Far below this area, tectonic plates are grinding past each other along the San Andreas Rift Zone, and just east of Cholame the fault line marks 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. The discovery of oil in the San Joaquin Valley in the early 1900s created demand for a pipeline to a coastal shipping point, leading the Union Oil Company and 150 independent oil producers to form the Producers Transportation Company; in 1909 crews dug 240 miles of trenches, laid an 8-inch pipeline, and built fifteen pumping stations to move heated crude 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 one another, and seismologists have found that an area on the San Andreas Fault near Parkfield, California, repeatedly produces magnitude 6.0 earthquakes about every 22 years, making Parkfield an international center of seismic research. The San Andreas Fault Observatory at Depth, with sensors placed nearly 2 miles deep in the Earth's crust, studies activity on and near the fault during an earthquake cycle, although predicting the timing and severity of future quakes remains elusive. During the Miocene Epoch, most of California was submerged below the sea, and around 15 million years ago thick layers of oil-forming organic material were being deposited. The Pacific Rim's Ring of Fire contains volcanoes and undersea trenches around most of the ocean's edge and accounts for about 90 percent of the world's earthquakes, while California's earliest reported earthquake was felt in 1769 by Gaspar de Portolá's expedition about 30 miles southeast of the Los Angeles area.