Earthquake

An earthquake is the shaking of the Earth's surface. It happens when energy is suddenly released in the lithosphere, the rigid outer layer of the planet. This creates seismic waves. People also call an earthquake a quake, tremor, or temblor. Earthquakes vary in intensity. Some are so weak that no one can feel them. Others are strong enough to throw objects and people into the air, damage important buildings, and destroy whole cities.

Earthquakes can happen naturally or be caused by human activities, such as mining, fracking, and nuclear weapons testing. The point where the rupture begins underground is called the hypocenter or focus. The ground directly above it is the epicenter. Most earthquakes are caused by geological faults, but they can also come from volcanism, landslides, and other seismic events. When a large earthquake's epicenter is under the ocean, the seafloor may move enough to cause a tsunami.

Some of the most significant earthquakes in history are the 1976 Tangshan earthquake in China, which caused many deaths, and the 1960 Valdivia earthquake in Chile. The Valdivia earthquake is the largest ever recorded, with a magnitude of 9.5. Earthquakes can cause ground shaking and soil liquefaction, which can lead to serious damage and loss of life.

People work to reduce earthquake risks through prediction, forecasting, and preparation. This includes seismic retrofitting and earthquake engineering, which design buildings to withstand shaking. Earthquakes have also shaped human culture, appearing in myths, religious beliefs, and modern media. Similar events called marsquakes and moonquakes have also been observed.

Terminology

An earthquake is the shaking of the surface of Earth. It happens when energy is suddenly released in the lithosphere. This energy creates seismic waves. Earthquakes may also be called quakes, tremors, or temblors. The word tremor is also used for non-earthquake seismic rumbling.

In its most general sense, an earthquake is any event that creates seismic waves. This can be natural or caused by people. Earthquakes are mostly caused when geological faults break. Other events can cause them too, such as volcanic activity, landslides, mine blasts, fracking, and nuclear tests. The point where an earthquake first breaks is called its hypocenter or focus. The epicenter is the point on the ground directly above the hypocenter. The seismic activity of an area describes how often earthquakes happen there, what kinds they are, and how big they are over a certain time. The seismicity of a place is the average amount of seismic energy released there.

Major examples

Main article: Lists of earthquakes

One of the most devastating earthquakes in recorded history was the 1556 Shaanxi earthquake. It struck Shaanxi, China, on 23 January 1556. Many people died. Afterwards, many more people were lost because they left the area, and because of disease and hunger. Most houses in the area were yaodongs, homes carved out of loess hillsides. Many people died when these homes collapsed. The 1976 Tangshan earthquake was the deadliest earthquake of the 20th century.

The 1960 Chilean earthquake is the largest earthquake ever measured on a seismograph. It reached a magnitude of 9.5 on 22 May 1960. The Good Friday earthquake happened on 27 March 1964. It was centered in Prince William Sound, Alaska, and released about half as much energy. The ten largest recorded earthquakes were all megathrust earthquakes. Only the 2004 Indian Ocean earthquake is also among the deadliest in history. The earthquakes that caused the most deaths were dangerous because they struck near crowded areas or the ocean. In the ocean, they can create tsunamis that destroy communities far away. The areas at greatest risk of heavy loss of life are places where powerful earthquakes are rare. They are also poor regions with weak building rules or rules that are not enforced.

Occurrence

Further information: Fault (geology) and Strike and dip

Main article: Depth of focus (tectonics)

Main article: Volcano tectonic earthquake

Main article: Tidal triggering of earthquakes

Main article: Aftershock

Main article: Induced seismicity

Tectonic earthquakes happen when enough energy builds up in rock and the rock breaks along a fault plane. The two sides of a fault would slide smoothly past each other if the surface had no bumps. Most faults do have bumps, so the rock sticks and then suddenly slips. As the plates keep moving, pressure builds up until it breaks through the bump. The stored energy is released as seismic waves, heat from friction, and cracking rock. This pattern of slow build-up followed by sudden movement is called the elastic-rebound theory. Only about 10 percent or less of an earthquake's total energy is released as seismic waves.

