What is an earthquake?

An earthquake is a sudden shaking of Earth's surface caused by the movement of rocks deep underneath.
Most quakes are unnoticeable by people on Earth's surface. Thousands of quakes occur every day but are too weak to be felt.

Video courtesy of Security Camera / Anadolu Agency via Getty Images

Earthquakes often happen without warning. Severe quakes can result in property damage, injury, and loss of life. They can cause fires, tsunamis, landslides and avalanches, and volcanic eruptions.

The size of an earthquake is known as its magnitude. The higher the number, the more powerful the earthquake.

Every year, on average:

Scientific instruments detect about 500,000 quakes worldwide.

A magnitude 8 quake happens somewhere on Earth.

About 50,000 people die as a result of earthquakes.

Where earthquakes happen

The lithosphere is the outermost layer of the Earth. It consists of the crust and uppermost mantle. The lithosphere is broken into extremely large slabs called tectonic plates. These plates move around on the molten layer beneath. For example, the North American Plate, which includes most of North America, Greenland, and part of Siberia, is approximately 75,900,000 square kilometers (29,305,000 square miles).

Looking at the map you can see the Earth’s surface divided into seven major plates and many minor plates. As you can see, earthquakes mostly (but not always) occur where these tectonic plates meet—the plate boundaries. Each boundary is made up of faults—fractures in the rock along which movement can take place. Movement of these plates is called seismic activity which is known as an earthquake.

Almost 80 percent of our planet's largest earthquakes occur along the Ring of Fire. This circles the edges of the Pacific Ocean.

Select the i for each plate to find out which direction it moves in and by how much.

Credit: Public Domain

The San Andreas Fault is especially visible from Earth’s surface. It is the boundary between the Pacific Plate and the North American Plate.

What causes earthquakes

We already know that earthquakes occur mostly, but not always, at the boundaries of tectonic plates. Let’s look at the mechanisms behind this.

1

The plates do not always move smoothly past each other at faults.

2

Sometimes plate edges get stuck because of friction but the rest of the plate keeps moving, very slowly.

3

The energy and pressure that move the plates get stored up at the edges.

4

When the energy and pressure build up enough to overcome the friction, the plates move past each other along the fault.

5

The energy and pressure are released in the form of seismic waves that shake the surface as they move through it - like ripples on water.

This sudden movement along the fault can cause the ground to

move forward and backward
heave up and down
shift from side to side -

an earthquake!

Did you know?

Plates move approximately at the speeds at which human fingernails grow.

Intraplate earthquakes

Less than 10 percent of all earthquakes occur far from plate boundaries. These are called intraplate earthquakes. These happen when pressure builds up and the earth’s crust is stretched or squeezed until it rips.

Destructive boundary - Convergent

Fault Type Observed
Thrust, or reverse, faults occur where plates collide and one side of the fault is pushed up and over the other.

What Happens at Plates
An oceanic plate subducts under a continental one. Earthquakes are generated within the subducting plate and where the two plates meet. Large magnitude earthquakes often occur at subduction zones, along with many small-scale ones.
Examples of Plates Involved
Nazca Plate (oceanic) subducts under the South American Plate (continental).

Plate Activity Results
Peru-Chile Trench

Image: A NASA Visible Earth map shows the location of a magnitude 8.0 earthquake off the coast of Peru.
NASA, Map by Robert Simmon based on data from the USGS

Continental collisions - Convergent

Fault Type Observed
Thrust, or reverse, faults occur where plates collide and one side of the fault is pushed up and over the other.

What Happens at Plates
One plate crumples over the other one, instead of being subducted.

Mountain ranges are formed here.
Examples of Plates Involved
Eurasian Plate and Indian Plate

Plate Activity Results
The Himalayas

NASA image by Jeff Schmaltz

Divergent

Fault Type Observed
Normal faults are common. Plate boundaries pull apart and Earth's crust is stretched. One side of the fault slips down.

What Happens at Plates
Plates move apart, and magma slowly rises toward the surface, eventually moving along fractures that appear.
Examples of Plates Involved
The African Plate is splitting.

Plate Activity Results
East African Rift

Astronaut photograph provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center – NASA.

Transform boundary

Fault Type Observed
A strike-slip fault occurs where plates meet and slide against each other horizontally.

What Happens at Plates
Plates move past each other with earthquakes generating close to the surface. Earthquakes are shallow but powerful.
Examples of Plates Involved
Pacific Plate and North American Plate.

Plate Activity Results
San Andreas Fault, California
North Anatolian Fault, Turkey

Image: The San Andreas Fault slashes the desolate Carrizo Plain.
Photograph by James P. Blair

Did you know?

The deadliest earthquake ever recorded occurred in 1556 and struck Shaanxi province, China. It killed an estimated 830,000 people. Afterward, buildings were made from softer materials like bamboo and wood rather than stone.

Predicting earthquakes

No scientists have ever predicted a major earthquake.

According to the United States Geological Survey (USGS), scientists can only calculate the probability that a significant earthquake will occur in a specific area within a certain number of years. For example, a major quake is likely in the San Francisco Bay region before 2030.

Seismologists—scientists who study earthquakes—use a range of equipment, including:

creepmeters to check for movement along faults;
tiltmeters to monitor changes in the slope of the land;
satellites to detect changes in the position of plates

Rather than attempting predictions, scientists focus their efforts on helping to improve the safety of buildings and structures.

Recording and locating earthquakes

Seismologists cannot predict earthquakes but once a quake has happened, they can pinpoint its starting point, or epicenter, with accuracy.

