日本語版 (Japanese)
This section explains the questions many people have about the JMA’s seismic intensity.
About seismic intensity
Here, I would like to briefly mention seismic intensity.
In Japan, the Japan Meteorological Agency uses the seismic intensity scale, which is divided into 10 levels (seismic intensity 0, seismic intensity 1, seismic intensity 2, seismic intensity 3, seismic intensity 4, seismic intensity 5-, seismic intensity 5+, seismic intensity 6-, seismic intensity 6+, and seismic intensity 7).
The seismic intensity is explained in a separate article if you would like to know more details.
Are only “seismic intensity 5” and “seismic intensity 6” marked “lower” and “upper”?
Those who watch Japanese earthquake information may wonder, “Why are only seismic intensity 5 and 6 expressed in slightly confusing terms such as 5-(5-lower), 5+(5-upper), 6-(6-lower), and 6+(6-upper)?” Many of you may be wondering “Why are only seismic intensity 5 and 6 expressed in a slightly confusing way?
So we will explain that as well.
The reason for the division into “lower” and “upper” seismic intensity 5 and 6 was triggered by the damage caused by the Great Hanshin-Awaji Earthquake.
In fact, field surveys of the Great Hanshin-Awaji Earthquake revealed that even in areas with the same intensity of 5 or 6, there were large differences in the extent of damage. In order to reflect the magnitude of the damage, the seismic intensity is subdivided into three levels (seismic intensity of 4.5 to 5.0 is classified as “seismic intensity 5-,” 5.0 to 5.5 as “seismic intensity 5+,” 5.5 to 6.0 as “seismic intensity 6-,” and 6.0 to 6.5 as “seismic intensity 6+).
At the time of the Great Hanshin-Awaji Earthquake, seismic intensity 7 was determined by on-site surveys, but since then, it has been determined when seismic intensity 6.5 or higher is measured by a seismic intensity meter.
Why is there no greater than seismic intensity 8?
Those who have extensive knowledge of earthquakes may already know that there are no seismic intensity 8 or higher, but many people, including those who are not interested in earthquakes, assume that there are seismic intensity 8 or higher, even among Japanese. So why is there no seismic intensity of 8 or higher? The following two reasons are given for this.
there has never been an instance of a tremor greater than 7 on the Japanese seismic scale (instrumental seismic intensity 7.5 or greater), and that there is no benefit in creating a seismic intensity of 8 or greater.
- There has never been an instance of a tremor greater than seismic intensity 7 (instrumental seismic intensity 7.5 or greater)
- There is no benefit in creating a seismic intensity of 8 or greater.
The earthquakes that have actually measured seismic intensity 7 on the instrumental seismic intensity scale are as follows.
As shown above, the maximum observed seismic intensity is far from instrumental seismic intensity 7.5, which is equivalent to seismic intensity 8, and the maximum instrumental seismic intensity is 6.7, which is equivalent to seismic intensity 8. Even if a seismic intensity of 8 exists, it will not be used unless a instrumental seismic intensity 7.5 or higher is observed, and tremors equivalent to seismic intensity 8 or higher are basically impossible to occur on the earth unless an unknown tremor phenomenon of a very large scale occurs.
Another reason is that it does not make sense to create an seismic intensity 8 or higher. In addition to instrumental seismic intensity, seismic intensity is also treated as an estimate of damage that can be used as a guide to some extent. If the seismic intensity is observed 5-lower or more on the Japanese seismic intensity scale, it is possible to gauge that some damage may have occurred, and if the seismic intensity is 7, which is the maximum seismic intensity, it is considered the maximum level of response in terms of disaster prevention. Therefore, there is not much advantage in making seismic intensity 8 or more.
Seismic intensity classes treated outside Japan
The seismic intensity levels reported on TV in Japan are based on the seismic intensity levels of the JMA, but overseas, different seismic intensity levels are used. The most representative is the “Mercalli seismic intensity class>/b>”, which is slightly more than the seismic intensity class of the JMA. The following is a guide to the revised Mercalli seismic intensity class, which is not based on measured seismic intensity, but rather is calculated from the damage, and therefore has no mathematical basis. Therefore, it cannot be easily compared with the seismic intensity classes established by the JMA, and this is even more true in Japan, where there are many buildings with high earthquake resistance.
Source: Prepared based on USGS website
The Mercalli seismic intensity class for the 2011 Tohoku earthquake is equivalent to IX (shaking equivalent to seismic intensity 7 on the JMA seismic intensity scale).