What causes earthquakes? The short answer
An earthquake is caused by a sudden slip on a fault, a crack in Earth's rocky outer shell.1 That shell is broken into giant pieces called tectonic plates, and they're always slowly moving. Their rough edges get stuck against each other because of friction. The rest of each plate keeps moving anyway, so the rock near the fault bends and stores up energy. When the stress finally beats the friction, the fault lurches, and the stored energy races out as seismic waves. Those waves are the shaking we feel.12
plates keep moving → fault stays stuck → rock bends → SNAP → seismic waves
How do earthquakes happen? Earth's cracked shell
Earth has four main layers: a solid inner core, a liquid outer core, a thick, hot mantle and a thin, rocky crust.3 The crust is only about 5 to 50 kilometers thick. Compared with the whole planet, that's like the skin on an apple.9
The crust and the stiff top of the mantle together make up the lithosphere. It isn't one smooth shell. It's cracked into about 15 major plates that fit together like a jigsaw puzzle.34 The plates drift on the mantle below, which is solid but flows very slowly over long stretches of time.34
They move only a few centimeters a year.4 Along California's San Andreas Fault, the Pacific Plate slides past the North American Plate at around 5 centimeters (2 inches) a year, which USGS points out is about as fast as your fingernails grow.1112 It sounds tiny, but over millions of years it adds up to thousands of kilometers.15
Why do the plates move at all? That is a whole story of its own: see what makes tectonic plates move.
Three ways plates meet
Plate boundaries and fault types from the British Geological Survey and USGS.412 Earthquakes happen at all three, but the biggest ones happen where plates collide.12
How are earthquakes formed? Stuck, bend, snap
Here's the key idea. Hold a plastic ruler at both ends and slowly bend it. It stores energy as it bends. Let one end slip and it springs straight again with a twang. Rock near a fault works the same way, only on a gigantic scale.24
- Stuck: the edges of the plates are rough, so the fault locks up.2
- Bend: the plates keep moving, so the rock on each side slowly bends. Where a fault stays locked, strain can build up for hundreds of years.1
- Snap: when the stress is stronger than the friction, the fault slips in seconds. The rock springs back into shape, but now it's shifted along the fault.45
- Shake: the stored energy spreads out as seismic waves, like ripples on a pond.2
This is called elastic rebound theory. Geologist Henry Fielding Reid came up with it after the great San Francisco earthquake of 1906.45 That earthquake ruptured 477 kilometers (296 miles) of the San Andreas Fault.16 Fences built straight across the fault had slowly been bent before it. In the quake they snapped straight again, but in two offset pieces. One fence near Bolinas was offset by about 8.5 feet (2.6 meters).5 It's still the main model scientists use for the earthquake cycle.16
After the main shock, the rock keeps settling, and that makes smaller aftershocks for days, months or even years.2
Other causes of earthquakes
Almost all big earthquakes are this kind of tectonic earthquake.9 Smaller quakes can also come from magma moving under volcanoes, from caves or mines collapsing, and from underground explosions.910 People can trigger quakes too. Pumping large amounts of wastewater deep underground has increased earthquake activity in some regions.9
Focus and epicenter: where an earthquake starts
The spot underground where the fault first slips is the focus, also called the hypocenter. The point on the surface directly above it is the epicenter.210 Earthquakes start anywhere from near the surface down to about 800 kilometers (500 miles) deep, in the crust or upper mantle. Deeper quakes usually cause weaker shaking at the surface.12
Seismic waves: P waves, S waves and surface waves
Wave speeds from Britannica; wave behavior from Caltech and IRIS.789 Because S waves can't travel through liquid, scientists worked out that Earth's outer core must be liquid: S waves never make it through.48
How far away was the earthquake? The P–S trick
P waves arrive first and S waves arrive later, and the farther away the earthquake is, the bigger the gap between them.7 That gap tells you the distance. Using the speeds near the surface (6 and 3.4 km/s), every second of gap works out to roughly 8 kilometers:
distance ≈ P–S gap (seconds) × 8 km
It's a rule of thumb, since real waves speed up as they go deeper. Seismologists use standard travel-time charts, and with three or more stations they can pin down exactly where the quake started.7
How are earthquakes measured?
Earthquakes are recorded by seismographs. A classic seismograph has a base fixed firmly to the ground and a heavy weight that hangs free. When the ground shakes, the base moves but the weight mostly stays still, and the difference between them is drawn as a wiggly line called a seismogram.2 A small quake makes a short, low wiggle. A big one makes a long, tall wiggle.2
Magnitude: why each step is a giant leap
An earthquake's size is its magnitude. Each quake gets one magnitude number, no matter where it's measured from.6 The scale is logarithmic:
- One whole number up = wiggles on the seismogram 10 times taller.6
- One whole number up = about 32 times more energy released.610
M7 vs M5: 32 × 32 ≈ 1,000 × the energy
Put another way, a magnitude 1 releases about as much energy as 6 ounces of TNT, and a magnitude 8 about as much as 6 million tons.10
You've probably heard of the Richter scale, introduced by Charles Richter in 1935. Today it's mostly used for small, local quakes. For everything else, scientists use the moment magnitude scale (Mw), which is based on how big the fault is and how far it slipped, and which stays accurate for the biggest earthquakes.610
Magnitude vs intensity
Magnitude is one number for the whole earthquake. Intensity describes how strong the shaking was at one particular place. It changes from place to place depending on distance and the ground underneath. In the US, intensity is measured on the Modified Mercalli scale, written in Roman numerals such as VI or IX.6
Table from Michigan Technological University.10 In total, about 500,000 earthquakes a year are detected worldwide. About 100,000 of them can be felt, and about 100 cause damage.11
What was the biggest earthquake ever recorded?