There are three main types of fault: normal, reverse, and strike-slip. The largest earthquakes, called megathrust earthquakes, happen along reverse faults where plates push together. Each step up in strength releases roughly thirty times more energy. Most of the world's earthquakes happen along the Pacific Ring of Fire. Volcanoes can also cause earthquakes, and some are caused by people, such as when fluids are pumped underground. Hundreds of thousands of earthquakes are detected each year, and about a hundred thousand of them can be felt. Small earthquakes happen much more often than large ones.

Measurement and location

Main articles: Seismic magnitude scales and Seismology

Main article: Epicenter

Scientists use several scales to describe how big an earthquake is. The Richter scale began in the 1930s. It is a simple measure of shaking, but it is rarely used today. The surface-wave magnitude was developed in the 1950s. It measures distant earthquakes and works better for large ones. The moment magnitude scale uses the seismic moment. This measures the size of the broken area, how far the fault slipped, and how stiff the rock is. The Japan Meteorological Agency seismic intensity scale, the Medvedev–Sponheuer–Karnik scale, and the Mercalli intensity scale are based on what people observe. Seismic waves travel through the Earth's interior. They can be recorded by seismometers far away.

Earthquakes send out P waves, S waves, and surface waves, including Rayleigh and Love waves. These waves travel through rock at different speeds. P waves are the fastest. S waves and surface waves cause most of the shaking damage. The time gap between when P and S waves arrive helps show how far away the epicenter is. The depth of the hypocenter can be roughly estimated.

Magnitude and intensity are different things. Magnitude is one number for the size of an earthquake at its source. Intensity is how strongly the ground shakes at each place. It depends on the distance from the source and on the rock or soil below. The first scale for measuring earthquake magnitudes was created by Charles Francis Richter in 1935. Later seismic magnitude scales kept the rule that each step up means ten times the shaking and about 32 times the energy. News reports often say "Richter magnitude," but most experts use the moment magnitude scale.

Earthquakes are also grouped by place. The world is divided into 754 Flinn–Engdahl regions. Reports include the magnitude, the time, the geographic coordinates, and the depth of the epicenter. Beno Gutenberg found the Earth's core in 1913 by studying seismograms. In 2016, scientists found that gravity measurements can detect earthquakes right away. They confirmed this using records from the 2011 Tohoku-Oki earthquake.

Effects

Shaking and ground rupture are the main effects of earthquakes. They can badly damage buildings and other solid structures. How much damage happens depends on the earthquake's magnitude, how far away the epicenter is, and the ground underneath. The ground can make the shaking stronger or weaker. Shaking is measured by ground acceleration. Ground rupture is when the ground visibly breaks and shifts along a fault. In big earthquakes, the ground can move by several meters. This is dangerous for large structures like dams, bridges, and nuclear power stations. Sometimes shaking makes wet sand lose its strength and act like a liquid. This is called soil liquefaction. For example, in the 1964 Alaska earthquake, many buildings sank into the ground and fell down.

Main article: Soil liquefaction

How much people and communities are affected depends on how strong the shaking is and on the community itself. Poorer and developing communities often have bigger and longer-lasting problems. People can be hurt or lose their lives. Roads, power, water, and communication systems can be damaged. Hospitals, police, and fire services can be lost. Buildings can collapse or be weakened. The Food and Agriculture Organization of the United Nations estimated the damage from earthquakes between 1991 and 2023 at US$336 billion. Earthquakes can also start fires by damaging electrical power or gas lines. Fires can be hard to stop when water pipes break. In the 1906 San Francisco earthquake, more damage was caused by fire than by the earthquake itself.

Further information: Landslide

A study of 162 earthquakes from 1772 to 2021 found that China had the most landslide deaths. Most of these were caused by the 2008 Sichuan earthquake. Peru, after the 1970 Ancash earthquake, and Pakistan, after the 2005 Kashmir earthquake, came next. China also had the largest area affected by landslides. Canada came next, after the 1988 Saguenay and 1946 Vancouver Island earthquakes.

