Final answer:
The Cascadia Fault Line involves the Juan de Fuca plate subducting beneath the North American plate, leading to significant seismic and volcanic activity, such as in the Cascade Range including Mt. St. Helens.
Step-by-step explanation:
The Cascadia Fault Line is analogous to the famous San Andreas Fault, but it is located off the West Coast of North America, including areas of Northern California, Oregon, Washington, and British Columbia in Canada. This fault is part of a complex boundary between the smaller Juan de Fuca plate to the west and the North American plate to the east.
The Cascadia Subduction Zone is where the oceanic Juan de Fuca plate is moving towards, and being forced beneath, the continental North American plate. This subduction leads to high levels of seismic and volcanic activity in the region, as exemplified by the Cascade Range, which includes volcanoes such as Mt. St. Helens.
This process can cause very powerful earthquakes and tsunamis similar to what happened in Japan. With the Pacific plate moving northward and the North American plate moving southward, there is a risk for significant natural events. The relative motion of these plates is several centimeters per year, which is a significant geological force over time.