About Earthquakes

What are Earhquakes?

Stress in the earth's crust can cause solid rock to deform elastically until it suddenly fractures and is displaced along the fracture , producing a fault. The faulting or a later abrupt movement on an existing fault causes an earthquake.

Features of an Earthquake

An earthquake has certain features and effects (below).

When the stressed parts of the earth suddenly fracture or shift, energy is released as shock waves, which move outward from the earthquake's focus like ripples in a pool of water. The focus of an earthquake is the point of initial movement, and the epicenter is the point on the surface directly above the focus. The epicenter may not be on the fault, either because the fault does not reach the surface or because the fault is not vertical (and, therefore it surfaces some distance away from the epicenter).

Measuring Earthquakes

One way of measuring the severity of an earthquake is by it's magnitude on a modified version of the Richter scale.The magnitude is a measure of the amount of energy released in the earthquake, as indicated by the amplitude (size) of the vibrations when they reach a recording instrument known as a seismograph (below).

Using this approach, seismologists rate earthquakes as insignificant (less than 4 on the Richter scale), minor (4.0 - 4.9), damaging (5.0 - 5.9), destructive (6.0 - 6.9), major (7.0 - 7.9), and great (over 8.0). Each unit on the Richter scale represents an amplitude that is 10 times greater than the next smaller unit, so a magnitude 6.0 earthquake is 10 times greater than a magnitude 5.0, and a magnitude 7.0 earthquake is 100 times greater than a magnitude 5.0 earthquake.

Earthquakes often have aftershocks that gradually decrease in frequency over a period of up to several months, and some have foreshocks from seconds to weeks before the main shock. The primary effects of earthquakes include shaking and,sometimes, a permanent vertical or horizontal displacement of the ground. These effects may have serious consequences for people and for buildings, bridges, freeway overpasses, dams, and pipelines. Secondary effects of earthquakes include various types of mass wasting (such as rockfalls and rockslides), urban fires, and flooding due to to subsidence of land. Coastal areas can also be severely damaged by large, earthquakes-generated water waves, called tsunamis (pronounced sue-na-me) that travel as fast as 950 kilometers (590 miles) per hour (below). Tsunamis occur when there is a sudden slump in the ocean bed during an earthquake and it forms a trough in the water surface subsequently followed by a crest and smaller waves. A more marked change of level in the sea bed can form a crest, the start of a tsunami which can have waves up to 60 meters high.

World distribution of Earthquakes

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