LOOKING INTO THE EARTH

Exploring the Earth Using Seismology

Earthquakes create seismic waves that travel through the Earth. By analyzing these seismic waves, seismologists can explore the Earth's deep interior. This fact sheet uses data from the 1994 magnitude 6.9 earthquake near Northridge, California to illustrate both this process and Earth's interior structure. The cutaway of Earth on the left shows the paths seismic waves traveled following the quake. The ground motion caused by the seismic waves is shown at the right (seismograms recorded at various locations around the world).

Keypoints:

• Seismic waves propogate outwards from an earthquake in all directions.
• Seismologists use the recordings of the ground motion caused by these waves to explore the Earth's deep interior.
• We do not see shear (S) waves passing through the outer core. Because liquids can not be sheared, we infer that the outer core is molten.
• We do, however, see S waves as they go through the inner core. Because the inner core transmits shear energy, we assume it is solid. 
• http://www.iris.edu/hq/inclass/fact-sheet/exploring_earth_using_seismology