S-waves are transverse, seismic waves.

The idea

In an S-wave the rock particles oscillate at right angles, perpendicular, to the direction of energy transfer, so the rock is sheared from side to side rather than squeezed. That is the same geometry as a wave on a rope or a light wave. Shearing only works where the material resists a change of shape, which solids do and liquids do not, so S-waves travel through solid rock but stop dead at a liquid boundary. They are slower than P-waves, roughly half to two thirds of the P-wave speed in the same rock, which is why they arrive second and are called secondary waves. The delay between the P and the S arrival at a station grows steadily with distance from the earthquake.

What earns the marks

• Stating that the oscillations are perpendicular to the direction of energy transfer.
• Saying that S-waves cannot travel through a liquid.
• Stating that S-waves travel more slowly than P-waves and arrive second.
• Linking the inability to travel through a liquid to a liquid not resisting a change of shape.

Common mistakes and how to put them right

• Writing up and down instead of perpendicular to the direction of energy transfer, which fails for a wave travelling upwards.

• Saying S-waves are slower because they are weaker.

• Claiming S-waves are absorbed by the liquid core rather than being unable to travel through it at all.

• Mixing up which wave is longitudinal and which is transverse.

Exam tips

• Answer the definition using the word perpendicular; sideways on its own is not credited.
• Say that a liquid cannot resist shearing, rather than that a liquid is too thin.
• When a question gives P and S arrival times, subtract them and comment on the distance.