What is an Earthquake?
[ click to see Earthquake Scenario section]
The outer hundred or so kilometres of the earth is made of rigid blocks or plates of rock which vary in area from car-size to the size of whole oceans or continents. All of these plates are moving past each other. Often the plates catch on their neighbours and stop moving for a time, however they are generally still being pulled or pushed and they flex or bend a little until they finally become unstuck and spring very rapidly past each other - up to many metres in only a few seconds. As this rapid movement occurs the elastic energy stored in the slightly bent plates is released catastrophically. The energy radiates outwards through the earth from the point where the plates became unstuck (the earthquake focus) in the form of many elastic waves. When one of these waves passes by a point on the earth the ground rapidly moves either up and down or from side to side before returning to its initial position. If the earthquake intensity (local movement of the ground) is large it can be felt by animals and people. However the majority of earthquakes are too small to be noticed by people or animals and can only be detected by seismographs like the computer here (many earthquakes are even too small to be detected by seismographs).
Because the energy that is released during an earthquake is distributed through an ever increasing volume of rock as it radiates from the focus, people close to the epicentre of an earthquake (the place on the earth's surface above the earthquake focus) will be more likely to feel ground movement than people further away. Eventually the energy dissipates so widely that the ground movement becomes impossible to record. Since the waves travel between 4-8 kilometres in one second people at different distances from the focus will feel the earthquake at different times - see figure below.
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Schematic vertical cross section through the lower South Island showing P waves radiating from a hypothetical earthquake focus under Fiordland. The waves move very fast (around 8 km/s) but still there will be a measurable difference between the times they arrive in different places. |
If an earthquake occurs in Fiordland (as happens many times each year) the primary wave will radiate at 8 kilometres per second in all directions (see figure above). Thus a person in Gore will feel the earthquake 19 seconds later (150km / 8km/s = 19 seconds) and a person in Dunedin will feel it 38 seconds later (300km / 8km/s = 38 seconds).
As pointed out earlier, several different kinds of elastic waves are released from an earthquake focus during an earthquake. Two of these waves, the primary (P) and secondary (S) waves, can easily be used to determine when and how far away an earthquake occurred. The P & S waves generally travel through the earth at 8km/s and 4.4 km/s respectively (depending on the nature of the rocks they are passing through). Hence the first wave to arrive at a point distant from an earthquake will be the P wave (followed by the S wave some seconds later) - see figure below.
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Schematic vertical cross section through the lower South Island showing the difference in velocity of P and S waves radiating from a hypothetical earthquake focus under Fiordland. The P-waves will reach Gore before the S-waves will. |
The S-P time gap between the arrival of the P waves and the S waves can be used to calculate the distance between the earthquake focus and the observation point in Gore. The distance in kilometres is equal to the S-P time multiplied by about 10. Thus the S-P time of 15 seconds observed at Gore suggests a distance of 150 kilometres to the focus (15 seconds x 10 = 150 km).
Since we know that the P waves travel at 8 km/s and that the earthquake focus was 150 kilometres away we can calculate that the earthquake occurred 19 seconds before it was first felt in Gore (150km / 8km/s = 19 seconds).
In order to trace the actual place of origin of any earthquake the arrival time and S-P separation is worked out for at least 3 seismograph computers which recorded that particular earthquake. In practice, all stations of the New Zealand National Seismograph Network are used to locate earthquakes buy the seismic observatory.
In fact we never initially know when or where an earthquake occurs, we only know when it was felt in different areas and what the S-P time was at these different places. A seismograph-computer in the Geology Department can be used to measure the S-P separation time for any earthquake waves that reach Dunedin. Using such information we can work out where and when an earthquake actually occurred.