section_path: Measurement and location/Seismic waves/Seismic wave arrival
section_name: Seismic wave arrival
paragraph_text: Seismic wave arrival
  
  As a consequence, the first waves of a distant earthquake arrive at an observatory via the Earth's mantle.
  
  On average, the kilometer distance to the earthquake is the number of seconds between the P- and S-wave times 8. Slight deviations are caused by inhomogeneities of subsurface structure. By such analysis of seismograms, the Earth's core was located in 1913 by Beno Gutenberg.
  
  S-waves and later arriving surface waves do most of the damage compared to P-waves. P-waves squeeze and expand the material in the same direction they are traveling, whereas S-waves shake the ground up and down and back and forth.