The Japan Meteorological Agency seismic intensity scale is a seismic scale used in Japan and Taiwan to measure the intensity of earthquakes. It is measured in units of shindo (震度, seismic intensity, "degree of shaking"). Unlike magnitude measures such as the Moment Magnitude Scale (Mw), and the earlier Richter magnitude scale, which attempt to describe the energy released by the earthquake, the JMA scale describes the degree of shaking at various points on the Earth's surface, and is analogous to the Mercalli intensity scale. The intensity of an earthquake is not totally determined by its magnitude, but varies with event's depth, and distance from the event; for example, a quake may be described as "shindo 4 in Tokyo, shindo 3 in Yokohama, shindo 2 in Shizuoka".
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The JMA operates a network of 180 seismographs and 627 seismic intensity meters and provides real-time earthquake reports to the media and on the Internet.
History
Japan experiences about 400 earthquakes every day, although the vast majority are shindo scale "0" or less and detectable only using specialist apparatus.
The Japan Meteorological Agency (JMA) first assigned a four-stage Shindo in 1884, with the levels: 微 (faint), 弱 (weak), 強 (strong), and 烈 (violent).
In 1898 this scale was changed to a numerical system, assigning earthquakes levels 0–7.
In 1908, the levels on this scale were given descriptions, and earthquakes were assigned levels based on their perceived effect on people. This scale was widely used during the Meiji period, and revised during the Shōwa period with the descriptions seeing an overhaul.
Following the Great Hanshin earthquake in 1995, the first time an earthquake had received the highest rating of 7 on the scale, levels 5 and 6 were divided in two, giving a total of 10 levels of earthquake: 0–4, lower/upper 5 (５弱、５強, "weak 5"/"strong 5"), lower/upper 6 (６弱、６強, "weak 6"/"strong 6") and 7.
The Shindo scale has been used in Japan from 1996 without change.
Scale explanation
The JMA Scale runs from 0 to 7, with 7 being the strongest. The Mercalli intensity scale is sometimes used together with the Shindo; however this is not usual practice in Japan. Real-time earthquake reports are calculated automatically from measurements of ground acceleration. The JMA reports the earthquake level based on the ground acceleration, measured automatically with seismic intensity meters. There is no simple and linear relationship between Shindo Number and peak ground acceleration as Shindo Number also depend on duration, so the values of the table below should only be regarded as an approximate orientation.
Comparison with other seismic scales
From year 1967 to 1970, 106 JMA offices observe seismic scales in both the JMA Shindo scale and Medvedev–Sponheuer–Karnik scale (MSK scale) simultaneously. The result of simultaneous observations shows that the JMA Shindo scale is more suitable in observing low seismic scale earthquakes whereas the MSK scale being more suitable to measure high seismic scale earthquakes. The research also suggested that, by letting JMA Shindo scale as J and MSK scale as M, for small earthquake with JMA Shindo scale up to 3, the relative MSK scale can be found by having M=1.5J+1.5 whereas for larger earthquake, M=1.5J+0.75.