Earthquakes and Volcanoes » Earthquakes: Focus, Epicentre and Seismic Waves

What is an earthquake?

You already know that plates stick, build up pressure and then jerk free (see What Happens at Each Plate Boundary). This lesson is about what happens next: where the earthquake starts, and how its shaking travels.

A good way to picture it is a plastic ruler. Bend the ruler slowly and it stores energy. Bend it too far and it snaps. The energy is released in one go and the ruler shakes. Rock under pressure does the same, only on a huge scale. The snap sends out waves of shaking in every direction.

Focus and epicentre

Every earthquake has two key points. One is underground and one is on the surface.

Key terms:

• Focus (hypocentre): the point underground where the rock snaps and the earthquake starts.
• Epicentre: the point on the surface directly above the focus.
• Aftershock: a smaller earthquake that follows the main earthquake, in the same area.
• Seismometer: an instrument that detects and records the shaking of the ground.

Think of a firework going off. The focus is where it explodes high in the air. The epicentre is the spot on the ground directly beneath it. The shaking is strongest at the epicentre, because that is the closest surface point to the focus.

Depth of focus

The focus can be shallow or deep. Earthquakes with a focus less than about 70 km below the surface are called shallow-focus earthquakes. Focus depths can reach several hundred kilometres at convergent boundaries, where a plate is sinking.

So shallow-focus earthquakes usually cause more damage than deep-focus earthquakes of the same size.

Seismic waves

The energy from the focus travels outwards as seismic waves. There are three types, and they are easy to tell apart by speed.

Wave	Order of arrival	Speed	Damage
Primary (P) waves	First	Fastest	Least damage. They push and pull the ground back and forth.
Secondary (S) waves	Second	Slower than P	More damage. They move the ground up and down and side to side.
Surface waves	Last	Slowest	Most damage. They roll along the surface and cause the strongest shaking.

P waves are the "primary" waves because they arrive first. S waves are "secondary" because they arrive second. Surface waves travel only along the top of the crust, from the epicentre outwards. They are slowest, but they carry the most energy near the surface, which is why they do the most harm to buildings, roads and bridges.

A simple way to remember the order is that the waves arrive in order of speed: P, S, then surface.

Shaking weakens with distance

Drop a pebble into a pond. Ripples spread out in rings and get smaller the further they go. Seismic waves behave in the same way. The shaking is strongest at the epicentre and gets weaker the further you are from it.

• Near the epicentre: strong shaking, the most damage.
• Far from the epicentre: the waves have spread out and lost energy, so the shaking is weaker and damage is lower.

The main pattern is simple: the closer to the epicentre, the worse the shaking.

Aftershocks

The main earthquake does not always finish the job. Rock around the fault has been moved, so it is left under new stress. It settles by slipping again in a series of smaller earthquakes called aftershocks. Aftershocks can continue for days, weeks or even months. They matter because they can bring down buildings already weakened by the main earthquake, and they can strike while rescuers are working.

How seismometers record earthquakes

A seismometer is a machine fixed to the ground with a heavy weight that hangs loosely. When the ground shakes, the weight stays almost still while the frame moves, and the difference is recorded as a wiggly line called a seismogram.

A seismogram tells a story over time:

1. A flat, quiet line before the earthquake.
2. Small wiggles when the fast P waves arrive.
3. Bigger wiggles when the S waves arrive.
4. The biggest wiggles when the surface waves arrive.
5. The wiggles slowly die away.

The further a seismometer is from the earthquake, the longer the gap between the P waves and the S waves. Scientists use this gap to work out how far away the earthquake was. Using records from three or more seismometers in different places, they can pinpoint the epicentre.

Exam-style question

Two made-up earthquakes, X and Y, were the same size and struck near the same size of town. Earthquake X had a focus 10 km below the surface. Earthquake Y had a focus 300 km below the surface.

(a) Identify the term for the point on the surface directly above the focus. [1 mark]

(b) State which type of seismic wave arrives first at a seismometer. [1 mark]

(c) Describe how a seismometer records an earthquake. [2 marks]

(d) Explain why Earthquake X is likely to cause more damage than Earthquake Y. [4 marks]