Two tectonic plates are grinding slowly past each other along the San Andreas Rift Zone, and just east of Cholame the highway crosses the fault line where the Pacific Plate, carrying much of the California coast and all of Baja California, moves northwest at about two inches a year past the North American Plate, which carries the rest of the continent. The discovery of oil in the San Joaquin Valley in the early 1900s created demand for a pipeline to move crude oil to a coastal shipping point, leading the Union Oil Company and 150 independent oil producers to form the Producers Transportation Company, whose crews began in 1909 to dig 240 miles of trenches and lay an 8-inch pipeline supported by fifteen pumping stations that kept heated crude flowing to the Union Oil Company wharf at Port Harford near Avila for loading onto tankers. Earthquakes occur when locked tectonic plates suddenly overcome friction and lurch past each other, and seismologists have found that an area on the San Andreas Fault near Parkfield, California, regularly produces magnitude 6.0 earthquakes about every 22 years, making Parkfield an international center of seismic research where the San Andreas Fault Observatory at Depth, with sensors placed nearly 2 miles into the Earth's crust, studies the fault during an earthquake cycle, although predicting future quakes and their severity remains elusive.