Final answer:
The remnants of the Farallon Plate, now represented mainly by the Juan de Fuca Plate, are being subducted along northern California and the Pacific Northwest, leading to the region's tectonic activity.
Step-by-step explanation:
The remnants of the Farallon Plate are being subducted along northern California and the Pacific Northwest. This plate is now mostly fragmented into smaller plates, with the main remnant being the Juan de Fuca Plate, which is responsible for the tectonic activity in this region, including the formation of the Cascade Range through subduction. As the Pacific Plate moves northward, it interacts with the various fragments of the ancient Farallon Plate, creating complex geological features along the West Coast of North America.