Seismic Waves - shock waves from an earthquake

Types of seismic Waves
 

P waves - travel by back and forth movement of rock  particles. S waves - travel by up and down movement of rock  particles.  These waves do not go through liquids. L waves - when P and s waves arrive at the earth's  surface, they become surface or L waves.  (They do the most damage during an earthquake.)

Rock density determines the speed at which a wave travels.  The more
dense the rock, the faster the speed of the seismic wave.

Different densities of rock cause speed changes.
Different densities of rock will also change the angle of a seismic wave. This is -
refraction - the bending of a (seismic) wave.
 

Andrija Mohorovicic found the boundary between the crust and the mantle.
How ?
At 30 to 35 km, seismic waves speeded up.  He inferred that that meant the
rocks got more dense and were probably different in their makeup.   This
boundary between the mantle and the core is called the MOHO.

There is a zone a certain distance from an earthquake where no waves
can be detected.  This is the shadow zone caused by the earth's core.

Meteorites are made of iron and nickel.  The theory is that since meteorites
are probably the remains of planets that have broken apart (most likely
their cores),  then earth's core is probably made of nickel and iron too.

Links
Project Moholehttp://www.nas.edu/history/mohole/
Project Mohole Pictures http://www.nas.edu/history/mohole/mohole_gallery.html
Build a simple seismograph
See a picture of a seismographhttp://www.exploratorium.edu/faultline/earthquakescience/eqscience5.html
See seismograph information