How do seismograms recorded during earthquakes help scientists learn about the composition of the Earth?
Earthquakes generate energy that travels through the earth in the form of seismic waves, much in the way that waves travel through water. Seismographs record the arrival of these seismic waves. The earth is composed of a great variety of materials of different densities and composition. Waves will move at different speeds through different materials. By recording when waves arrive and knowing how far you are from the earthquake you can calculate how fast the wave was moving and then infer the type of material that the waves were moving through.
