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Seismic intensity and magnitude

The terms “seismic intensity” and “magnitude” are used in the TV news and newspaper, when the earthquake occurs.

The seismic intensity is the intensity index of the quake at each observation point, which is shown by ten scales and is called as the seismic intensity scale.

For the feature of the quake of each level, see Table.

By contrast, magnitude characterizes the size of earthquake by measuring the energy released.

If the seismic intensity is different at the observation points, the magnitude is the same everywhere.

Under the same magnitude, the seismic intensity is smaller than that near the epicenter.

In general, the earthquakes of magnitude 3 to 5, 5 to 7, and more than 7 are named as small, middle and big earthquake. Specially, the earthquake more than magnitude 8 is called as mega earthquake.

If the magnitude becomes 0.2 times bigger, twice energy is released.

Seismic
intensity 		Feature of the quake
1 ・People can not feel the quake.
2 ・Only some people keeping still can feel the quake.
3 ・Most people in the building feel the quake.
・Some people awake.
・Hanging objects such as lamp slightly swing.
4 ・Most people awake.
・Hanging objects such as lamp considerably swing.
・Unstable ornaments fall occasionally.
5-
lower 		・Some people find it difficult to move.
・Most unstable ornaments fall.
・Dishes in a cupboard and books on a bookshelf fall.
・Furniture moves.
・Windowpanes are broken occasionally.
5-
upper 		・Many people find it difficult to move.
・Most dishes in a cupboard and most books on a bookshelf fall.
・Furniture fall occasionally.
6-
lower 		・People find it difficult to keep standing.
・A lot of unfixed furniture moves and falls.
・Less earthquake-resistant houses collapse occasionally.
6-
upper 		・People can not keep standing and move without crawling.
・Most unfixed furniture moves and falls.
・Many, less earthquake-resistant houses collapse.
7 ・People can not move at will.
・Most furniture moves to a large extent and some jumps up.
・Highly earthquake-resistant buildings collapse occasionally.
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