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A seismograph records the shock waves given off by an earthquake. Seismology is the study of earthquakes and how their shock waves travel through the Earth. Seismic waves produced by deliberate explosions are one of the main methods of underground exploration, used by seismologists to look for oil wells and geological faults. They can even help with finding long-buried meteorite craters. For example, the Chicxulub Crater, which was formed by an meteorite impact that probably caused the extinction of the dinosaurs 66 million years ago, was located using maps made by seismologists to find oil.

P-waves travel through both the mantle and the core, whereas S-waves are slower and cannot travel through the liquid outer core.

Probing the Earth

Shock waves from earthquakes provide a picture of the interior layers of the Earth. When there is a quake, two different kinds, P-waves (primary waves) and S-waves (secondary waves), travel all over the globe from the focus of the quake. Seismographs and seismometers are devices for measuring them.    
Both kinds of waves travel through the Earth, but at different speeds according to the layers they travel through. P-waves travel through both the mantle and the core. Seismometers receive them on the far side of the globe about 20 minutes after the quake. S-waves are slower and cannot travel through the liquid outer core.
By noting times the waves take to travel from quakes to various seismometers around the world, seismologists have been able to work out the exact depth of the boundaries between the crust and the mantle and the mantle and core. They can also tell whether the rocks deep inside the Earth are solid or liquid.

Seismologists discovered the crater formed by the meteorite that crashed to Earth 65 million years ago, and is thought to have caused the extinction of the dinosaurs.

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