What is ground shaking?

Ground shaking is the vibration of the land surface as seismic waves pass through it, and it is the direct cause of most earthquake damage.

What determines how strongly a particular site is shaken?

The magnitude of the earthquake, the distance from the epicentre and the depth of the focus, together with the local geology and the duration of the shaking.

Give an example of a city damaged by seismic amplification.

Mexico City in 1985, where buildings founded on the soft sediments of a drained lake bed collapsed even though the epicentre lay some 350 km away.

Liquefaction is the loss of strength of saturated, loosely packed sand or silt during shaking, when the water pressure between the grains rises until the material behaves like a liquid.

What are two visible effects of liquefaction?

Buildings tilt or sink into the ground while lighter buried structures such as pipes and tanks float upwards, and grey silt is forced to the surface through cracks.

WJEC A-Level Geography 601-8455-3

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Tectonic Hazards

Earthquake hazards and ground behaviour

2 things to know
AS Unit 1 and A2 Unit 4

Overview

Ground shaking, why soft sediment increases earthquake damage and city damaged by seismic amplification.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain why soft sediment increases earthquake damage.

Key term

Define liquefaction.

Questions from this subtopic

Each one with the model answer underneath.

1
What is ground shaking?
Model answer
Ground shaking is the vibration of the land surface as seismic waves pass through it, and it is the direct cause of most earthquake damage.
2
What determines how strongly a particular site is shaken?
Model answer
The magnitude of the earthquake, the distance from the epicentre and the depth of the focus, together with the local geology and the duration of the shaking.
3
Explain why soft sediment increases earthquake damage.
Model answer
Loose, water-rich sediment amplifies the seismic waves and prolongs the shaking compared with solid bedrock, so buildings standing on it are shaken harder and for longer.
4
Give an example of a city damaged by seismic amplification.
Model answer
Mexico City in 1985, where buildings founded on the soft sediments of a drained lake bed collapsed even though the epicentre lay some 350 km away.
5
Define liquefaction.
Model answer
Liquefaction is the loss of strength of saturated, loosely packed sand or silt during shaking, when the water pressure between the grains rises until the material behaves like a liquid.
6
What are two visible effects of liquefaction?
Model answer
Buildings tilt or sink into the ground while lighter buried structures such as pipes and tanks float upwards, and grey silt is forced to the surface through cracks.

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