MENDOCINO FRACTURE ZONE

submarine fracture zone in the Earth's surface in the eastern Pacific Ocean, extending westward from about 120 mi (190 km) off Cape Mendocino, California, for at least 2,500 mi to latitude 38 N, longitude 175 W. In most places the fracture zone is expressed topographically as the steep southern slope of an asymmetric ridge with a gentler northern slope. Smaller, shorter ridges and scarps parallel the main scarp to the north and south along its length. Off California the main scarp is about 10,500 ft (3,200 m) high; toward its western end the height is only about 8,400 ft. Regional depths of the seafloor north of the fracture zone are consistently shallower than to the south, the difference varying between 2,600 and 3,900 ft. Magnetic intensities of seafloor rocks indicate an apparent lateral offset of 700 mi along the fracture zone; rocks north of the fracture zone are 23,000,000 to 27,000,000 years younger than rocks to the south. This apparent displacement is the scar of transform faulting that accompanied seafloor spreading, a process still going on along the Gorda Ridges north of the fracture zone. This spreading, estimated to proceed at a rate of 1.1 in. (2.9 cm) per year on either side of the Gorda Ridges, results in earthquakes along the fracture zone, from the crest of the ridge eastward to the coast.