An earthquake is the effect of seismic waves, which create vibrations of the Earth’s crust, and can be manmade or the result of natural phenomena. Most naturally occurring earthquakes are caused by tectonic plates shifting. The plates rub against each other, collide, or slide under each other. These movements release energy. Part of the energy released is seismic waves. The energy also heats the area near the fault line between the plates and breaks up rock. There are actually several different types of seismic waves – surface waves and two types of body waves. The body waves are primary and secondary waves. Surface waves are body waves that have reached the surface. They cause the most damage because they have the most intense vibrations. Although most earthquakes are caused by the movement of tectonic plates, there are also other causes including underground explosions, structures collapsing, the impact of meteors, and volcanic eruptions.
Many earthquakes occur every year; however, most of them are not powerful enough to cause any serious damage. In fact, many earthquakes are too slight to even be felt. Major earthquakes do occur however and cause terrible damage. Because most earthquakes are not major, many people do not realize exactly how frequently earthquakes occur. More than 3 million earthquakes occur each year, which means approximately 8,000 earthquakes each day.
Most earthquakes occur in specific regions. This is because they usually happen along the borders of tectonic plates. That is why so many earthquakes happen in California – there is a major fault line near the coast of California.
Even though major earthquakes do not happen that often, they can cause substantial damage. One reason why earthquakes cause so much damage is that they can trigger other natural disasters, such as tsunamis and volcanoes, which are also linked to tectonic plates. Additionally, earthquakes can also trigger fires when gas or electrical lines are damaged and floods when dams break. Some famous earthquakes that resulted in terrible damage to property and life include the 1906 San Francisco Earthquake and the earthquake that destroyed Pompeii in 62 AD.
Scientists measure earthquakes using a seismometer, which is an instrument that has been around for centuries. There is also a method of measuring the intensity of an earthquake. It is known as the Richter scale. The scale starts at one and goes up in intensity. Although there is no upper limit to the scale, most people set ten as the upper limit because no earthquakes equal to or greater than ten have been recorded.
Universe Today has articles on famous earthquakes and the Richter scale.
Astronomy Cast has an episode on Earth you will definitely enjoy.