Parts of a trench:

  • Outer arc high.  This will often have some small normal faults and earthquakes as the upper part of the plate extends as it bends.
  • Trench--the topographic low point.  The depth varies with speed of subduction, age of the subducting crust, and the sediment supply to fill in the trench.
  • Accretionary prism: sediment scraped off the subducting plate, with a series of  imbricate thrust faults.  The oldest fault will be on top, and the youngest at the bottom, closest to the trench.
  • Forearc basin: sediment from the magmatic arc.
  • Island arc/volcanic/magmatic arc
  • Back arc basin (and spreading center).  Only present when the overriding plate has oceanic crust.



Image from Wikipedia

The Benioff zone is the zone of dipping earthquake hypocenters which defines the subducting plate.

The largest and most damaging megathrusts will be relatively close to the surface.

The deepest subduction zone earthquakes, at only a few trenches, extend down to about 600 km.


last revision 11/6/2016