There are some unusual features at the Cascade subduction zone. The probable explanation lies in the rate of convergence between the Juan de Fuca and North American Plates. These two plates converge at 3-4 centimeters per year at present. This is only about half the rate of convergence of 7 million years ago. Plate tectonic setting of the Cascades.
Yellow rings indicate recorded earthquakes.
Black lines show faults active within past 10,000 years. The small Juan de Fuca Plate and two platelets, the Explorer Plate and Gorda Plate are the meager remnants of the much larger Farallon oceanic plate. The Explorer Plate broke away from the Juan de Fuca about 4 million years ago and shows no evidence that it is still being subducted. The Gorda platelet split away between 18 and 5 million years ago and continues to sink beneath North America. More than 3000 vents erupted during the most recent volcanic episode that began 5 million years ago.