G7-Earthquakes – Outward Expression of Earth’s Dynamic Interior-800-900L-04

At the moment of an earthquake, the energy that had been stored is released and radiates outward in all directions in the form of seismic waves. These seismic waves are like ripples in a pond. They move through Earth, and when they reach the surface, they shake everything around: the ground, our homes, and even us. These same waves also travel through Earth, which helps scientists learn about what they could not otherwise see.

As an earthquake (like the one that hit San Francisco in 1989) occurs, it sends out two types of waves, P and S waves. You might think of them like lightning and thunder. P waves travel very quickly through rock, like lightning. S waves are more like the thunder as they travel slowly. In a storm, the time it takes between seeing the lightning and hearing the thunder can tell you how far away you are from the strike. A similar technique using P and S waves can be used to find the distance of an earthquake. Perhaps even more interesting, these same waves inform scientists about Earth’s interior.

From this data, they are able to construct a fairly accurate model, but how?

P waves can travel through any type of material. S waves can only travel through solids. What scientists noticed was that P waves could be collected almost anywhere. S waves did not pass directly through the Earth. This could mean only one thing. This was clear evidence that some part of Earth’s interior must be liquid. By collecting more and more data, the size of the outer core could be estimated. Today, scientists have learned a lot about what they cannot see. To do so, they use a very simple instrument called a seismograph.

Seismographs are used to record the P and S waves generated from an earthquake. By using three or more seismographs, the location and intensity of an earthquake can be found. A seismograph is nothing more than a heavy pendulum bob supported by a base that is anchored to the ground. As the earth shakes, the pendulum bob does not move while the base connected to the ground does. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.