Here's a breakdown:

* Subduction Zones: These are areas where one tectonic plate slides beneath another. The Juan de Fuca Plate is subducting beneath the North American Plate at the location of Mt. St. Helens.

* Fault: A fault is a fracture in the Earth's crust where there has been movement. The subduction zone itself is a major fault.

* Volcanoes: Subduction zones are known for producing volcanoes because the descending plate melts, generating magma that rises to the surface. Mt. St. Helens is a stratovolcano, formed by the accumulation of lava flows and ash.

In short: Mt. St. Helens is located on a fault within the subduction zone where the Juan de Fuca Plate is diving beneath the North American Plate. This tectonic activity is the primary reason for its volcanic activity.