How earthquakes are graded

Updated on Mar 14, 2003 11:57 AM IST

Earthquakes are measured by seismographs but according to scientists, they are also graded from I to XII in terms of intensity and impact as felt by humans.

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Earthquakes are measured by seismographs but according to scientists, they are also graded from I to XII in terms of intensity and impact as felt by humans.

A Grade I quake is not felt except by a very few people under specially favourable circumstances.

A Grade II quake is felt only by those who live on top floors of multi-storeyed buildings. Ceiling fans and chandeliers may swing.
 
A Grade III quake is quite noticeable indoors, specially on the upper floors but many people do not recognise it as a quake. It can slightly rock stationary vehicles.

Many people tend to feel the tremors indoors in a Grade IV quake. If it occurs at night, some people wake up. Dishes and windows swing violently. Walls make a creaking sound. Also, stationary automobiles rock noticeably.

Nearly everyone feels a Grade V tremor. Many wake up. Wall plaster cracks, dishes and windows break. Light objects overturn and pendulum clocks can stop.

There is no way that one can miss a Grade VI quake. Tremors leave many frightened, forcing them to rush outdoors. Heavy furniture is displaced and at times plaster falls off.

A Grade VII quake does not spare even well-designed buildings. Ordinary structures and those built poorly suffer damage. Motorist can feel the impact while driving.

Specially designed structures are damaged during a Grade VIII quake. Loss is considerable. Poorly-built structures collapse and panel walls are thrown out. Heavy furniture overturns.

A Grade IX quake causes a lot of damage to specially designed structures. In some cases, buildings are shifted off foundations. Ground cracks up and underground pipes break.

When a Grade X quake strikes, foundations of buildings are ripped off. The ground cracks badly and landslides are caused.
 
If a Grade XI quake hits, a few, if any, masonry structures remain standing. Bridges are destroyed, broad fissures appear on the ground and underground pipelines are rendered useless. Railway lines are twisted out of shape.

A Grade XII quake leaves nothing untouched. It's total devastation. Ground leaps in waves, lines of sight and levels are distorted and objects thrown high up in the air.

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