The San Francisco 1906 earthquake was caused by a rupture on the San Andreas Fault, a continental transform fault that forms part of the tectonic boundary between the Pacific Plate and the North American Plate. The fault is characterized by mainly lateral motion where the western (Pacific) plate moves northward relative to the eastern (North American) plate. The 1906 rupture propagated both northward and southward from its epicenter for a total of about . The San Andreas Fault runs the length of California from the Salton Sea in the south to Cape Mendocino to the north, a distance of about . The earthquake ruptured the northern third of the fault for a distance of about . The maximum observed surface displacement was about ; however, geodetic measurements show displacements of up to in some places. The most recent analysis by the United States Geological Survey (USGS) shows that the most likely epicenter of the 1906 earthquake was very near Mussel Rock on the coast of Daly City, an adjacent suburb just south of San Francisco.