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Earth Science
Grade 6-12
Earthquake Waves (P, S, Surface) Cheat Sheet
A printable reference covering P waves, S waves, surface waves, wave speed, arrival times, and earthquake location for grades 6-12.
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Earthquake waves are vibrations that carry energy through Earth after rocks break along a fault. This cheat sheet helps students compare P waves, S waves, and surface waves using speed, motion, and where each wave can travel. Understanding these waves is important because seismologists use them to locate earthquakes and learn about Earth's interior.
Key Facts
- P waves are primary waves, travel fastest, and move particles back and forth in the same direction the wave travels.
- S waves are secondary waves, travel slower than P waves, and move particles perpendicular to the direction the wave travels.
- Surface waves travel along Earth's surface and usually cause the strongest shaking and the most damage near the epicenter.
- P waves can travel through solids, liquids, and gases, but S waves can travel only through solids.
- Wave speed can be calculated with speed = distance / time when distance and travel time are known.
- Distance to an earthquake can be estimated from the time gap between arrivals: larger S-P time means the epicenter is farther away.
- Epicenter location requires data from at least three seismograph stations using circles drawn around each station.
- The focus is the underground point where the earthquake starts, and the epicenter is the point on Earth's surface directly above it.
Vocabulary
- Seismic wave
- A seismic wave is an energy wave produced by an earthquake or other sudden movement in Earth.
- P wave
- A P wave is the fastest seismic body wave and compresses and expands material in the direction it travels.
- S wave
- An S wave is a slower seismic body wave that shakes material side to side or up and down at right angles to its travel direction.
- Surface wave
- A surface wave is a seismic wave that moves along Earth's surface and often produces strong ground motion.
- Epicenter
- The epicenter is the point on Earth's surface directly above the earthquake focus.
- Seismograph
- A seismograph is an instrument that detects and records earthquake waves.
Common Mistakes to Avoid
- Confusing P waves and S waves is wrong because P waves arrive first and move by compression, while S waves arrive later and move material perpendicular to travel direction.
- Saying S waves travel through liquids is wrong because liquids do not have enough rigidity to transmit shear motion.
- Using only one seismograph station to locate an epicenter is wrong because one station gives only a distance, not a unique location.
- Assuming the biggest wave always arrives first is wrong because surface waves often have large amplitudes but arrive after P and S waves.
- Mixing up focus and epicenter is wrong because the focus is underground where the earthquake begins, while the epicenter is on the surface above it.
Practice Questions
- 1 A P wave travels 600 km in 100 s. What is its speed in km/s?
- 2 At one station, the P wave arrives at 10:15:20 and the S wave arrives at 10:16:05. What is the S-P arrival time difference?
- 3 If Station A has an S-P time of 20 s and Station B has an S-P time of 50 s, which station is farther from the epicenter?
- 4 Why do scientists use both P waves and S waves to learn that Earth's outer core is liquid?