How Do Earthquakes Happen?

What You'll Learn

• Earth's outer shell is made of giant puzzle pieces called tectonic plates
• These plates are always slowly moving — about as fast as your fingernails grow
• When plates push together, pull apart, or slide past each other, pressure builds up
• When the pressure releases, energy rushes out as waves that shake the ground — that's an earthquake
• Most earthquakes are too small to feel; only big ones cause damage
• Scientists measure earthquakes with a tool called a seismograph

Words to Know

Tectonic plates

Giant pieces of Earth's outer shell that fit together like a puzzle

"Earth has about 15 major tectonic plates."

Earthquake

A sudden shaking of the ground when energy is released from moving plates

"The earthquake made the table wobble!"

Fault line

A crack in the ground where two plates meet and push against each other

"The San Andreas Fault is a famous fault line."

Seismograph

A tool that measures how strong an earthquake is

"The seismograph drew big wiggly lines!"

Epicenter

The spot on the surface right above where an earthquake starts

"The epicenter is where the shaking is strongest."

Check What You Learned

Fun Facts

• Earth's outer shell (the crust) is broken into giant pieces called tectonic plates

  Source: USGS / National Geographic

• There are about 15 major tectonic plates on Earth

  Source: USGS

• Tectonic plates move at about the same speed your fingernails grow — 1 to 5 inches per year

  Source: USGS

• When plates push together, pull apart, or slide past each other, pressure builds up over time

  Source: USGS

• An earthquake happens when built-up pressure is suddenly released, sending energy waves through the ground

  Source: USGS

• Most earthquakes are too small to feel — there are over 500,000 detectable earthquakes every year, but only about 100 cause damage

  Source: USGS