese eras * Jōwa (Heian ...

with the descriptions seeing an overhaul. Following the Great Hanshin Earthquake of 1995, the first quake to generate shaking of the scale's strongest intensity (7), intensities 5 and 6 were each redefined into two new levels, reconfiguring the scale into one of 10 increments: 0–4, 5-lower (5–), 5-upper (5+), 6-lower (6–), 6-upper (6+), and 7. This scale has been in use since 1996.

Scale overview

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 The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the ef ...

, which is not commonly used in Japan. Real-time earthquake reports are calculated automatically from seismic-intensity-meter measurements of

throughout an affected area, and the JMA reports the intensities for a given quake according to the ground acceleration at measurement points. Since there is no simple, linear correlation between ground acceleration and intensity (it also depends on the duration of shaking), the ground-acceleration values in the following table are approximations.

Intensity 7

The Intensity 7 ( 震度7) is the maximum intensity in the Japan Meteorological Agency seismic intensity scale, covering earthquakes with an instrumental intensity (計測震度) of 6.5 and up. The effects of Intensity 7 earthquakes include generally throwing people off by the shaking and making it impossible to move at will. The intensity was made in the wake of the

1948 Fukui earthquake The occurred in Fukui Prefecture, Japan. The magnitude 6.8 quake struck at 5:13:31 p.m.( JDT) on June 28, 1948. The quake's hypocenter was approximately 10 km north-northeast of Fukui, in the present-day neighborhood of Maruoka, Sakai City. ...

. The

1995 Great Hanshin earthquake The , or Kobe earthquake, occurred on January 17, 1995, at 05:46:53 JST (January 16 at 20:46:53 UTC) in the southern part of Hyōgo Prefecture, Japan, including the region known as Hanshin. It measured 6.9 on the moment magnitude scale and h ...

was the first earthquake to observe a seismic intensity 7. Below is a list of earthquakes with Intensity 7.

Comparison with other seismic scales

A 1971 study that collected and compared intensities according to the JMA and the Medvedev–Sponheuer–Karnik (MSK) scales showed that the JMA scale was more suited to smaller earthquakes whereas the MSK scale was more suited to larger earthquakes. The research also suggested that for small earthquakes up to JMA intensity 3, a correlation between the MSK and JMA values could be calculated with the formula MSK = JMA1.5 + 1.5, whereas for larger earthquakes the correlation was MSK = JMA1.5 + 0.75.

References

External links