What's the Japanese “Seismic Intensity” (shindo) scale for measuring earthquakes, and why does it matter?
Like with many other things, Japan likes to be a little different when it comes to measuring earthquakes.
Switch on the TV after a tremor and, almost immediately, you will be greeted with an image like this:
If you’re not used to it, you may see this image and be a little confused…
Essentially, the image tells you four important things:
- The Epicentre (震源 shingen): This is marked with the big X. The text on top of the screen tells you the epicentre of the quake is in the western part of Kanagawa Prefecture (神奈川県西部).
- The Depth of the earthquake (深さ fukasa): The image tells you the earthquake occurred 10km (10キロ) below the surface of the earth. Generally speaking, shallower quakes tend to be more damaging.
- The Magnitude reading（マグニチュード): The Magnitude reading of this quake is 4.9.
- The Seismic Intensity (震度 shindo): The image shows several numbers in different colours (5+, 3, 2, 1) spread across the map, the highest being 5+. These are the seismic intensity, or shindo, measurements which vary according to your location.
So what’s the difference between Magnitude and Seismic Intensity (shindo)?
- Magnitude (a.k.a. the Richter Magnitude scale) measures the amount of energy released at the earthquake’s epicenter. The scale can go up to as high as 9.5 (this was the largest ever recorded in Chile in 1960). Magnitude is used internationally.
- Seismic Intensity (a.k.a. shindo) measures the degree of shaking at a certain point on the earth’s surface as a number. The bigger the number, the bigger the tremor. This scale is used only in Japan and Taiwan.
This means that an earthquake will have only 1 Magnitude reading, but will have several shindo measurements.
The degree of shaking will vary depending on how far away from the epicenter the measurement is taken. It will also depend on the depth of the earthquake. Generally speaking, the further away you are from the epicentre, the lower the “shindo” reading.
And this is why when TV reports show live earthquake information in Japan, there are different numbers shown on the map. These numbers represent the different degrees of shaking in different areas.
To provide another real-life example:
- The Great East Japan Earthquake on 11th March 2011 measured 9.0 on the Magnitude scale.
- The seismic intensity in Miyagi Prefecture, which was nearest the epicenter, was 7 (the highest number possible).
- The seismic intensity in Tokyo however, was 5+, because the capital is hundreds of kilometers away from the epicenter.
The degree of shaking also depends on the depth of the epicenter.
Generally speaking, the shallower the epicenter, the stronger the shaking. So two Magnitude-8.0 earthquakes in the same area may have different shindo readings depending on the epicenter depth.
For more about disasters in Japan, see “Fukushima’s Nuclear Exclusion Zone: 7 Years On” or “Japan Floods 2018: How To Help” or “Hokkaido Earthquake 2018”. You can also see the Explore Tohoku project which follows my 600km walk along Japan’s tsunami-affected coastline to document the region’s recovery, 6 years on from the 2011 Great East Japan Earthquake.