Cape Mendocino, located on the Lost Coast entirely within Humboldt County, California, USA, is the westernmost point on the coast of California. The South Cape Mendocino State Marine Reserve and Sugarloaf Island are immediately offshore, although closed to public access due to their protected status. Sugarloaf Island is cited as California's westernmost island.
Named by early Spanish navigators in honor of Antonio de Mendoza, Viceroy of New Spain, Cape Mendocino has been a landmark since the 16th century, when Manila Galleons followed the prevailing westerlies across the Pacific to the Cape, then followed the coast south to Acapulco, Mexico. The Cape Mendocino Light was lit December 1, 1868, standing on eight prefabricated panels sent up from San Francisco; an automated light stood near the original location but was removed in 2013.
The Cape Mendocino region of California's north coast is one of the most seismically active regions in the contiguous United States. Three earthquakes with epicenters nearby at Petrolia and offshore west of Cape Mendocino, 25–26 April 1992, were outstanding, one reaching 7.2 Mw; they demonstrated that the Cascadia subduction zone is both capable of producing large earthquakes and generating tsunamis. Many geologists and seismologists believe that the main shock in the 1992 sequence may be a forerunner of a much more powerful earthquake in the Pacific Northwest.
Offshore of Cape Mendocino lies the Mendocino Triple Junction, a geologic triple junction where three tectonic plates come together. The San Andreas Fault, a transform boundary, runs south from the junction, separating the Pacific Plate and the North American Plate. To the north lies the Cascadia subduction zone, where the Gorda Plate is being subducted under the margin of the North American plate. Running west from the triple junction is the Mendocino Fault, the transform boundary between the Gorda Plate and the Pacific Plate.
- "Cape Mendocino". Geographic Names Information System. United States Geological Survey.
- California Department of Fish and Wildlife , California Protected Marine Areas, 14 March 2013
- California Department of Fish and Wildlife, Sugarloaf Island Special Closure, 2015
- Sugarloaf Island Special Closure, California Marine Sanctuary Foundation, accessed July 12, 2015
- Rowlett, Russ. "Northern California". Lighthouses of the United States. University of North Carolina at Chapel Hill. Retrieved 9 September 2012.
- USGS. "Cape Mendocino, California Earthquakes". Retrieved 2009-10-21.
- Kathy Moley. "Why we have earthquakes: a unique geologic setting". Retrieved 2009-10-21.