The San Andreas fault in California is very distinct in the Carrizo Plain east of the city of San Luis Obispo, CA. Many faults can not be seen at the Earth's surface like this.
Click on image for full size

Why Do Earthquakes Happen?

When the giant blocks of rock which, because of plate tectonics, move in different directions, they are bound to bump into each other. These blocks of rock come in contact at faults. Sometimes they slide smoothly past each other along a fault. But other times the blocks of rock get stuck - the rough surfaces of rock snag, preventing movement along the fault. That might lead to an earthquake.

There might be no movement along a fault for a long time if the blocks of rock are hitched together. However, plate tectonic force continues to push the rocks so the energy continues to grow. The energy builds over decades, centuries, and sometimes even over millennia.

Eventually the energy is released as an earthquake when the force is large enough. The rock breaks, often very deep underground, and moves into a new position. Vibrations called seismic waves travel outward in all directions from the point where the energy was released, known as the focus. Like a stone tossed into a pond that sends concentric circles of ripples outward, the seismic waves radiate from the focus of the earthquake. These seismic waves are what people on the surface of the Earth feel when they are in an earthquake.

There are different types of seismic waves. Some rumble the ground surface for hundreds or even more than a thousand miles. Other types of seismic waves travel through the planet. While people in Cuba can't feel an earthquake that shakes Japan, instruments called seismographs can record the seismic waves that have traveled through the planet.

Sometimes small earthquakes are caused when fluids are pumped underground.

Last modified April 29, 2016 by Jennifer Bergman.

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Plate Tectonics

Many forces cause the surface of the Earth to change over time. However, the largest force that changes our planet's surface is the movement of Earth's outer layer through the process of plate tectonics....more

What Is an Earthquake?

The expression "on solid ground" is often used to describe something as stable. But sometimes the solid ground underfoot is not stable. It moves as Earth's tectonic plates move. Sometimes it moves gradually....more

Seismic Waves: Moving and Shaking During an Earthquake

During an earthquake, energy is released in waves that travel from the earthquake's focus or point of origin, in the form of seismic waves. The seismic waves radiate from the focus like ripples on the...more

Haiti Earthquake January 2010

A major earthquake causing widespread devestation and extensive loss of life struck the nation of Haiti on January 12, 2010. The earthquake had a magnitude of 7.0. Haiti is on the island of Hispaniola...more

How Do Plates Move?

Earthís center, or core, is very hot, about 9000 degrees F. This heat causes molten rock deep within the mantle layer to move. Warm material rises, cools, and eventually sinks down. As the cool material...more

The Great San Francisco Earthquake of 1906

At 5:12 am on Wednesday April 18, 1906 most people in San Francisco, CA were still asleep. But they were about to wake up very suddenly. The Earth shook violently - an earthquake. It lasted for only about...more

Whatís That Mineral?

Each type of mineral is made of a unique group of elements that are arranged in a unique pattern. However, to identify minerals you donít need to look at the elements with sophisticated chemical tests....more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA