Plate Tectonics - Earthquakes

Earthquakes

What are earthquakes?

Earthquakes are vibrations caused by earth movements at plate boundaries and at major fault lines (cracks in the earth’s surface).

They can occur at all four major plate boundaries but the most severe earthquakes are normally found at conservative and destructive plate boundaries.

The main cause of earthquakes is friction. When plates try to move, they are stopped by friction. This can be with another plate or with a mantle. The forces pushing the plate keep pushing, until there is so much force that it overcomes friction and the plate suddenly moves forward.

The Richter Scale

This measures the magnitude of a tremor (how powerful it is) using an instrument called a seismograph.

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’.

Epicentre and focus

The focus is the point at which the rock moves normally underground.

Seismic waves start at the focus.

The epicentre is directly above the focus near the earth’s surface.

How can we predict earthquakes?

Water levels can rise in wells and lakes because of cracks in the rock.

A tilt meter can check any movement within the rocks.

Foreshocks before the main quake can be detected by a seismometer.

Animals can act strangely before the earthquake.

Building in earthquake zones

This is San Francisco in the USA.

San Francisco is near the San Andreas Fault and therefore the city experiences earthquakes.

This skyscraper has been built to be ‘earthquake-proof’.

Its wide base lowers the centre of gravity of the building and makes it more stable.