Learn how vibrations that shake the ground when the earth is relieving stresses and pressures beneath its surface becomes an earthquake.
Throughout history the earth has moved relieving stresses and pressures beneath its surface. The vibrations that shake the ground when this happens are what we know as an earthquake. With so many areas of the earth populated today, this can have a devastating effect on people and their property. Hundreds of people died in 1906 when the city of San Francisco was destroyed by an earthquake. Although today science has made it possible to measure the magnitude of earthquakes, unfortunately there is no way to predict when they will strike.
Earthquakes are caused when the plates of the earth push against each other building pressure. When the force becomes too great, the rock snaps as the plates jump to their new positions. Plates can remain in one place for years or even centuries until the stress builds again. As the breaking point arrives it sends vibrations through the earth's crust that spread from the point of the break in a manner similar to ripples on a lake after a splash. The vibrations shake the earth's surface causing either minute tremors or wide spread destruction. The focus point, or point from which an earthquake occurs, is usually in the area of a deep fault. This is the area on the earth's surface where most of the damage occurs. As you move away from the focus point or epicenter there will be less and less damage.
The shockwaves produced by an earthquake can vary. P waves produce a back and forth pushing motion, while S waves produce a side to side shaking. L waves are the long waves traveling along the surface that cause actual damage. Seismographs are instruments that pick up these waves and help scientist calculate the strength of an earthquake, as well as the actual location where it occurred.
The Mercalli scale is a twelve point scale used to measure the intensity of an earthquake. A one on this scale can only be felt by instruments. A two will cause light bulbs to swing and can be felt in an upstairs area. A three is when the tremors can be felt at any location indoors. A four will rattle windows like a truck hitting the building and can be felt both inside and out. A five cracks plaster and wakes you if you are sleeping. A six will break windows or dishes and cause plaster to fall. A seven makes it difficult to stand and will cause loose bricks to fall. An eight will make it difficult to steer your car and bring down chimneys. A nine cracks the ground and causes buildings to collapse. A ten smashes buildings or houses and creates landslides. An eleven bends bridges and railway lines while destroying underground pipes. A twelve is registered when objects are thrown into the air and everything is destroyed. Of course, using this scale will depend on your location in relationship to the epicenter of the earthquake.