Earthquakes

An earthquake is a weak to violent shaking of the ground produced by the sudden movement of rock materials below the earth’s surface.

There are two ways by which we can measure the strength of an earthquake: magnitude and intensity. Magnitude is proportional to the energy released by an earthquake at the focus. It is calculated from earthquakes recorded by an instrument called seismograph.
Intensity on the other hand is the strength of an earthquake as perceived and felt by people in a certain locality. It is a numerical rating based on the relative effects to people, objects, environment and structures in the surrounding. The intensity is generally higher near the epicenter.
We should educate ourselves to avoid confusion.

Intensity I

Shaking: Scarcely Perceptible
Description: Perceptible to people under favorable circumstances. Delicately balanced objects are disturbed slightly. Still Water in containers oscillates slowly.

Intensity II

Shaking: Slightly Felt
Description: Felt by few individuals at rest indoors. Hanging objects swing slightly. Still Water in containers oscillates noticeably.

Intensity III

Shaking: Weak
Description: Felt by many people indoors especially in upper floors of buildings. Vibration is felt like one passing of a light truck. Dizziness and nausea are experienced by some people. Hanging objects swing moderately. Still water in containers oscillates moderately.

Intensity IV

Shaking: Moderately Strong
Description: Felt generally by people indoors and by some people outdoors. Light sleepers are awakened. Vibration is felt like a passing of heavy truck. Hanging objects swing considerably. Dinner, plates, glasses, windows and doors rattle. Floors and walls of wood framed buildings creak. Standing motor cars may rock slightly. Liquids in containers are slightly disturbed. Water in containers oscillate strongly. Rumbling sound may sometimes be heard.

Intensity V

Shaking: Strong
Description: Generally felt by most people indoors and outdoors. Many sleeping people are awakened. Some are frightened, some run outdoors. Strong shaking and rocking felt throughout building. Hanging objects swing violently. Dining utensils clatter and clink; some are broken. Small, light and unstable objects may fall or overturn. Liquids spill from filled open containers. Standing vehicles rock noticeably. Shaking of leaves and twigs of trees are noticeable.

Intensity VI

Shaking: Very Strong
Description: Many people are frightened; many run outdoors. Some people lose their balance. motorists feel like driving in flat tires. Heavy objects or furniture move or may be shifted. Small church bells may ring. Wall plaster may crack. Very old or poorly built houses and man-made structures are slightly damaged though well-built structures are not affected. Limited rockfalls and rolling boulders occur in hilly to mountainous areas and escarpments. Trees are noticeably shaken.

Intensity VII

Shaking: Destructive
Description: Most people are frightened and run outdoors. People find it difficult to stand in upper floors. Heavy objects and furniture overturn or topple. Big church bells may ring. Old or poorly-built structures suffer considerably damage. Some well-built structures are slightly damaged. Some cracks may appear on dikes, fish ponds, road surface, or concrete hollow block walls. Limited liquefaction, lateral spreading and landslides are observed. Trees are shaken strongly.

Intensity VIII

Shaking: Very Destructive
Description: People are panicky. People find it difficult to stand even outdoors. Many well-built buildings are considerably damaged. Concrete dikes and foundation of bridges are destroyed by ground settling or toppling. Utility posts, towers and monuments mat tilt or topple. Water and sewer pipes may be bent, twisted or broken. Numerous landslides and rockfalls occur in mountainous and hilly areas. Boulders are thrown out from their positions particularly near the epicenter. Fissures and faults rapture may be observed. Trees are violently shaken. Water splash or stop over dikes or banks of rivers..

Intensity IX

Shaking: Destructive
Description: People are forcibly thrown to ground. Most buildings are totally damaged. bridges and elevated concrete structures are toppled or destroyed. Numerous utility posts, towers and monument are tilted, toppled or broken. Water sewer pipes are bent, twisted or broken. The ground is distorted into undulations. Trees are shaken very violently with some toppled or broken. River water splashes violently on slops over dikes and banks.

Intensity X

Shaking: Completely Devastating
Description: Practically all man-made structures are destroyed. Massive landslides and liquefaction, large scale subsidence and uplifting of land forms and many ground fissures are observed. Changes in river courses and destructive seiches in large lakes occur. Many trees are toppled, broken and uprooted.

Magnitude 2.5 or less

Earthquake Effects: Usually not felt, but can be recorded by seismograph.

Magnitude 2.5 to 5.4

Earthquake Effects: Often felt, but only causes minor damage.

Magnitude 5.5 to 6.0

Earthquake Effects: Slight damage to buildings and other structures.

Magnitude 6.1 to 6.9

Earthquake Effects: May cause a lot of damage in very populated areas.

Magnitude 7.0 to 7.9

Earthquake Effects: Major earthquake. Serious damage.

Magnitude 8.0 or greater

Earthquake Effects: Great earthquake. Can totally destroy communities near the epicenter.