Earthquake Notes

Earthquakes occur at plate boundaries.
Earthquakes occur at Ext.
- Name:
  1. A location in Europe that has earthquakes.
  2. A location in Asia.
  3. Three locations that border the Pacific ocean.
Aim: To understand how earthquakes occur and how they are measured.

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.

Pacific Plate
Indo-Australian Plate
Plates are moving in different directions.
Plates try to slide past each other.
Movement is not constant – plates stick together.
A large amount of pressure builds up.
Pressure is suddenly released and the plates move, causing an earthquake.
As land is not being created or destroyed at these margins, there are no volcanoes.

Earthquakes occur at all plate margins, as well as along fault lines that spread out from the main fault. They are particularly violent at destructive and conservative plate margins.
Over time, stress builds up at these plate margins as plates rarely move continually. When this is suddenly overcome the plate moves, causing the rocks to fracture and creating an earthquake.
The longer a fault remains locked, the more stress builds up and the greater the chance of it having a major earthquake.

Earthquakes produce two main waves which travel at different speeds (body waves which travel through the Earth and surface waves).
P-waves are the fastest and travel through both solids and liquids.
S-waves are slower and can only travel through solids.
Love waves shake the ground at right angles to the direction of movement; they are faster than Rayleigh waves.
Rayleigh waves produce both horizontal and vertical ground movement, occurring in a rolling motion. These are what often cause the most damage.

Earthquakes are measured on a seismometer.
The intensity or size of the earthquake is measured on the Richter scale. This records an earthquake’s magnitude.
The Richter scale is logarithmic, so a magnitude 7 earthquake is 10 times more powerful than a magnitude 6.
An earthquake larger than magnitude 5 is likely to cause some structural damage to buildings.
Earthquake intensity (degree of surface shaking) is measured using a qualitative scale called the Mercalli 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’.

The Mercalli scale measures how much damage is caused by the earthquake based on observations.
It is measured on a scale between 1 and 12.