Shop Windows to the Universe

Earth Science Rocks! Select one of our four cool NESTA t-shirts from our online store, and express your love of Earth and space science!

What Is an Earthquake?

The expression “on solid ground” is often used to describe something as stable. Usually the solid ground underfoot seems very stable. But sometimes it is not.

"The ground seemed to twist under us like a top while it jerked this way and that, and up and down and every way," wrote a person describing the experience of being in the large 1906 earthquake in San Francisco, CA.

Earthquakes happen as large blocks of the Earth’s crust move suddenly past one another because of the force of plate tectonics. These blocks of the Earth’s crust meet at cracks called faults. Sometimes those pieces do not slide smoothly past one another. There can be friction along the fault – jagged edges that snag the blocks of rock. This makes it difficult for them to move past each other. Sometimes they get stuck together temporarily. When the pieces of rock overcome the snags, energy is released. The release of energy causes shaking at the ground surface.

The location inside the Earth where an earthquake begins is called the focus. The point at the Earth’s surface directly above the focus is called the epicenter. The strongest shaking happens at the epicenter.

Each year, more than a million earthquakes occur worldwide. Most of these are so small that people do not feel the shaking. But some are large enough that people feel them, and a few of those are so large that they cause significant damage.

Earthquakes can cause damage to things like buildings, bridges, and roads. Earthquakes can cause landslides and mudslides, too. If a large earthquake happens under the ocean it can cause a tsunami – a giant ocean wave or series of waves.

Scientists can figure out whether an earthquake is likely to happen in a place by studying plate tectonics, the faults underground, and the history of the area’s earthquakes. However, unlike weather events, earthquakes can not be forecast ahead of time.

Last modified January 19, 2010 by Randy Russell.

Shop Windows to the Universe Science Store!

Print copies of Spring 2011 issue of The Earth Scientist, focusing on modernizing classroom seismology education, are available in our online store. Thanks to IRIS, the issue is also available as a free pdf online at NESTA!

Windows to the Universe Community

News

Opportunities

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

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

Plate Tectonics

The main force that shapes our planet’s surface over long amounts of time is the movement of Earth's outer layer by the process of plate tectonics. This picture shows how the rigid outer layer of the...more

Scientists Return to Haiti to Assess Possibility of Another Major Quake

A team of scientists from the United States was invited to visit Haiti in late January 2010 to look into the cause of the magnitude 7 earthquake that happened there. While there, the geologists will collect...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

Study of Glacial Earthquakes Shakes Up Idea of How Ice Streams Move

New research shows that a 7,000-square-mile region of the Whillians Ice Stream in West Antarctica moves more than two feet twice every day in an earthquake-like pattern equal to a Magnitude 7 earthquake....more

How Do Plates Move?

Plates at our planet’s surface move because of the intense heat in the Earth’s core that causes molten rock in the mantle layer to move. It moves in a pattern called a convection cell that forms when...more

Composition of Rock Sediment Affected the Strength of Earthquakes in Sumatra

Earthquakes occur regularly in Sumatra, Indonesia because it is located near the boundary of two of Earth's tectonic plates. These earthquakes occur at 'subduction zones,' which are areas where one tectonic...more

It’s Not Your Fault – A Typical Fault, Geologically Speaking, That Is

Some geologic faults that appear strong and stable, slip and slide like weak faults, causing earthquakes. Scientists have been looking at one of these faults in a new way to figure out why. In theory,...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