The Effect of Sub-surface Deposits
For a given intensity of an earthquake, the amount of damage caused is dependent on the nature of the rocks and deposits of an area.
Solid, competent rock such as granite or limestone allows the vibrations to pass through easily. They are transferred without causing much damage to the rock or to structures built on it. The amplitude of L waves is comparatively low.
Weaker rocks such as clay or poorly cemented sandstone absorb some of the energy and may therefore be deformed. Buildings built on weaker rock are more likely to suffer damage.
Unconsolidated sands and gravels can oscillate freely. The L wave amplitude is higher and damage is greater than on consolidated rock and the intensity is therefore greater.

Earthquake Magnitude - the Richter Scale
The Richter Scale (after 1904) measures the magnitude of the earthquake by recording the amplitdue of the earthquake waves. The range in energy released by different earthquakes is very large and so the values are converted on a logarithmic scale.

On the Richter scale each increase of 1 in the scale means that the amount of enegy released by the earthquake increases by a factor of around 30. There is no theoretical limit to the amount of energy which an earthquake can release and therefore no upper limit on the Richter scale. The maximum recorded value so far is 8.9.

	Created by siobhan.quirk over 11 years ago

Undergraduate Geology - Part 1 (Earthquakes) Note on Measuring Earthquakes, created by siobhan.quirk on 13/05/2013.