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the surface, but on those that go down into the earth. When an earthquake occurs, the waves are detected
by monitoring stations all over the world, not just by those close to where the earthquake occurs. Detection is possible worldwide because seismic waves move right through the earth.
By studying the time it takes for earthquakes
to reach distant parts of the planet, it is possible to work out what the rocks are like between where the earthquake occurred and where it was monitored.
Using the nature of material in the earth’s crust as a starting point, it is possible to work out how long it should take an earthquake to be detected at distant stations. In general, waves move faster through more dense material and slower through less dense material. Observations showed that waves arrived much earlier than expected. This could only happen if the interior of the earth was more dense than the crust.
By looking at patterns of P waves and S waves, it is clear that S waves are absent from traces recorded on the side of the earth immediately opposite to the earthquake. S waves cannot travel through liquids, leading to the conclusion that the core of the earth must be liquid.
There is also a zone where neither P nor S waves are recorded. It is known as a shadow zone. It can only occur if the mantle and core bend the P waves as the P waves pass through the zone. It is possible to use this information to work out the size and the nature of the various layers inside the earth.
In this way, from earthquake traces alone, it has been possible to find out the thickness of both the core and the mantle, and to find out what they are made of.
PS
Because P waves travel faster than
S waves, the time interval between the arrival of the P waves and the
S waves increases away from the focus. Studying traces such as this allows scientists to calculate the distance to the earthquake as well as the structure of the earth.
Time between arrivals
P wave
S wave
20
PS
PS