しんど【震度】, マグニチュード


■ The 震度 of an earthquake is the intensity of its accompanying ground shaking at a particular location. Usually the 震度 is higher near the epicenter, or 震源地(しんげんち), and lower farther away.

Sometimes translated as "seismic intensity," 震度 is measured in 10 gradations, from 0, at which no vibration can be felt by human beings, through 1, 2, 3, 4, 5弱(じゃく), 5強(きょう), 6弱, 6強, and 7, the last of which entails heavy destruction of buildings and other man-made structures. When an earthquake occurs in Japan, the 震度 at various locations are reported within minutes on television, the radio, and the Internet. A full description of the 震度 levels in Japanese and English can be found here. The 震度 scale does not seem to be used outside Japan.

The Japanese word マグニチュード, which is derived from the English "magnitude," indicates a measure that is used by the Japanese Meteorological Agency. It is abbreviated "M" in the Japanese media, while scientists prefer to have it abbreviated MJ. Like the similar "Richter scale" magnitude, it represents the amount of energy released in an earthquake.

The term "Richter scale" is often used in English to describe the magnitude of an earthquake; however, seismologists refer to several types, each calculated in a different manner but all nevertheless intended to be a measure of the energy released. The scale seismologists prefer today is the called モーメントマグニチュード in Japanese and "moment magnitude" in English. Moment magnitude is abbreviated MW.

Unless it is clear from the context that マグニチュード refers to a different scale than that used by the Meteorological Agency, it is best translated as "magnitude" and at least once qualified as being according to the Meteorological Agency's measurement.

It is important to note that a high マグニチュード--unlike a high 震度--does not necessarily correlate to significant destruction, as the earthquake's focus (震央 しんおう, the place where the initial rupture takes place) may be deep underground or far out to sea.

◇ 最近地震が多発していますが,地震が起きると震度〜,マグニチュード〜といいます。では,この震度とかマグニチュードというのは何なのでしょうか?
震度というものは,各場所での揺れの大きさで,昔は気象庁の人が揺れを感じ震度〜と決めていましたが,地震計が各地に設置されたので,地震計で測られています。震度は0から7まであり,5と6にはそれぞれ5弱,5強,6弱,6強とあるので,震度階は10段階あります(1996年に8段階から10段階になりました)。最近頻発している震度5強の地震ですと,「非常な恐怖を感じる」地震だそうです。
それに対してマグニチュードというものは,地震そのものの大きさ(規模)を表す数値で,言い換えると地震が放出したエネルギーの大きさということもできます。このマグニチュードも昔は,震源から100km離れた場所での震度とされていましたが,都合よく100kmの地点の震度を測れませんし,地盤の状態などで震度も変化してしまうので,現在では数値計算にて求められています。このマグニチュードの値は対数値なので(10を底とする対数です),マグニチュードが1大きくなると地震の持つエネルギーは,およそ30倍になります。
(The above comparison of 震度 and マグニチュード was found here in October 2000.)


This entry was created by Tom Gally, with additional contributions by Jim Lockhart.


Created 2000-10-08. Definition and explanation sharpened up by JL 2000-10-09 and 2000-10-10.


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