Seismic waves are produced by earthquakes.

The idea

An earthquake happens when rock under stress suddenly slips along a fault, releasing stored energy. This sends mechanical waves out through the Earth in all directions from the focus, known as seismic waves. They are detected at the surface by seismometers. Two types travel through the body of the Earth: P-waves, which are longitudinal, and S-waves, which are transverse. P-waves are faster, so they arrive first. The gap between the two arrivals grows with distance, letting records from several stations locate the earthquake. While P-waves cross the whole planet, S-waves cannot travel through the liquid outer core, creating a shadow zone. This distinction provides crucial evidence for the Earth's internal liquid structure.

What earns the marks

• Saying that seismic waves are produced when rock slips and releases energy at an earthquake.
• Naming the two body waves as P-waves and S-waves.
• Saying that P-waves reach a seismometer before S-waves because they travel faster.
• Explaining that S-waves cannot travel through the liquid outer core, unlike P-waves.

Common mistakes and how to put them right

• Describing seismic waves as electromagnetic, or as travelling only along the surface.

• Claiming that both P-waves and S-waves can travel through the entire Earth including the liquid core.

• Confusing the seismometer record with the earthquake itself.

Exam tips

• Remember P for primary and first to arrive, S for secondary and later; the names encode the order.
• When reading a seismogram, use the gap between the P and S arrivals, not the height of the trace.