Interpreting Seismograms: The Wonder Wiggles

The best pictures we have of the Earth's interior come from the field of SEISMOLOGY, in which we analyze the way in which vibrations propogate through the Earth.

Some definitions:

Seismometers work just like a phonograph needle; or, if you like, a speaker in reverse: Inertial mass on a spring with a magnet moving through a coil of wire.

The waves produced by an earthquake arrive at a point as distinct "phases". We can use these phases to tell us a lot about the earthquake source and the earth structure.

Example: location of an epicenter can be accomplished by the S-P time.

Aside: We know now that most earthquakes are shallow (<15 km depth) but there are belts of deep earthquakes. How can this be?

More sophisticated processing can reveal the nature of the rupture process.