「マグニチュード」と「震度」

It came to me to write this after today’s really bad earthquake and tsunami up north. Things in Iwate and Miyagi are especially bad with cars seen with half their body in the sea and ships being brought to shore floating alongside washed away houses. According to the news, the 震度 was 7 and マグニチュード (magnitude) 8.8 or M8.8.

This reminded me of a lesson some time ago when I was still at Yamasa. We were reading a news article on earthquake and our teacher brought up the point on the difference between 震度 and マグニチュード. I used to think they were the same so for those who think like I did, here’s some little GK about it.

マグニチュード or magnitude, is the actual size of the earthquake measuring how far it spread based on the Richter scale. So, if the magnitude of the earthquake is 8.8, it is 8.8 and nothing else. However, 震度 is the actual movement felt in a particular area. Like I said, the 震度 was around 7 in Miyagi prefecture but it was around 3 in Aichi prefecture. If this is not clear enough, I found a very good analogy of the difference online.

If we liken an earthquake to a light bulb, the actual brightness of the bulb is the magnitude, but how bright each individual experiences depends on their location and proximity to the bulb. The varying brightness at different locations is the 震度 as shown in the illustration below.

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