What are the causes and effects of earthquakes

Earthquakes mainly happen on or near to plate boundaries, along a fault, or crack, in the rock.

An earthquake is a sudden violent movement of the Earth’s surface.
As the plates slide under, over or past each other, friction causes the plates to lock together.
As the pressure increases, weaknesses or fault lines in the earth's crust close to the boundaries begin to fail.
The fault line breaks and the stored energy is released and travels outwards from the focus as seismic waves.
Earthquakes occur near the Earth’s surface and go to a depth of about 700 km.
Below 700 km the rocks become too hot and flexible to break. They just bend very slowly.

The epicentre of the earthquake is on the earth's surface directly above the focus and is likely to suffer the greatest amount of damage.
The closer the focus is to the crust the greater the damage. Power of the earthquake reduces the further away it is from the focus.
The strength of shaking at the surface is much less for earthquakes that happen deeper into the lithosphere.
Shallow earthquakes are found at mid-ocean ridges while at subduction zones earthquakes range from shallow to deep.
Major earthquakes are often preceded or followed by smaller earthquakes called foreshocks and aftershocks.
Foreshocks can be a warning of a bigger earthquake to come.
Aftershocks are particularly dangerous.

- Buildings, already damaged

Earthquakes also happen in rock beneath the seabed – can trigger a tsunami (a giant wave).

The Richter Scale

The earthquake's energy is recorded by a seismometer, which measures the energy released by the earthquake using the Richter Scale (1-10).
On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. Although the Richter Scale has no upper limit, the largest earthquake ever recorded was in 1960 in Chile. It measured 9.5 on the Richter Scale.
It is a logarithmic scale which means that a size ‘6’ on the Richter Scale is 10 times larger than a size ‘5’ and 100 times larger than a size ‘4’.

The Mercalli scale

The Mercalli scale (I-XII) is a scale used to measure the damage of an earthquake and what people experience.