Figure 1 Figure 1. The Juan de Fuca plate is produced by spreading at the Juan de Fuca ridge several hundred kilometres offshore.
Great earthquakes occur on the thrust fault contact.
Cascade volcanoes such as Mt. Garibaldi and Mt. St. Helens are produced by melting above the downgoing plate where it reaches a depth of about 100 km. Figure 2 Figure 2. Earthquakes produced by most fault zones follow the Gutenberg-Richter relation. The average number of earthquakes per year decreases with increasing magnitude. The Cascadia thrust fault is unusual in that it produces no small earthquakes, only infrequent very large events.