An earthquake is a violent shaking of the Earth's crust caused by the sudden relief of enormous stresses. This leads to the crust 'snapping'.
The focus of the earthquake is where the 'snapping' begins and can occur many kilometres below the Earth's surface.
Deep focus earthquakes cause less damage and are felt less than shallow focus ones.
This is why the earthquake at Market Raison in Lincolnshire on 27th February 2008, measuring 5.2 at the surface, was felt so widely, but not severely.
The epicentre is the point of ground surface immediately above the focus - this is where the greatest damage occurs.
Radiating out from this point are shock waves - primary longitudinal waves, then stronger, transverse secondary waves, then the surface waves of both types.
The Richter Scale
This is a logarithmic scale used to measure the magnitude of an earthquake. The scale ranges from 0-12, (but this can be increased if necessary) but those below 3.5 can only be detected by seismographs.
Each number in the scale increases by 10x the previous value.
A less powerful earthquake can do more damage and destruction than a stronger one.
Earthquake Hazards
1) Geology:
The effect of an earthquake is much stronger on weak sand and clays than on solid rock.
The 1989 San Francisco earthquake caused more damage to an area that was built on an unstable landfill site.
Mexico, 1985: Part of Mexico City had been built on silt and peat that had accumulated when the site was previously a shallow lake. The shock waves brought water to the surface, turning the silt into mud. 10,000 buildings, all without rock foundations, collapsed into the mud.
2) Location:
Generally, the greatest damage occurs if the earthquake focus is close to the Earth's surface.
The distance from the epicentre is important; there is a distance-decay effect.
3) Population Density:
A densely populated area will experience more death and injury than a sparsely populated area e.g. Los Angeles, San Francisco and Bam (Iran). This can be linked to the time of day: busy rush-hour traffic, people asleep in bed, weekday/weekend.
4) Building Design:
Strong correlation between number of deaths and building design.
The Bam earthquake in 2003, had an estimated death toll of 30,000. Many of the buildings were constructed out of mud and clay, and collapsed on the occupants. Many also suffocated as a result of brick dust.
Building designs in earthquake-prone regions of MEDCs are very high-tech and include features such as reinforced foundations deep in bedrock and reinforced lift shafts with tensioned cables.Automatic shutters come down to prevent passers-by from being showered with glass fragments.
5) Wealth of a Country:
MEDCs obviously have the financial resources and technology to be able to limit earthquake damage.
There is more money to spend on earthquake proofing buildings, and spending money on research, prediction and prevention.
The Modified Mercalli Intensity Scale
Developed by 1981 by Harry Wood and Frank Newmann.
Uses Roman Numerals from I-XII.
Doesn't have a mathematical basis, but is a subjective ranking based on observed effects, ranging from imperceptible shaking to catastrophic destruction.
Structural engineers and architects will use this scale to assess the damage of buildings and infrastructure, in addition to evaluating future building design, and improving plaaning.
E.g.
II - Felt only by a few persons at rest, especially on upper floors of buildings. (weak)
V - Felt by nearly everyone, many awakened. Some dishes and windows broken. Pendulum clocks may stop. (moderate)
IX - Damage considerable in specially designed structures. Well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. (violent).
Where and Why do Earthquakes Occur?
At destructive plate margins, the pressure resulting from the sinking of the subducting plate and its subsequent melting can trigger strong earthquakes as this pressure is periodically released.
Earthquakes that occur at constructive plate boundaries tend to be less severe than those at destructive or conservative plate margins. The friction and pressure caused by the plates moving apart is less intense than at destructive plate marins.
Earthquakes tend to be of greater strength at conservative plate margins. Here, where the plates slide past each other, the plates tend to stick for periods of time. This causes stresses and pressure to build. The release of the pressure occurs in a sudden, quick release of the plates and the result is an earthquake.