Seismic Waves

Types of Seismic Waves

Seismic waves are vibrations that travel through the Earth, usually caused by earthquakes or volcanic activity. There are three main types:

• Primary (P) waves: These are longitudinal waves, meaning the particles in the Earth vibrate parallel to the direction the wave travels. P waves are the fastest seismic waves and can travel through solids, liquids, and gases.
• Secondary (S) waves: These are transverse waves, where particles vibrate perpendicular to the wave direction. S waves are slower than P waves and can only travel through solids, not liquids or gases.
• Surface waves: These waves travel along the Earth's surface and cause most of the damage during earthquakes. There are two types:
  • Love waves – move side to side, causing horizontal shaking.
  • Rayleigh waves – move in an elliptical rolling motion, similar to ocean waves, causing both vertical and horizontal ground movement.

Note: Surface waves have transverse components but their motion is more complex, including elliptical motion for Rayleigh waves, which is different from the simple transverse motion of S waves.

Wave Properties of Seismic Waves

Seismic waves show different properties depending on their type and the material they travel through:

• Longitudinal and transverse motion: P waves are longitudinal, compressing and expanding the material in the direction of travel. S waves and surface waves are transverse, moving particles perpendicular to the direction of travel.
• Speed differences in solids and liquids: P waves travel faster in solids than in liquids because particles in solids are more tightly packed and can transmit vibrations more quickly. S waves cannot travel through liquids because liquids cannot support shear (sideways) forces.
• Wave travel through Earth's layers: Seismic waves travel through different layers of the Earth (crust, mantle, outer core, inner core). P waves can pass through all layers, but S waves are stopped by the liquid outer core, creating a shadow zone where no S waves are detected.

For instance, P waves travel at about 8 km/s in the Earth's crust, while S waves travel slower at around 4.5 km/s. This difference helps scientists identify the type of wave and the material it has passed through.

Detection and Measurement

Seismic waves are detected and recorded using specialised instruments:

• Seismometers and seismographs: A seismometer detects ground vibrations, and a seismograph records these vibrations as a trace on paper or digitally. The trace shows the arrival times of different seismic waves.
• Recording wave arrival times: P waves arrive first on a seismogram, followed by S waves, then surface waves. The time difference between P and S wave arrivals helps determine the distance to the earthquake epicentre.
• Locating earthquake epicentres: By comparing arrival times from at least three different seismograph stations, scientists can triangulate the exact position of the earthquake epicentre on the Earth's surface.

Example: If a seismograph records the P wave arriving at 12:00:00 and the S wave at 12:00:30, the 30-second difference can be used to estimate how far away the earthquake occurred. For example, since P waves travel faster, the greater the time difference, the further the earthquake.

Applications of Seismic Waves

Seismic waves are valuable tools beyond just detecting earthquakes:

• Studying Earth's interior structure: By analysing how seismic waves travel through the Earth, scientists have mapped the layers inside the Earth, including the crust, mantle, outer core, and inner core. Changes in wave speed and behaviour reveal the properties of these layers.
• Earthquake analysis and safety: Understanding seismic waves helps engineers design buildings and structures that can withstand earthquake shaking, improving safety in earthquake-prone areas.
• Use in oil and mineral exploration: Controlled seismic waves (similar to small earthquakes) are used in exploration. By sending waves into the ground and measuring their reflections, geologists can locate oil, gas, and mineral deposits underground.