What are seismic waves in simple terms?

Seismic waves are vibrations that move through Earth when energy is suddenly released, usually during an earthquake. They spread out from the earthquake like ripples spreading out from a splash in water.

What causes seismic waves?

Most seismic waves are caused by earthquakes, which happen when rocks underground break or slip along a fault, a crack in Earth's crust. They can also be caused by volcanic eruptions or human activities like large explosions, but earthquakes are the main source.

What are the main types of seismic waves?

The main types are P-waves, S-waves, and surface waves. P-waves are the fastest and arrive first, S-waves arrive next and move the ground side to side or up and down, and surface waves travel along the surface and often cause the most damage.

How do scientists use seismic waves to study Earth?

Scientists record seismic waves with seismographs and measure how long different waves take to reach different places. By seeing how the waves speed up, slow down, or change direction, they can map where an earthquake started and learn about Earth's internal layers.

What Are Seismic Waves? Definition and Examples

Kid-friendly explanation

Seismic waves are like invisible waves that move through the ground when Earth shakes. When an earthquake happens, these waves spread out in all directions. They can make the ground roll, buildings shake, and even ring tools in a toolbox. Scientists use them to learn what is deep inside Earth!

Worked examples

Example 1 (simpler): Feeling different seismic waves in a city

Imagine an earthquake starts under a town. The place where it starts inside Earth is called the focus, and the spot on the surface right above it is called the epicenter.

First, fast seismic waves called P-waves (primary waves) reach the town. People might feel a quick bump or a sudden jolt, like a truck hit the building. A short time later, slower S-waves (secondary waves) arrive and make the ground move more side to side. Finally, surface waves, which travel along Earth's surface, reach the town and can cause the biggest shaking, making things sway and roll.

Even though all of these are seismic waves, they move in different ways and arrive at different times, which helps scientists figure out where the earthquake started.

Example 2 (more advanced): Using seismic waves to find the earthquake location

Suppose a seismograph, which is a tool that records ground shaking, is in a city. During an earthquake, it records that P-waves arrive at 0 seconds and S-waves arrive 40 seconds later.

Scientists know that in this region, S-waves travel more slowly than P-waves. A bigger time gap between P-wave and S-wave arrivals means the seismograph is farther from the earthquake. Using charts or computer models that compare the P-wave and S-wave time difference to distance, they find that a 40-second difference matches a distance of about 400 km from the earthquake's epicenter.

By combining this information from at least 3 different seismograph stations, scientists can draw circles on a map and find the point where the circles meet. That point is the earthquake's epicenter, all based on how seismic waves traveled through Earth.

Going deeper

As students move into higher grades, seismic waves become a key part of understanding how Earth works inside. Different types of seismic waves travel at different speeds and through different materials. For example, S-waves cannot travel through liquids, but P-waves can.

By studying how seismic waves bend, speed up, slow down, or disappear as they move through Earth, scientists have learned that Earth has layers, like the crust, mantle, outer core, and inner core. This connects seismic waves to ideas like plate tectonics, because earthquakes mostly happen where plates meet and move.

Seismic waves also help explain how earthquakes can change the shape of land at the surface, creating or affecting a landform such as a mountain, valley, or new crack in the ground.

Common mistakes

Mistake: Thinking all seismic waves are the same and cause the same kind of shaking.
Correction: There are several types of seismic waves, such as P-waves, S-waves, and surface waves, and each moves in a different way and causes different kinds of motion.
Mistake: Believing seismic waves only happen during big, damaging earthquakes.
Correction: Seismic waves occur during all sizes of earthquakes, even very small ones that people cannot feel but seismographs can record.
Mistake: Thinking seismic waves only move along Earth's surface like water waves on a pond.
Correction: Some seismic waves travel deep through Earth's inside, not just along the surface, which is how scientists study Earth's inner layers.

Everyday connection

Seismic waves are at work whenever there is an earthquake that makes the news or shakes a nearby area. Even if your child has never felt an earthquake, they may have seen videos of buildings swaying or roads cracking. Those motions are caused by seismic waves moving through the ground and up into the structures people use every day.

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What are seismic waves?