Earthquakes, including the recent major quakes in Chile and Haiti, release energy that radiates two kinds of seismic waves: compression waves (P waves) and shear waves (S waves). P waves alternately compress and release rocks in the direction the waves are moving. S waves move rocks perpendicular to the direction the waves are moving. "P waves travel faster, but S waves cause most of the damage because they are larger amplitude," says Jeff J. McGuire, associate scientist in Geology & Geophysics at WHOI. By measuring these waves, scientists can locate an earthquake’s source.
(Illustration by Jayne Doucette, Woods Hole Oceanographic Institution)