| Earthquakes |
| Earthquakes are caused by the relief of stress
built up along faults in the rocks of the Earth's crust. A fault may produce
frequent, small slips, or may store up energy like an elastic band until
a large movement occurs. The movement produces a shock in the rock, and
waves of compression travel out in all directions from the location of
the movement (the focus), usually some way underground. These waves are
referred to as body waves, since they move through the body of the rock.
When these waves reach the surface, the surface can be disturbed, producing
surface waves, the slower-moving disturbances which cause most destruction.
The point on the surface vertically above the focus is referred to as
the epicentre, and it is here that the surface disturbance is usually
greatest.
Compressional body waves travel at about 6 km per second in the upper portions of the crust, producing a backward-and-forwards shaking in the direction of their travel. Shear body waves are also produced, typically travelling at 3.5 km/s, resulting in sideways movements perpendicular to the direction of travel. Since the compressional waves always arrive first, they are referred to as primary, or P waves, and the shear waves are called secondary, or S waves. The last to arrive are the damaging surface waves, which move up-and-down. Sensitive seismic recording equipment can detect and distinguish P and S waves from very distant earthquakes, making it possible, by comparing timings from widely-separated sites, to pinpoint the epicentre and depth. Much of our knowledge about the interior structure of the Earth is also inferred from the analysis of seismic signals produced by earthquakes. The energy released by a large earthquake can be similar to that produced by a large nuclear bomb. It is possible to measure the elastic energy stored in rocks before and after an earthquake and thus find out how much energy was released, but in practice this is very difficult to do. Instead, the energy released is estimated by measuring the maximum amplitude of the P waves, the duration of a wave-cycle and the time delay between the arrival of P and S waves. The scale of measurement is called the Richter magnitude scale, after the seismologist who developed the method. The Richter scale is logarithmic, so that an increase of one unit corresponds to a tenfold increase in the amplitude. This would normally equate to a hundred-fold increase in energy, but larger quakes tend to produce longer wavelengths, reducing the average energy increase to 30 times. The difference between a magnitude 4 and magnitude 7 event is thus 30x30x30 = 27,000 times more energy. A magnitude 4.5 event is enough to rattle window frames, but a magnitude 7 event will cause bridges to twist and masonry buildings to collapse. Quakes above magnitude 8.0 occur once every 5-10 years and are totally destructive. The largest recorded Richter magnitudes to date are about 8.6, corresponding to about 10,000 bombs of the size dropped on Hiroshima in 1945. Luckily, these events were in sparsely-populated areas. |
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