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Why do they happen?

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When Eartquake events occure, they reek havoc, but why do they occur ?

Earthquakes by Andy Higgs (University of Wolverhampton)

Earthquakes, or seismic tremors, occur at a rate of several hundred per day around the world. A worldwide network of seismographs (machines that record movements of the Earth) detects about 1 million small earthquakes per year. (Encarta)

 Very large earthquakes, such as the 1964 Alaskan earthquake, which caused millions of dollars in damage, occur worldwide once every few years. (Encarta)

 Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California, and the 1995 tremor in Kbe, Japan, occur about 20 times a year. Moderate earthquakes also cause millions of dollars in damage and can harm many people.

Earthquakes occur along a variety of faults, often miles underground and under the ocean, it is along this boundaries of friction and energy that these events occur.  The faults, or fractures, 'rub' together to create enormous amounts of energy, which fuel the movement of the earths crust.

 

There are a variety of fault movements that can occur ;

These fault movements are known as the following -

strike-slip,normal, and thrust faults

            Source- http://earthquake.usgs.gov/image_glossary/fault.html

The type or form of movement can often dictate the stength of scale of the eartquake.  Thrust movement often tends to create the most destructive energy and consequently the most intesnive earthquake. 

 

 

 

 

Seismographs and seismic waves -
 
Sensitive seismographs are the principal tool of scientists who study earthquakes. Thousands of seismograph stations are in operation throughout the world, and instruments have been transported to the Moon, Mars, and Venus. Fundamentally, a seismograph is a simple pendulum. When the ground shakes and plate tectonics emit seismic waves, the base and frame of the instrument move with it, but intertia keeps the pendulum bob in place. It will then appear to move, relative to the shaking ground. As it moves it records the pendulum displacements as they change with time, tracing out a record called a seismogram - Earthquake hazards Program.
 
 

seismograph.gif

A basic Seismograph

Be a seismologist!

Seismologists hold vital knowledge to predicting and relating earthquake events. Although simple in design, a basic seismograph can deliver vitsl information.  Seismic waves can be recorded using electronic methods in observationaries.

The Richter Scale ;
 
Seismologists use a magnitude scale to express the seismic energy released by each earthquake.  The values recorded are known as the Richter Scale.  Here are the typical effects of earthquakes in various magnitude ranges;

Richter         Earthquake
Magnitudes      Effects

Less than 3.5   Generally not felt, but recorded.

3.5-5.4         Often felt, but rarely causes damage.

Under 6.0       At most slight damage to well-designed buildings.
   Can cause major damage to poorly constructed buildings
  over small regions.

6.1-6.9         Can be destructive in areas up to about 100 kilometers
across where people live.

7.0-7.9         Major earthquake. Can cause serious damage over larger areas.

8 or greater    Great earthquake. Can cause serious damage in areas several
hundred kilometers

The average earthquake that we experience on a daily basis bears no real time effects on our day to day to day lives.