Further information: Conflagration

Main article: Tsunami

Tsunamis are large sea waves. They happen when a huge amount of water suddenly moves, including when an earthquake occurs at sea. In the open ocean, they can travel 600 to 800 kilometers per hour, depending on how deep the water is. They can flood nearby coasts within minutes. They can reach far shores hours after the earthquake. Floods can also follow if dams are damaged or if landslides block rivers and then collapse.

Further information: Flood

The land below Sarez Lake in Tajikistan could face a huge flood. This could happen if the landslide dam formed by an earthquake, known as the Usoi Dam, fails in a future earthquake. The flood could affect about five million people.

Management

Earthquake prediction is part of seismology, the science of earthquakes. It tries to say the time, place, and magnitude of future earthquakes. Many methods have been tried, but even with much research by seismologists, scientists cannot yet make reliable predictions for a specific day or month. Many people believe that earthquakes are preceded by earthquake weather in the early morning. Earthquake forecasting is often treated differently from prediction. It estimates the chance of strong earthquakes in an area over years or decades. For well-understood faults, scientists can estimate the chance that a section may break during the next few decades.

Earthquake warning systems can give a region notice of an earthquake that has started, before the ground begins to move. This may let people find safe places before they feel the shaking. Earthquake engineering aims to predict how earthquakes will affect buildings, bridges, tunnels, roads, and other structures, and to design them to reduce damage. Existing buildings can be made stronger through seismic retrofitting. Earthquake insurance can help owners with money after losses. Emergency management plans help governments and organizations reduce risks. Artificial intelligence may help check buildings and plan safety steps. The Igor expert system is part of a mobile laboratory used to check buildings in Lisbon, Rhodes, and Naples. People can also secure water heaters and heavy items, learn where utility shutoffs are, and learn what to do when shaking starts. Near large bodies of water, preparation includes the possibility of a tsunami caused by a large earthquake.

In culture

From ancient Greece, in the 5th century BCE, until the 14th century CE, people often thought earthquakes were caused by "air (vapors) in the cavities of the Earth." The Greek philosopher Anaxagoras was one of those who shared this idea. Pliny the Elder called earthquakes "underground thunderstorms." Thales of Miletus (625–547 BCE) is the only known person from that time who believed they were caused by tension between the earth and water.

Many myths also explained earthquakes. In Norse mythology, earthquakes came from the struggle of the god Loki, who was punished after the death of Baldr, the god of beauty and light. In Greek mythology, Poseidon was the god who caused earthquakes. In Japanese mythology, Namazu (鯰) is a giant catfish who makes the ground shake. In Taiwanese folklore, the Tē-gû (地牛) is a giant earth buffalo who does the same thing. The New Testament also mentions earthquakes in Matthew's Gospel, both after the death of Jesus (Matthew 27:51, 54) and at his resurrection (Matthew 28:2).

Today, people often think of earthquakes through stories about big cities that were badly damaged, such as Kobe in 1995 or San Francisco in 1906. In stories, earthquakes usually come suddenly, without warning. Many stories begin with the earthquake and focus on what happens right after, as in Short Walk to Daylight (1972), The Ragged Edge (1968), or Aftershock: Earthquake in New York (1999). Heinrich von Kleist's classic story The Earthquake in Chile describes the damage done to Santiago in 1647. Haruki Murakami's collection of short stories After the Quake shows how the Kobe earthquake of 1995 affected people's lives. The most famous imaginary earthquake is the "Big One," which people imagine might happen someday on California's San Andreas Fault. It appears in Richter 10 (1996), Goodbye California (1977), and San Andreas (2015), among other works.

Outside of Earth

Main article: Quake (natural phenomenon)

Quakes similar to earthquakes have been seen on other worlds, such as Mars and the Moon. These are called marsquakes and moonquakes.

Quakes also happen on stars. Neutron stars are a type of star left behind after a large star explodes as a supernova. One starquake came from a neutron star called SGR 1806−20, which is very far away, about halfway across the Milky Way from Earth. The quake happened long ago. When its light reached Earth in 2004, scientists learned that the quake gave off a huge amount of energy.