Further Research
Earthquake definitions
1. EARTHQUAKE: A SERIES OF SHOCKS GENERATED AT A POINT WITHIN THE EARTH'S CRUST OR MANTLE.
An earthquake occurs when the forces driving blocks on the opposite sides of a fault exceed the frictional forces that lock the blocks in place. The earthquake rupture does not take place all at once. Instead, the rupture begins at one place on the fault plane and then moves along the fault plane.
2. FOCUS: THE POINT AT WHICH THE SHOCK WAVES ARE GENERATED.
3. EPICENTRE: THE POINT ON THE SURFACE OF THE EARTH DIRECTLY ABOVE THE FOCUS.
Three types of shock wave are generated:
i. P - WAVES
TRAVEL THROUGH SOLIDS AND LIQUIDS
ARE REFLECTED AND REFRACTED
TRAVEL FASTEST AND ARE FIRST TO BE RECORDED BY A SEISMOGRAPH
ii. S - WAVES
- HIGH FREQUENCY
- SHORT WAVELENGTH
- TRANSVERSE WAVES
- TRAVEL IN ALL DIRECTIONS FROM THE FOCUS
CANNOT TRAVEL THROUGH LIQUIDS
ARE REFLECTED AND REFRACTED
THE SECOND SET OF WAVES TO BE RECORDED BY A SEISMOGRAPH
SHOW EARTH'S CORE
iii. L - WAVES
- LOW FREQUENCY
- TRANSVERSE WAVES
- TRAVEL THROUGH THE SURFACE LAYERS OF THE CRUST
SHOW EARTH'S CORE
4. SEISMOGRAPH: AN INSTRUMENT USED TO RECORD EARTHQUAKE SHOCKS. IT HAS TO BE ISOLATED FROM OTHER TYPES OF SHOCK, e.g. THOSE PRODUCED BY TRAFFIC. DIFFERENT SEISMOGRAPHS ARE USED TO RECORD: HORIZONTAL MOVEMENT, ON EAST-WEST AND NORTH-SOUTH AXES; AND VERTICAL MOVEMENT.
THE TRACE PRODUCED BY A SEISMOGRAPH IS CALLED A SEISMOGRAM.
i. Earthquake magnitude: the Richter Scale
ii. Intensity
Earthquakes may be given an intensity value as shown by the scale below (a simplified version of the Mercalli Earthquake Intensity Scale).
I. Not felt except by a very few under especially favourable circumstances.
II. Felt by s few persons at rest, especially on upper floors of buildings.
III. Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognise it as an earthquake.
IV. During the day, felt indoors by many; outdoors by few. At night some awakened.
V. Felt by nearly everyone; many awakened. Some dishes, windows broken; a few instances of cracked plaster; unstable objects overturned.
VI. Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.
VII. Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures. Some chimneys broken.
VIII. Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls.
IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings.
X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Some landslides.
XI. Few, if any (masonry), structures remain standing. Bridges destroyed.
XII. Damage total. Lines of sight and level distorted. Objects thrown upward into the air.
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