An earthquake emits elastic energy in the form of waves (seismic waves!) that travel throughout the planet. We measure these waves with seismometers when they interact with the surface and cause the ground to move (ground motion).  We derive all kinds of information from the recorded waves, on the nature of the earthquake (the source) and the path that the waves travelled from the source to the seismometer.

Here is a movie of the Magnitude 7 earthquake near Niigata in Japan, on July 16, 2007.

We added the sonified data to this animation that was originally made by researchers at the Earthquake Research Institute, University of Tokyo

These waves are in the “near field”, the very large amplitude waves that cause damage to buildings and provoke landslides, and tsunamis when the motion under the ocean is large enough. But these waves propagate through the planet and are recorded by seismometers all over, as illustrated in this simulation:

 

Here is how we represent the *real* data visually and sonically, when recorded on a large array of seismometers, from the USArray project run by the Earthscope Program at NSF. This movie contains data from two earthquakes, the Mag. 7 Haiti earthquake (near Port-au-Prince) and the Mag. 9 Tohoku earthquake, so you can compare the waves from a nearby but smaller earthquake and a much larger but much more distant earthquake:

You can see these patterns… the waves travel first in one direction and then in another direction ! Why is that?

Here is a simulation of the wave propagation after the Tohoku earthquake:

but that was only two hours…

In fact, those waves keep moving– the Earth keeps vibrating for much longer than that after a large earthquake (the duration of the vibrations are proportional to the magnitude).. Here is another simulation for much longer times:

[ we need a movie of real time ! ]