Measuring Earthquakes

Earthquake Intensity - the Mercalli scaleEarthquakes used to be mreausred by noticeable effects of surface vibrations in the area. This depends on: strength of earthquake at its origin or focus distance from focus and epicentre nature of rocks and sediments that the buildings are constructed on standard and design of building Instrumental - not normally felt. Birds and animals uneasy - >3.5 magnitude Feeble - felt by a few persons at rest - 3.5-4.0 Slight - vibrations like large lorry passing. Felt by people at rest - 3.5-4.0 Moderate - felt indoors by many. Cars rocks - 4.0-5.0 Rather strong - Sleepers wakened. Some windows broken - 4.0-5.0 Strong - small bells ring. Trees sway. Loose objects fall - 5.0 Very Strong - difficult to stand up. People run outdoors. Walls crack - 5.5 Destructive - Partial collapse of ordinary buildings. Chimneys fall - 6.0 Ruinous - ground cracks, buildings shift off foundations. Pipes break - 6.5 Disastrous - landslides, buildings damaged. Many buildings destroyed - 7.0 Very disastrous - few buildings stand, bridges destroyed, landslides - 7.5 Catastrophic - damage total. Waves formed on ground surface - 8.1+

The Effect of Sub-surface DepositsFor 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 ScaleThe 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.

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