Earthquake

San Francisco, California earthquake, sometime in 1906

An earthquake is the sudden movement or trembling of the Earth's tectonic plates, that creates the shakes of the ground. This shaking can destroy buildings and break the Earth's surface. A sudden shake underwater is called a tsunami.

Earthquakes are caused by disturbances in the balance of the earth. Earthquakes are also caused by reactions that result from events that can't be seen operating beneath the Earth's crust..An earthquake's first point of rupture is called its focus. The epicenter is the point at ground level directly above the hypocenter.

People who study earthquakes are called seismologists. There can be many earthquakes over a short period of time in a small area. The sudden release of tension in the tectonic plates sends waves of energy that travel through the Earth. Seismology studies the cause, repeats, type, and size of earthquakes.

Earthquakes are measured using watching from seismographs. The magnitude of an earthquake and the power of shaking is usually reported on the Richter scale. The Richter Scale was invented by Charles Francis Richter in 1935. On the scale, 2 is scarcely noticeable, and magnitude 5 (or more) creates damage over a wide area.

An earthquake under the ocean can destruction as the earthquake itself in mountainous areas. Earthquakes can also create landslides. Earthquakes are part of the Earth's natural rock cycle. The affect of an earthquake can be measured by a seismometer. It detects the shakes caused and it puts these movements on a seismograph. The strength, or magnitude, of an earthquake, is measured using the Richter scale. The Richter scale is numbered 0-10. The largest earthquake ever measured was a 9.5. An earthquake of 10 has never been recorded.

Scientists cannot predict an earthquake before it happens. But we do know where earthquakes might happen in the future, like close to fault lines. An earthquake under the ocean can create a tsunami, this can create just as much destruction as the earthquake itself.

Zones

Earthquakes zones are divided into some parts of the world. The first is the Pacific belt. This part is the biggest part of seismic belt which means the largest activities of earthquakes caused by the movement of the earth's lithosphere as well as the volcanoes. The zone shape the most volcanoes located on the nations around the pacific ocean.

The second part is the alpide. It is the seismic activities from Sumatra, Himalayan, europan to outer atlantic.

History

Earthquakes sometimes hit cities and kill hundreds or thousands of people. Most earthquakes happen along the Pacific Ring of Fire but the biggest ones mostly happen in other places. Tectonically active places are places where earthquakes or volcanic eruptions are quite repeated.

Causes of an earthquake

Main article: plate tectonics

Earthquakes are caused by tectonic movements in the Earth's crust. The main cause is when tectonic plates ride one over the other, causing orogeny (mountain building), and severe earthquakes.

The boundaries between moving plates form the largest fault surfaces on Earth. When they stick, motion between the plates leads to increasing stress. This continues until the stress rises and breaks, suddenly allowing sliding over the locked portion of the fault. This releases the stored energy as shock waves. The San Andreas fault in San Francisco, and Rift valley fault in Africa are faults like this. 1. Volcanic Earthquakes: Earthquakes which are caused by volcanic eruptions are quite devastating. However, these are confined to areas of active volcanoes. 2. Collapse Earthquakes: In areas of intense mining activity, often the roofs of underground mines collapse and minor tremors take place. These are called collapse earthquakes.

Earthquake fault types

Model of ancient seismometer with pendulum sensitive to ground tremors. In Luoyang in 133 AD, it detected an earthquake 400 to 500 km (250 to 310 mi) away

There are three main types of geological fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal faults occur mainly in areas where the crust is being extended. Reverse faults occur in areas where the crust is being shortened. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other.

Earthquake clusters

Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors which cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.

A foreshock is an earthquake that happens before a larger earthquake, called the mainshock.

An aftershock is an earthquake that happens after a previous earthquake, the mainshock. An aftershock is in the same place of the main shock but always of a smaller magnitude. Aftershocks are formed as the crust adjusts to the effects of the main shock.

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 sequence is obviously the main shock, 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 happen 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 happen over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern happened in the North Anatolian fault in Turkey in the 20th century.

Tsunami

Main article: Tsunami

Play media

Animation of the 2011 Sendai tsunami.

A Tsunami or a chain of fast moving waves in the ocean caused by powerful earthquakes is a very serious challenge for people's safety and for earthquake engineering. Those waves can inundate coastal areas, destroy houses and even swipe away whole towns. This is a danger for the whole mankind.

Unfortunately, tsunamis cannot be prevented. However, there are warning systems which may warn the population before the big waves reach the land to let them enough time to rush to safety.

Earthquake-proofing

Main article: Earthquake engineering

Earthquake-proof buildings are constructed to withstand the destructive force of an earthquake. This depends upon its type of construction, shape, mass distribution, and rigidity. Different combinations are used. Square, rectangular, and shell-shaped buildings can withstand earthquakes better than skyscrapers. To reduce stress, a building's ground floor can be supported by extremely rigid, empty columns, while the rest of the building is supported by flexible columns inside the empty columns. Another method is to use rollers or rubber pads to separate the base columns from the ground, allowing the columns to shake parallel to each other during an earthquake.

To help prevent a roof from collapsing, builders make the roof out of light-weight materials. Outdoor walls are made with stronger and more reinforced materials such as steel or reinforced concrete. During an earthquake, flexible windows may help hold the windows together so they don’t break.

Other websites

• PBS NewsHour - Predicting Earthquakes
• USGS – Largest earthquakes in the world since 1900 Archived 2009-03-03 at the Wayback Machine
• The Destruction of Earthquakes Archived 2009-02-08 at the Wayback Machine - a list of the worst earthquakes ever recorded
• Recent Quakes WorldWide Archived 2017-09-01 at the Wayback Machine
• Earthquake Citizendium
• Real Time Seismicity European-Mediterranean Seismological Centre (EMSC)