Earthquakes and Plate Subduction

Background:

• Geophysicists categorize earthquakes by focus depth.
  • shallow earthquakes: 0 to 70 km depth.
  • medium depth earthquakes: 70 to 300 km
  • deep focus earthquakes: 300-700 km
• Geophysicists discovered an interesting pattern when the locations of earthquakes near ocean trenches were plotted.
• They saw that the depth of earthquakes changes with increasing distance from an ocean trench.
• Plotting the locations of earthquakes in three dimensions allowed them to "see" a subducting plate at a convergent plate boundary.
• This pattern helped support theories regarding subduction of tectonic plates and deep focus earthquakes.