Earthquakes

There are as many as a million earthquakes in a single year. Most of them take place underneath the sea. It's hard to believe but few of them cause any damage. But if an earthquake happens near a large city with soft ground, it is going to do lots of damage. An earthquake's energy may be 10,000 times greater than the first atomic bomb. The strength of an earthquake is measured on a scale of numbers called the Richter Scale. The Richter Scale has numbers that measure the strength of the earthquake.

Large earthquakes, called Tsunamis, cause waves that sweep up on land and cause even greater destruction.
According to the plate tectonics theory, the earth consists of about 20 rigid plates. Earthquakes occur when these plates shift and move slowly past one another. This movement stretches and squeezes the plates. If the force becomes too great, the rocks rupture or break, and shift, causing an earthquake. Most earthquakes occur near plate boundaries.
Faults are breaks in the earth either above land or in water.  Most faults are not able to seen. But some, like the San Andreas Fault in California, are visible.
Near the focus, vibrations of Seismic waves can be destructive. When an earthquake occurs, waves of energy move outward from the focus in all directions The focus is the place where the rupture begins. Seismic waves are shear waves which shake buildings vertically.
Building which can withstand earthquakes
People have created buildings that can withstand earthquakes. Above is an example of a building which was built to withstand an earthquake. There is a building in San Francisco, California called the Transamerica Building which was built to withstand earthquakes.  It has a special platform that allows the building to move with the vibrations of the earth if an earthquake hits. When an earthquake occurs, waves of energy move outward from the focus in all directions.
Compressional waves are sound waves, and travel at a speed of 5 miles a second!! They move horizontally, which is the opposite of Seismic waves.
The deadliest earthquake was recorded in 526 A.D.  It occured in what is now Turkey and killed 250,000 people. Another large earthquake was in Northern China in 1976. It killed 240,000 people!!
Earthquakes occur in two great belts- the Circum-Pacific Belt and the Alphide Belt. Each dot on this map represents 5 earthquakes during 9 years. The Circum- Pacific Belt accounts for more than three-fourths of the earth's earthquakes. Most of the other earthquakes occur in the Alphide Belt, which cuts across Europe and Asia.

The map below showing the red and black lines are where earthquakes occur.  These are where the fault lines are located.

Earthquake Experiment