An earthquake is shaking of the surface of the earth, caused by sudden movement in the Earth’s crust. When two large pieces of the earth’s crust suddenly slip, it causes shock waves to shake the surface of the Earth in the form of an earthquake.
Earthquakes are usually quite brief but may repeat. They are the result of a sudden release of energy in the Earth’s crust. This creates seismic waves, which are waves of energy that travel through the Earth. The study of earthquakes is called seismology. Seismology studies the frequency, type, and size of earthquakes over a period of time.
There are large earthquakes and small earthquakes. Large earthquakes can take down buildings and cause death and injury. Earthquakes are measured using observations from seismometers. The magnitude of an earthquake and the intensity of shaking is usually reported on the Richter scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 or more causes damage over a wide area.
An earthquake under the ocean can cause a tsunami. This can cause just as much death and destruction as the earthquake itself. Landslides can happen, too.
Earthquakes usually occur on the edges of large sections of the Earth’s crust called tectonic plates. These plates slowly move over a long period of time. Sometimes the edges, which are called fault lines can get stuck but the plates keep moving. Pressure slowly starts to build up to where the edges are stuck and, once the pressure gets strong enough, the plates will suddenly move to cause an earthquake.
There are three main types of geological fault that may cause an earthquake – normal, reverse (thrust) and strike-slip.
Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors that cause little to no damage, but earthquakes can recur in a regular pattern.
A foreshock is an earthquake that occurs before a larger earthquake, called the mainshock. A foreshock is in the same area of the mainshock but always of a smaller magnitude.
An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. Aftershocks are formed as the crust adjusts to the effects of the mainshock.
Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequences is obviously the mainshock, therefore none have notably higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.
Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern occurred in the North Anatolian fault in Turkey in the 20th century.
Shock waves from an earthquake that travel through the ground are called seismic waves. They are most powerful at the center of the earthquake, but they travel through much of the earth and back to the surface. They move quickly at 20 times the speed of sound.
Scientists use seismic waves to measure how big an earthquake is. They use a device called a seismograph to measure the size of the waves. The size of the waves is called the magnitude.
To tell the strength of an earthquake scientists use a scale called the Moment Magnitude Scale or MMS (it used to be called the Richter Scale). The larger the number on the MMS scale, the larger the earthquake. We usually won’t even notice an earthquake unless it measures at least a 3 on the MMS scale. Here are some examples of what may happen depending on the scale:
The place where the earthquake starts, below the surface of the earth, is called the hypocenter. The place directly above this on the surface is called the epicenter. The earthquake will be the strongest at this point on the surface.
