Earth Science Standards Of Learning (SOL) Practice Test

The key idea is how different seismic waves move through Earth's materials and what that implies for their travel times. P waves are the fastest and act like compressions that push and pull in the direction of travel. Because they can move through solids, liquids, and gases, they arrive first at a seismic station after an earthquake. S waves, on the other hand, move by shearing the material—shaking rocks side to side perpendicular to the direction of travel. Liquids can’t support this kind of shear, so S waves cannot propagate through liquids at all. Because they travel slower than P waves, they arrive next, making them the second to reach the station. Surface waves (which travel along the Earth's surface) typically arrive after the body waves (P and S) because they move more slowly and are constrained to the surface layer. So the waves that are second to arrive and cannot travel through liquids are the shear (S) waves.