How are earthquakes classified by the depth of their focus?

Shallow-focus earthquakes occur down to about 70 km, intermediate-focus ones from about 70 to 300 km, and deep-focus ones from 300 km down to about 700 km.

Why are shallow-focus earthquakes usually the most damaging?

The energy has less rock to pass through before it reaches the surface, so it is less weakened on the way and the shaking at the epicentre is far more intense.

Describe how a P wave travels through rock.

A P wave is compressional, pushing and pulling the rock backwards and forwards in the same direction as the wave is moving.

Why do P waves arrive first at a seismometer?

They are the fastest seismic waves, travelling at around 6 to 8 km s⁻¹ in the crust.

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Tectonic Hazards

Earthquake characteristics and seismic waves

2 things to know
AS Unit 1 and A2 Unit 4

Overview

Focus of an earthquake, how a P wave travels through rock and motion caused by an S wave.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Define the focus of an earthquake.

Key term

Define the epicentre.

Questions from this subtopic

Each one with the model answer underneath.

1
Define the focus of an earthquake.
Model answer
The focus, or hypocentre, is the point underground at which the rock first fractures and the stored energy is released.
2
Define the epicentre.
Model answer
The epicentre is the point on the Earth's surface lying directly above the focus.
3
How are earthquakes classified by the depth of their focus?
Model answer
Shallow-focus earthquakes occur down to about 70 km, intermediate-focus ones from about 70 to 300 km, and deep-focus ones from 300 km down to about 700 km.
4
Why are shallow-focus earthquakes usually the most damaging?
Model answer
The energy has less rock to pass through before it reaches the surface, so it is less weakened on the way and the shaking at the epicentre is far more intense.
5
Describe how a P wave travels through rock.
Model answer
A P wave is compressional, pushing and pulling the rock backwards and forwards in the same direction as the wave is moving.
6
Why do P waves arrive first at a seismometer?
Model answer
They are the fastest seismic waves, travelling at around 6 to 8 km s⁻¹ in the crust.

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