Final answer:
The west coast's high hazard ratings result from tectonic plate activities and volcanic actions, with California affected mainly by the San Andreas Fault and the Pacific Northwest shaped by the subduction of the Juan de Fuca plate contributing to its volcanic landscape.
Step-by-step explanation:
The west coast of the United States has the highest hazard ratings primarily due to tectonic plate boundaries and related geological activities such as earthquakes and volcanic activity. The San Andreas Fault is a transform boundary between the Pacific and North American plates, significantly impacting California's geography. In the Pacific Northwest, the Juan de Fuca plate converges with the North American plate leading to the creation of the Cascade Range and the potential for large-scale eruptions like that of Mount St. Helens.
Despite both areas experiencing seismic and volcanic hazards, California is mainly affected by movements along the San Andreas Fault, which poses a significant risk of earthquakes. In contrast, the Pacific Northwest's volcanic activity is shaped by the subduction of the Juan de Fuca plate, causing not only earthquakes but also giving rise to the volcanic Cascade Range. The different types of plate boundaries and their movements lead to distinct geological hazards in these regions.