The hypocenter (or focus), the place where the movement first occurred on the fault, can be hundreds of kilometers below the Earth's surface or just beneath it. The epicenter is the point on the surface directly above the hypocenter.

When the quake begins, seismic waves travel rapidly outward in all directions.
Primary waves (P waves) travel at the fastest speeds and spread through the crust from the hypocenter, alternately squeezing and stretching the rock as they travel through it.
Secondary waves (S waves) move slower than P waves and vibrate at right angles to the direction of wave travel. They distort the rock.
Surface waves follow at the slowest speed, causing intensive damage as they move along the Earth's surface.
Seismologists compare the arrival times of P waves and S waves at seismic stations to determine a quake's location.
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The waves are measured on sensitive instruments called seismographs. Older versions are anchored to the ground and have a pen attached to them, so when the Earth moves during a quake the device moves and the pen makes zigzag tracings on paper. Modern versions are all computerized and instantly record activity at the seismic station.

The resulting pattern, or seismogram, reveals to experts:
- where the quake began
- how long it lasted
- how much energy was released
To locate the epicenter of an earthquake, we need to look at seismograms from three different recording stations.
Experts compare the difference in arrival times of the P waves at the different stations. Then they compare the difference in arrival times of the S waves. This enables them to calculate the distance the earthquake is from the seismograph.

Measuring earthquake intensity

Seismologists used to assign magnitudes to earthquakes using the Richter scale.

Recently, scientists have begun to use the more precise moment magnitude scale, which measures the total energy released by a quake.

The moment-magnitude scale is logarithmic, so an increase of one unit means an earthquake is 10 times bigger, with about 30 times the energy produced.

Small earthquakes have about the same value on the Richter scale and the moment magnitude scale, so Richter is fine for those. But, larger earthquakes are better measured as moment magnitude, where Richter becomes less accurate.

All the earthquake magnitudes you will see here use the newer moment magnitude scale.

Moment magnitude	Frequency of Earthquakes per year *estimated	Energy in Joules	How much energy?
9	less than 1	2,000,000,000,000,000,000	The amount of energy used every year in the United Kingdom.
8	1	63,000,000,000,000,000	Almost half the energy a hurricane releases in one day.
7	17	2,000,000,000,000,000	About the same energy contained in two billion candy bars.
6	134	63,000,000,000,000	Around the same energy as 3.3 Hiroshima-sized atomic bombs.
5	1,313	2,000,000,000,000	Around the same energy contained in two trillion candy bars.
4	13,000*	63,000,000,000	Around the same energy as six tons of TNT.
3	130,000*	2,000,000,000	Twice the amount of energy in a lightning bolt.
2	1,300,000*	63,000,000	About the same energy contained in 63 candy bars.
1	millions	2,000,000	About the same energy contained in two candy bars.


Magnitude	9.1	8.8	7.0	6.1
Location	Andaman Islands	Maule, Chile	Haiti	Christchurch, New Zealand
Date	12/26/2004	02/27/2010	01/12/2010	02/22/2010

Moment magnitude 9

Frequency of Earthquakes per year: less than 1

Energy in Joules:
2,000,000,000,000,000,000
The amount of energy used every year in the United Kingdom.

Example:
Magnitude: 9.1
Location: Sumatra-Andaman Islands
Date: 12/26/2004
Moment magnitude 8

Frequency of Earthquakes per year: 1

Energy in Joules:
63,000,000,000,000,000
Almost half the energy a hurricane releases in one day.

Example:
Magnitude: 8.8
Location: Maule, Chile
Date: 02/27/2010
Moment magnitude 7

Frequency of Earthquakes per year: 17

Energy in Joules:
2,000,000,000,000,000
About the same energy contained in two billion candy bars.

Example:
Magnitude: 7.0
Location: Haiti
Date: 01/12/2010
Moment magnitude 6

Frequency of Earthquakes per year: 134

Energy in Joules:
63,000,000,000,000
Around the same energy as 3.3 Hiroshima-sized atomic bombs.

Example:
Magnitude: 6.1
Location: Christchurch, New Zealand
Date: 02/22/2010
Moment magnitude 5

Frequency of Earthquakes per year: 1,313

Energy in Joules:
2,000,000,000,000
Around the same energy contained in two trillion candy bars.
Moment magnitude 4

Frequency of Earthquakes per year: 13,000*
*estimated

Energy in Joules:
63,000,000,000
Around the same energy as 6 tons of TNT.
Moment magnitude 3

Frequency of Earthquakes per year: 130,000*
*estimated

Energy in Joules:
2,000,000,000
Twice the amount of energy in a lightning bolt.
Moment magnitude 2

Frequency of Earthquakes per year: 1,300,000*
*estimated

Energy in Joules:
63,000,000
About the same energy contained in 63 candy bars.
Moment magnitude 1

Frequency of Earthquakes per year: millions

Energy in Joules:
2,000,000
About the same energy contained in two candy bars.

Did you know?

In March 2011, a moment magnitude 9 earthquake in Japan moved the Earth’s axis up to 25 centimeters (10 inches) and moved the main island, Honshū, 2.4 meters (7.9 feet) closer to the United States. The tsunami it triggered claimed approximately 20,000 lives.

What is an earthquake?

Every year, on average:

Where earthquakes happen

What causes earthquakes

Did you know?

Intraplate earthquakes

Faults and boundaries

Did you know?

Predicting earthquakes

Recording and locating earthquakes

Measuring earthquake intensity

Did you know?

Time to create an earthquake