P Wave vs. S Wave

55s Novice

Why do P waves arrive first?

When an earthquake occurs, seismic waves, including P and S waves carry energy away from the hypocenter in all directions. The P wave propagates with a velocity of about six kilometers per second in rock, while the S wave is slower at four kilometers per second. This video explores how this difference wave speeds results in staggered arrivals that, in turn, provides information about how far away the earthquake was from the seismograph.  

Keypoints:

  • P waves travel faster than S waves.
  • The farther the seismic waves travel, the larger the gap between the P and S wave becomes. 
  • When recorded, the arrival times of the P wave and the S wave provides information about how far away the earthquake occurred from the seismograph.