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?
Understanding Earthquake Waves (P, S, Surface)
Rock can transmit several kinds of disturbance because it is elastic over small movements. A compression pulse briefly packs mineral grains closer together, then lets them spread apart. A shear pulse changes the shape of the material, much like pushing the top of a deck of cards sideways.
These motions are not the same as chunks of Earth travelling from the fault to a city. Most particles jiggle around a resting position while the energy moves onward.
This distinction helps students read wave diagrams correctly. The arrow showing travel direction is not always the direction in which the ground itself moves.
Wave paths are rarely straight for their whole journey. Earth is made of layers with different temperatures, pressures, densities, and compositions. When a wave enters a layer where its speed changes, its path can bend.
This bending is called refraction. Some energy can bounce from a boundary, producing reflected waves. Seismologists study these changes to map structures far below drilling depth.
One major clue comes from the outer core. Shear motion does not pass through its liquid iron rich material.
Compression waves do continue through it, though their paths bend strongly. Patterns of missing or delayed signals around the globe provide evidence for a liquid outer core and a solid inner core.
A seismograph turns ground motion into a record called a seismogram. The instrument contains a heavy mass that tends to remain nearly still when the ground and the instrument frame move. The relative movement is recorded over time.
Scientists identify the first small arrivals, then later larger motions. This can be difficult when background vibration from wind, traffic, ocean waves, or earlier earthquakes makes the trace messy. One station can estimate distance, but it cannot determine a unique direction.
Its possible earthquake locations form a ring around the station. Records from several stations narrow the answer where their distance rings meet. Real results may not meet at one exact point because Earth models, timing, and measurements contain uncertainty.
The shaking felt at the surface depends on more than the earthquake size. Loose wet sediment can amplify motion or lose strength during intense shaking. This process is called liquefaction, and it can make buildings tilt or sink.
Hillsides may slide when repeated motion weakens soil and rock. Buildings have natural vibration periods, so a structure can sway especially strongly when the ground motion has a similar rhythm. Engineers use this knowledge when designing foundations, bridges, and tall buildings.
When learning from seismograms, pay attention to the time scale, the amplitude scale, and the station location. For speed problems, keep distance and time units consistent before dividing distance by travel time. Larger wave height does not automatically mean a farther earthquake, since local ground conditions and instrument distance affect the recorded signal.