Earth Science middle-school May 20, 2026

How Earthquakes Travel Through Rock

Waves that reveal Earth’s inside

Cutaway view of Earth showing earthquake waves spreading from a fault through layers of rock to seismic stations on the surface.

An earthquake starts when rock breaks and slips along a fault. That sudden motion sends energy through the ground as waves. Some waves squeeze rock back and forth, while others shake it side to side.

Big Idea. NGSS MS-ESS3-2 connects earthquake wave data to patterns that help people understand and reduce natural hazards.

During an earthquake, the ground can shake many miles from the break in the rock. That happens because the break does not move the whole planet at once. It releases energy that travels outward through rock. Scientists use instruments called seismometers to record that motion. The records show that not all earthquake waves act the same way. Some waves move fast and can pass through solids and liquids. Other waves move slower and only travel through solids. This difference helps scientists locate earthquakes and study Earth’s hidden layers. It also explains why shaking can feel different from place to place. Rock type, distance, and wave speed all matter. In middle-school Earth science, this question connects a real event to evidence, models, and hazard planning. Earthquake waves are not just shaking. They are clues moving through rock.

A sudden slip starts the waves

Diagram of two rock blocks slipping along a fault, with waves spreading outward from a focus below the surface. — A fault slip releases energy into surrounding rock

Most earthquakes begin at a fault. A fault is a crack where blocks of rock can move past each other. The blocks may stick for years while forces in Earth’s crust keep pushing. Stress builds in the rock, like a bent ruler held in place. When the rock finally slips, stored energy is released. That energy spreads away from the starting point in every direction. The starting point inside Earth is called the focus. The point directly above it on the surface is called the epicenter. The slip may last only seconds, but the waves can travel across a continent. Near the fault, shaking is usually strongest. Farther away, the waves spread out and lose energy. This is why distance from the fault matters, but it is not the only factor.

An earthquake wave begins when stored energy is released by moving rock.

P-waves squeeze and stretch

P-wave diagram showing rock particles moving back and forth parallel to the direction of wave travel. — P-waves push and pull rock in the travel direction

The fastest earthquake body waves are P-waves. The letter P stands for primary, because these waves usually arrive first at a seismometer. A P-wave moves rock particles back and forth in the same direction the wave travels. Picture a spring being squeezed and stretched along its length. That is the basic motion. P-waves can travel through solid rock, liquid, and gas. This makes them useful for studying the whole planet. In rock, they move faster where the material is stiff and hard to compress. They slow down in materials that are easier to squeeze. People may feel P-waves as a quick jolt before stronger shaking arrives. The motion is often small, but the timing is important. The first arrival helps scientists estimate how far away the earthquake began.

P-waves arrive first because they usually travel fastest through Earth.

S-waves shake side to side

S-wave diagram showing a wavy path through rock and particles moving up and down across the direction of travel. — S-waves move rock across the travel direction

S-waves are the second main kind of body wave. The letter S stands for secondary, because these waves arrive after P-waves. An S-wave moves rock particles at right angles to the direction the wave travels. If the wave moves forward, the particles may move up and down or side to side. This motion changes the shape of the rock for a moment. Solids can handle that kind of shape change. Liquids cannot hold their shape in the same way. That is why S-waves do not travel through liquids. This fact is one reason scientists know Earth has a liquid outer core. S-waves are slower than P-waves, but they can cause strong shaking. Buildings and bridges must be designed with sideways motion in mind. The wave type matters for safety.

S-waves cannot pass through liquid, so they reveal what Earth is made of.

Arrival times locate the quake

Map view with three seismic stations and distance circles crossing near an earthquake epicenter. — Three stations can locate an epicenter

A seismometer records ground motion as a wiggly line. The first small wiggles are often P-waves. Larger wiggles from S-waves arrive later. The time gap between the first P-wave and the first S-wave tells scientists the distance to the earthquake. A larger gap means a longer distance. One station can tell distance, but not direction. Three or more stations can locate the epicenter. Scientists draw a circle around each station. Each circle shows a possible distance to the earthquake. Where the circles meet is the best estimate of the epicenter. This method is called triangulation. It works because P-waves and S-waves travel at different speeds through rock. The same idea is used in real earthquake monitoring networks. Timing turns shaking into a map location.

The P-wave and S-wave time gap helps measure distance to an earthquake.

Rock changes the shaking

Cross section showing earthquake waves traveling through hard bedrock and soft sediment, with stronger surface shaking above the soft layer. — Local ground can change earthquake shaking

Earthquake waves do not travel through all ground in the same way. Hard bedrock often carries waves quickly. Soft sediment can slow waves and make shaking last longer. Loose, wet soil can shake strongly because particles move more easily. In some places, waves bounce, bend, and overlap near the surface. That can make one neighborhood shake more than another, even at the same distance from the fault. This is why hazard maps include local ground conditions. Engineers use wave data to design safer buildings, bridges, and pipelines. Emergency planners use the same data to prepare drills and warning systems. Students can model this idea with trays of different materials, such as sand, gravel, and clay. The model is simple, but the pattern is real. The path through rock shapes the shaking people feel.

The same earthquake can feel different because waves interact with local ground.

Vocabulary

Fault: A crack or zone in rock where blocks can move past each other.
Focus: The point inside Earth where an earthquake begins.
Epicenter: The point on Earth’s surface directly above the focus.
P-wave: A fast earthquake wave that squeezes and stretches material in the direction it travels.
S-wave: A slower earthquake wave that shakes material across the direction it travels and cannot move through liquid.
Seismometer: An instrument that records ground motion from earthquakes and other vibrations.

In the Classroom

Slinky wave model

20 minutes | Grades 6-8

Students use a slinky to model P-waves by pushing and pulling along its length. They model S-waves by moving one end side to side, then compare particle motion and wave direction.

Find the epicenter

35 minutes | Grades 6-8

Give students three paper seismograms with different P-wave and S-wave arrival gaps. Students convert each gap to distance using a simple travel-time chart, then draw circles on a map to locate the epicenter.

Ground material shake test

30 minutes | Grades 6-8

Students place small block models on trays of sand, gravel, and firm clay. They tap each tray the same way and compare how the blocks move on different materials.

Key Takeaways

• Earthquakes release energy when rock suddenly slips along a fault.
• P-waves travel fastest and squeeze material in the direction they move.
• S-waves arrive later, shake material side to side, and cannot pass through liquid.
• Seismologists use P-wave and S-wave arrival times to estimate earthquake location.
• Rock type and local ground conditions affect how strong shaking feels at the surface.

Content generated with AI assistance and reviewed by the LivePhysics editorial team. See sources below for original references.

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How Earthquakes Travel Through Rock

A sudden slip starts the waves

P-waves squeeze and stretch

S-waves shake side to side

Arrival times locate the quake

Rock changes the shaking

Vocabulary

In the Classroom

Slinky wave model

Find the epicenter

Ground material shake test

Key Takeaways

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Sources and Further Reading