Seismic waves: P, S and L waves

Overview

Three types of seismic wave, P wave and S wave.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What are the three types of seismic wave, and how are they grouped?

   Model answer

   P waves and S waves are body waves, which travel through the interior of the Earth from the focus outwards in all directions. L waves are surface waves, which travel only along the Earth's surface, spreading outwards from the epicentre. The distinction matters because body waves are the ones that reveal the internal structure of the Earth, while surface waves are the ones that do most of the damage to buildings.

2. 2

   Describe the P wave.

   Model answer

   The P or primary wave is the fastest seismic wave, travelling at roughly 6-8 km per second in the crust, so it always arrives first, which is what its name records. It is a compressional or longitudinal wave: the rock particles vibrate backwards and forwards in the same direction that the wave is travelling, alternately squeezing and stretching the material, in the same way that a sound wave travels through air. It can pass through solids, liquids and gases. It has a relatively small amplitude and high frequency, so it is felt as a sharp jolt or bang and does comparatively little damage.

3. 3

   Describe the S wave.

   Model answer

   The S or secondary wave is the second to arrive, travelling at roughly 0.6 of the speed of a P wave, around 3-4.5 km per second in the crust. It is a shear or transverse wave: the rock particles vibrate at right angles to the direction the wave is travelling, so the material is shaken from side to side and up and down. It can travel only through solids. Its amplitude is greater than that of the P wave, so the side-to-side shearing it produces causes significantly more damage to structures.

4. 4

   Describe the L wave.

   Model answer

   The L wave, or long or surface wave, is the slowest of the three and therefore arrives last, travelling at around 2-4 km per second and only along the Earth's surface rather than through its interior. It has by far the largest amplitude and the longest wavelength, and its energy dies away comparatively slowly with distance because it is confined to a thin surface layer. It moves the ground in a rolling motion and in a side-to-side sideways shearing motion, and because of its large amplitude and low frequency it causes most of the destruction in an earthquake.

5. 5

   Why do the three wave types arrive in the order P, S then L?

   Model answer

   Their order of arrival is fixed by their speeds, which are determined by the physical mechanism of each wave and by the properties of the rock. Compression and expansion of rock is resisted more strongly than shearing, so the compressional P wave travels fastest, at roughly 1.7 times the speed of the shearing S wave. Surface waves are slower still than both body waves, and they must also travel around the curve of the surface rather than through the interior, so they cover a longer path. The gap between the arrivals therefore widens the further a station is from the focus.

6. 6

   Rank the three seismic waves by the damage they cause, and explain the ranking.

   Model answer

   L waves cause the most damage, S waves the next most, and P waves the least. Damage depends chiefly on amplitude and on the direction of ground movement. P waves have small amplitudes and push and pull buildings along the direction of travel, which structures resist well because they are designed to carry vertical and compressive loads. S waves have larger amplitudes and shake structures sideways, which they resist much less well. L waves have the largest amplitudes of all, produce a slow rolling and lateral motion of the ground surface, and last longest, so they progressively weaken and topple buildings, and their low frequency is especially damaging to tall structures and to soft ground.

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