The Ground Is Moving! How Earthquakes Happen

One morning in San Francisco in 1906, the whole city shook. Buildings fell. Streets cracked open. People were very scared. What made the ground move like that? An earthquake! The Earth's ground can shake and rumble, and in this lesson you will find out exactly why.

In this lesson, you will: - Learn what causes earthquakes - Find out what tectonic plates are - Discover what scientists who study earthquakes are called - Learn what to do if an earthquake happens near you

The ground feels solid under your feet. But deep under the ground, the Earth is made of giant pieces called tectonic plates. Think of the Earth like a hard-boiled egg with cracks in the shell. Those cracked pieces are like tectonic plates — but they are HUGE, as big as whole continents! These plates move very, very slowly — about as fast as your fingernails grow. Most of the time you cannot feel them moving at all. But sometimes two plates bump into each other or slide past each other. That creates a big shaking called an earthquake!

When tectonic plates suddenly slip or bump, the energy from that movement travels through the ground as waves. These waves are called seismic waves. Seismic waves move like ripples in a pond when you throw a rock. They start at a spot underground called the focus and travel outward in all directions. On the ground above the focus is a spot called the epicenter. That is where the shaking usually feels strongest. During an earthquake you might feel the floor shaking or rolling, hear a deep rumbling sound, see things falling off shelves, and hear buildings creaking. Most earthquakes last only a few seconds to about a minute. But they can feel much longer when you are in the middle of one!

Earthquakes happen most often near the edges of tectonic plates, where plates push against each other or slide past each other. One of the most famous earthquake zones is called the Ring of Fire. It goes all the way around the Pacific Ocean, touching Japan, Alaska, California, and many other places. Some earthquakes are tiny — so small only a seismograph can detect them. Others are very big and can cause a lot of damage. Scientists use a scale called the Richter scale to measure how strong an earthquake is, from a tiny 1.0 to a giant 9.0 or bigger.