Earthquakes, Faults, and Seismic Waves
When the ground suddenly shakes, you are feeling energy released deep within the Earth. Earthquakes happen along breaks in the crust called faults, and the energy travels outward as seismic waves. Understanding how earthquakes work is both practical and a common earth science test topic.
This lesson covers faults, earthquakes, and the waves they send out.
A fault is a crack in Earth’s crust where blocks of rock can move. An earthquake is the shaking caused when stress along a fault is suddenly released. That energy travels outward as seismic waves, which scientists detect and measure to study earthquakes.
What causes an earthquake?
Tectonic plates are constantly pushing, pulling, and grinding against each other. Where they meet, the rock can crack, forming a fault. Along a fault, the moving plates can get stuck by friction while stress keeps building. When the stress finally overcomes the friction, the rock snaps and slips suddenly, releasing energy. That sudden release is an earthquake, and the shaking is strongest near the point where it begins.
What are seismic waves?
The energy from an earthquake travels through the ground as seismic waves. There are two main types that arrive first. P waves (primary waves) are the fastest and can travel through solids and liquids, arriving first. S waves (secondary waves) are slower and travel only through solids. Because the two waves travel at different speeds, scientists can use the delay between them to figure out how far away an earthquake occurred.
| Wave | Speed | Travels through |
|---|---|---|
| P wave | Fastest, arrives first | Solids and liquids |
| S wave | Slower, arrives second | Solids only |
How are earthquakes measured?
Scientists record seismic waves with an instrument called a seismograph, which traces the ground’s motion. From these records they can locate the earthquake and measure its strength, or magnitude. Studying how seismic waves travel and bend has also revealed Earth’s internal structure, since the waves change speed and direction as they pass through different layers.
Watch: A Short Video Lesson
SurferToday walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for earthquake questions
- Recall that faults are cracks where rock blocks move.
- An earthquake is the sudden release of built-up stress.
- Match the waves: P waves are fastest and go through solids and liquids; S waves are slower and only through solids.
- Use the delay between P and S waves to find distance.
- A seismograph records and measures the shaking.
Practice questions
- What is a fault?
- What causes the ground to shake in an earthquake?
- Which seismic wave arrives first?
- Which wave can travel through liquids?
- What instrument records earthquakes?
- True or false: S waves travel through both solids and liquids.
Answers:
- A crack in Earth’s crust where blocks of rock can move.
- The sudden release of built-up stress along a fault.
- The P wave.
- The P wave.
- A seismograph.
- False. S waves travel only through solids.
Where this fits
Earthquakes happen along the plate boundaries driven by plate movement, and seismic waves reveal Earth’s internal layers. Find all topics on the ASVAB General Science Learning Hub.
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