05 Seismic waves and their recording
Seismic waves are recorded at observing stations by the seismograph.
Types of waves
- Primary waves (P): longitudinal compressional waves — particles vibrate in the direction of travel. They are the fastest and arrive first, and travel through solids, liquids and gases.
- Secondary waves (S): transverse waves with crests and troughs at right angles to the direction of travel. Slower than P, and they travel through solids only — proving the rigidity of the medium.
- Surface waves (L): complex, long waves of large amplitude travelling near the surface. The slowest, arriving last, and responsible for the total destruction of buildings.
What the waves revealed about the Earth's interior
- The passage of P waves through the whole core with a change in their speed allowed scientists to identify a molten outer core and a solid inner core.
- The inability of S waves to cross the outer core proved that it is liquid (molten).
- Analysis of these waves divided the core into an outer core (about 2,100 km) and an inner core (radius about 1,386 km).
Earthquake intensity
The intensity of an earthquake and its destructive effect are measured on the Modified Mercalli scale.