The geology of the North Coast is distinct from the rest of California. Geologists and seismologists find this region of special interest because of the Mendocino triple junction, a place where three plates, the Gorda, the North American and the Pacific, are in contact. This unique geologic setting makes the North Coast vulnerable to earthquakes from a variety of sources.
Black arrows show the relative motion at plate boundaries.
The Cascadia subduction zone is the boundary between the Gorda and Juan de Fuca plates offshore and the North American plate. The white triangles show the locations of active volcanoes, common along subduction zones. A vertical slice along the southern part of the Gorda plate where it is being subducted beneath the North American plate. The megathrust is the locked boundary between them. The San Andreas Fault South of the triple junction, the Pacific plate moves northwest relative to the North American plate.