A magnitude 9.5 in Chile on May 22, 1960, on a fault almost 1,000 miles long. The largest ever in the United States was a magnitude 9.2 in Alaska in 1964.1112 A magnitude 10 is thought to be impossible: USGS says no known fault is long enough, and one that was would have to stretch around most of the planet.12
Where do earthquakes happen?
Earthquakes can happen almost anywhere, but most happen along plate boundaries.2 The busiest zone by far is the circum-Pacific belt, or "Ring of Fire", around the edge of the Pacific Ocean. About 90% of all earthquakes, and about 81% of the largest ones, happen there.1112 Next comes the Alpide belt, running from Java and Sumatra through the Himalayas and the Mediterranean, with about 17% of the largest quakes. Third is the mid-Atlantic Ridge.12
Common misconceptions
"There's such a thing as earthquake weather." No. Earthquakes happen about equally in hot, cold and rainy weather. The idea goes back to Aristotle, who thought winds trapped in underground caves caused quakes.
12
"California will fall into the sea." It won't. The Pacific and North American plates slide sideways past each other, and the fault is only about 15 km deep. Instead, Los Angeles and San Francisco will slowly creep toward each other and be neighbors in about 15 million years.
111215
"Scientists can predict earthquakes." No one has ever predicted a major earthquake. USGS can only give the chance of a big quake in an area over a number of years.
12
"A magnitude 8 is twice as strong as a 4." It's four steps up. That's 10,000 times taller wiggles and roughly a million times more energy.
6
"Stand in a doorway" or "run outside." Both are outdated advice. Experts agree that the best action in most situations is Drop, Cover and Hold On.
13
What to do during an earthquake: Drop, Cover, Hold On
- Drop onto your hands and knees, right where you are, so the shaking can't knock you over.
- Cover your head and neck with one arm. Get under a sturdy table or desk if there's one close by.
- Hold on until the shaking stops.
Steps from the Earthquake Country Alliance's ShakeOut and USGS.1312 If you're outside, move into the open, away from buildings, trees and power lines. After the quake, expect aftershocks.1214 Earthquakes strike without warning, so practice with your family or class before one happens.1314
FAQ
What causes earthquakes, in simple words?
Earth's outer shell is broken into big pieces called tectonic plates that slowly move. Where two plates meet, their rough edges get stuck while the rest keeps moving, so the rock bends and stores energy. When the stuck spot suddenly slips, the rock snaps back and sends out seismic waves. That shaking is an earthquake.
How do earthquakes happen step by step?
1) Plates move a few centimeters a year. 2) At a fault, friction locks the edges. 3) The rock on either side slowly bends, storing energy, sometimes for hundreds of years. 4) The stress beats the friction and the fault slips in seconds. 5) The stored energy races out as P, S and surface waves that shake the ground.
Where do 90% of the world's earthquakes happen?
Around the rim of the Pacific Ocean, in the circum-Pacific belt or "Ring of Fire". USGS says about 90% of all earthquakes, and about 81% of the largest ones, happen there. The next busiest zone is the Alpide belt, from the Mediterranean through Turkey, Iran and the Himalayas to Sumatra.
How are earthquakes measured?
Seismographs record the ground shaking as a wiggly line called a seismogram. Scientists use recordings from many stations to work out the location and the magnitude, today usually the moment magnitude (Mw). Each whole number up means about 10 times taller wiggles and about 32 times more energy. Shaking at a particular place is described separately as intensity.
What are signs that an earthquake is coming?
Unfortunately there are no reliable ones. USGS says no scientist has ever predicted a major earthquake, and most large quakes have no warning signs at all. Scientists can only say how likely a big quake is in an area over years or decades, which is why being prepared matters.
Can earthquakes be prevented?
Natural earthquakes can't be stopped, because the plates will keep moving. What we can prevent is much of the damage: stronger buildings, securing heavy furniture and knowing to Drop, Cover and Hold On. Some small quakes are caused by people, for example by injecting wastewater deep underground, and those can be reduced.
What should you do during an earthquake?
Drop onto your hands and knees, Cover your head and neck (under a sturdy table if one is close), and Hold On until the shaking stops. Don't run outside or stand in a doorway while the ground is shaking. If you're outdoors, move away from buildings, trees and power lines.
Sources
- USGS, What is an earthquake and what causes them to happen?
- USGS Earthquake Hazards Program, The Science of Earthquakes
- NASA Space Place, What Is an Earthquake?
- British Geological Survey, What causes earthquakes?
- USGS, Reid's Elastic Rebound Theory
- USGS, Earthquake Magnitude, Energy Release, and Shaking Intensity
- IRIS / EarthScope, How are earthquakes located?
- Encyclopaedia Britannica, Seismic wave
- Caltech Science Exchange, What Happens During an Earthquake?
- Michigan Technological University, How Are Earthquakes Measured?
- USGS, Cool Earthquake Facts
- USGS FAQs: Where do earthquakes occur?; Is there earthquake weather?; Will California fall into the ocean?; Can you predict earthquakes?; Can megaquakes happen?; What should I do during an earthquake?
- Earthquake Country Alliance / Great ShakeOut, Drop, Cover, and Hold On
- Ready.gov, Earthquakes
- USGS, This Dynamic Earth: Understanding plate motions
- USGS, The Great 1906 San Francisco Earthquake