The JMA Earthquake Scale

The standard Japanese seismic intensity scale

The most advanced earthquake intensity scale is used by the Japanese government agency in charge of earthquakes, the Japan Meteorological Agency or JMA. It differs from older scales in a profoundly scientific way, and I think its approach should be adopted more widely.

A Thermometer Analogy

Older intensity scales (the Mercalli scale used in America, the European Macroseismic scale, the Omori scale once used in Japan, and others) were devised to turn human observations into somewhat more objective data.

To use these scales, you gather reports of people's perceptions and the behavior of buildings and objects and turn them into numerical intensities, which can then be crudely translated into physical data (ground accelerations) that scientists and engineers can use. They're like the original Fahrenheit temperature scale, which was based on human body temperature and was later adjusted to a more fundamental standard.

But the JMA scale is built the opposite way and is therefore more robust. To use this scale, you gather precise physical measurements (accelerations) reported by seismometers and turn them into strictly defined numerical intensities that scientists and engineers can use, and these numbers can then be translated into perceptual terms that regular people can use. The JMA scale is more like the Celsius temperature scale, which sets the zero and 100 degree marks at the freezing and boiling points of water.

Both types of seismic intensity scales are equally useful for the public, but the JMA scale is more useful for seismology and earthquake engineering.

The Hidden JMA Scale: Instrumental Intensity

The JMA scale is tied to the practical problem of judging the behavior of structures during earthquakes. It begins with carefully selected data: ground motions recorded by seismometers mounted on the ground or on the first floor of buildings.

The ground-motion records from these instruments are filtered to favor vibrations between 0.5 and 10 hertz, the frequencies that most strongly affect buildings. Then every 10 seconds of the filtered data stream, containing 1000 measurements, is tested with an algorithm that is tuned to detect a genuine earthquake motion—up, down or sideways. To make sure a signal is genuine, the 29 highest accelerations in the 1000 data points are discarded and the 30th-highest reading (A30) is selected. With that, the instrumental intensity is calculated with the equation

I = 2 × log(A30) + 0.94

Instrumental intensity I is the hidden JMA scale that scientists and engineers use. The scale is semilogarithmic, so that acceleration rises by 10 times for each two levels of I. An I of 1 corresponds to an acceleration of 1.1 centimeters per second per second (1.1 cm/s2), an I of 3 is 10.7 cm/s2, and an I of 5 is 107 cm/s2 (which is pretty intense and generally damaging). You can see that at higher levels of I, just a half-unit difference is quite significant. The intensity scale used by the public takes that into account.

The Public JMA Intensity Scale

In the JMA intensity scale for the general public, the "1" level (shindo 1)includes values of I between 0.5 and 1.5, just where people can barely feel shaking.

It corresponds to a range of accelerations between 0.6 and 1.9 cm/s2). The "3" and "5" levels each represent a tenfold increase (to 6–19 and 60–191 cm/s2 respectively).

The shindo-5 level covers a wide range of shaking from the viewpoint of buildings and people. For that reason, shindo 5 and 6 are both split into "upper" and "lower" halves. You'll see these levels rendered as "lower/upper 5" or "5L/5U" or "5–/5+" or "–5/+5" in news stories (all are pronounced shindo go jaku/kyoo, and 6–/6+ is shindo roku jaku/kyoo). Shindo 7, the top level, is rare and corresponds to total destruction, so the distinction isn't necessary.

The 5 level is where things start falling down: books and shelves at lower 5, bureaus and brick walls at upper 5. At the 6 level, people fall down: windows shatter at lower 6, walls at upper 6.

In the 2011 Tohoku earthquake, nearly all of northern Honshu island experienced intensities of lower 5 and higher, but only a small area north of Sendai suffered shindo 7.

Many Japanese websites carry the detailed table of the JMA intensity levels. If you live there or plan a long visit, this information is well worth studying.

Acceleration (cm/s2)
Instrumental intensity I0.
Instrumental intensity I<0.50.5–1.51.5–2.52.5–3.53.5–4.54.5–5.05.0–5.55.5–6.06.0–6.5>6.5
JMA Intensity (shindo)012345L5U6L6U7