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 one Magnitude reading, but will have several shindo measurements, because the degree of shaking will vary depending on how far away from the epicenter the measurement is taken .
And this is why when TV reports show live earthquake information, there are different numbers shown on the map. These numbers represent the different degrees of shaking in different areas.
To provide one real-life example:
– The Great East Japan Earthquake on 11 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.
The Japanese shindo scale has 10 levels going from 0 to 7 (5 and 6 are divided into ‘weak’ and ‘strong’).
Here is a useful graphic to illustrate the 10 different levels.
For more information on the seismic intensity scale, see this page by the Japan Meteorological Agency.