Earthquakes

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Overview of Earthquakes

An earthquake is a sudden or violent movement within the Earth’s crust followed by a series of shocks. An earthquake happens when the Earth’s plates move and cause the ground to shake.

Terminology

The focus is where the pressure is released underground and where the energy radiates out from. This is the place with the strongest waves that cause the most damage.
The epicentre is the point directly above the centre of the earthquake on the earth’s surface.
Seismic waves are when energy is released from the focus in shock waves.
- The most damage will occur at the places where the shock waves are strongest (i.e. closest to the epicentre).

Caused by release of built up stress

The plates on the Earth’s crust are always moving and so tensions can build up between rocks.
This stress can suddenly be released when the strain overcomes the elasticity of the rock.
This release of stress can cause parts of the surface to experience an intense shaking motion that lasts for a few seconds.

Transfer of energy

Large amounts of energy are released.
Much of this energy is transferred vertically to the surface and then moves outwards from the epicentre (point on surface above the focus) as seismic waves
After the moment of fracture, rocks may regain their original shape but in a new position.

Types of Earthquakes

Earthquakes happen at various depths and in various environments. The depth of focus (point below the surface where an earthquake occurs) of an earthquake is significant.

Shallow-focus earthquakes

Shallow-focus earthquakes are those where the focus is 0-70km under the Earth's surface.

Deep-focus earthquakes

Deep-focus earthquakes are those where the focus is 70-700km under the Earth's surface.
Deep-focus earthquakes are caused by previously subducted crust moving towards the core, heating up or decomposing.
Deep-focus earthquakes are generally less damaging than shallow-focus because the shock waves have to travel further and so cause less shaking at the surface.

Underwater earthquakes

Underwater earthquakes move the seabed, which causes water to get displaced. If enough water is displaced, tsunamis (series of enormous waves) can happen.
Underwater earthquakes can be shallow- or deep-focus. The deeper the focus, the bigger the tsunami.
Waves travel fast in deep water, and so tsunamis can strike shores with little warning. This can catch people off-guard and cause many deaths.

Earthquake Waves

Earthquakes can create three different types of wave that have varying effects:

P-waves

An earthquake can produce P-waves (primary waves) when the tectonic stress energy is released.
- P-waves are the fastest waves but they cause the least damage.

S-waves

Some earthquakes produce S-waves (secondary waves) that make the ground shake violently.

L-waves

The waves that create significant damage, including crustal fracturing, are L-waves (Love waves).
- These waves only travel across the surface and have a large amplitude.