San Andreas Fault

• 1. Introduction

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  The San Andreas Fault runs roughly 750 miles through the state of California, and the ground on its two sides moves in opposite directions. Geologists call it a continental right-lateral strike-slip transform fault. In plain terms, the land on either side of the line slides sideways past the other, rather than pulling apart or crashing together. The fault forms part of the boundary between the Pacific plate and the North American plate. Averaged along its whole length, the slip is between 0.79 and 1.38 inches a year.

  That sounds small. Yet the fault has raised mountain ranges and produced earthquakes that people still study. Scientists traditionally divide it into three segments, and each one behaves differently.

  This documentary follows three questions. Who first recognized the fault as a single continuous feature, and how did they prove it? Why does one stretch creep quietly while another stays dangerously locked? And why, after decades of intense study, can nobody say when it will break next?

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• 2. Lawson's Map

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  In 1895, Professor Andrew Lawson of UC Berkeley identified a fault in Northern California that would later carry the name San Andreas. Eleven years later, the 1906 San Francisco earthquake gave him a new task. Lawson was handed the job of working out where the disaster had begun.

  He started by surveying and mapping offsets along the surface ruptures. Fences and roads had been sliced in half, and each cut marked a spot where the ground had jumped. When he plotted these offsets on a map, they formed a near perfect line. That line sat on top of the fault he had already found, so he concluded that this fault was the origin of the earthquake. He also decided that it extended all the way into Southern California.

  The line passed through San Andreas Lake, a sag pond. An extensional step over in the fault had created a natural depression, and water settled into it. Many people assume Lawson named the fault after this lake. His reports from 1895 and 1908 say otherwise: he named it after the surrounding San Andreas Valley.

  In 1953, geologists Mason Hill and Thomas Dibblee published a paper proposing that the fault could carry hundreds of miles of lateral movement. The idea was considered radical at the time. Modern plate tectonics has since vindicated it.

  To see what Lawson's line looks like on the ground, the best place to start is its southern end, near Bombay Beach.

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• 3. Walking the Fault

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  Box Canyon, beside the Salton Sea, shows layers of rock tilted upward by the southern end of the fault. This southern segment, also called the Mojave segment, begins near Bombay Beach. It then runs along the southern base of the San Bernardino Mountains and crosses Cajon Pass. From there it heads northwest along the northern base of the San Gabriel Mountains, which the fault's own movement helped raise.

  In Palmdale, travelers can examine the fault at a roadcut for the Antelope Valley Freeway. The trace continues alongside Elizabeth Lake Road to the town of Elizabeth Lake. Near Gorman, Tejon Pass and Frazier Park, it begins to bend northward in a curve known as the Big Bend. Scientists think this restraining bend is where the fault locks up in Southern California, with an earthquake-recurrence interval of roughly 140-160 years.

  Northwest of Frazier Park lies the Carrizo Plain, a long treeless plain where much of the fault is plainly visible. The Elkhorn Scarp marks the fault trace along most of its length there. The southern segment stretches from Parkfield in Monterey County to the Salton Sea and could produce an 8.1-magnitude earthquake. At its closest, it passes about 35 miles northeast of Los Angeles. A quake of that size would kill thousands of people in Los Angeles, San Bernardino and Riverside. It would also cause hundreds of billions of dollars in damage.

  The central segment runs northwest from Parkfield to Hollister, and it behaves very differently. Apart from the southern section and the parts through Parkfield, the fault here shows aseismic creep. It slips continuously, without producing earthquakes.

  At Hollister the northern segment begins. It crosses the Santa Cruz Mountains and heads up the San Francisco Peninsula. Near Mussel Rock at Daly City, it goes offshore, close to the approximate epicenter of the 1906 earthquake. It returns onshore at Bolinas Lagoon, just north of Stinson Beach in Marin County. Next it slips underwater through Tomales Bay, the linear trough that separates the Point Reyes Peninsula from the mainland. It passes just east of Bodega Head through Bodega Bay, dips underwater again and comes back onshore at Fort Ross.

  From Fort Ross the fault runs overland, partly forming the linear valley that the Gualala River flows through. It leaves the land again at Point Arena and follows the coast underwater to Cape Mendocino, where it bends west. Around the San Francisco Bay Area, several significant sister faults run more or less parallel to it. Each of them can produce a destructive earthquake of its own.

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• 4. Origins and Motion

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  About 30 million years ago, in the mid-Cenozoic, a spreading center between the Pacific plate and the Farallon plate began to reach the subduction zone off North America's western coast. The Farallon plate has since been mostly subducted. Its remnants include the Juan de Fuca, Rivera, Cocos and Nazca plates. Because the Pacific plate moved differently relative to North America than the Farallon did, the spreading ridge itself began to be subducted. That created a new relative motion and a new style of deformation along the boundaries. The same process may have driven deformation of the Basin and Range. It may also explain the separation of the Baja California peninsula and the rotation of the Transverse Ranges.

  The main southern section of the San Andreas proper is only about 5 million years old. Its earliest known ancestor was the Clemens Well-Fenner-San Francisquito fault zone, active around 22-13 million years ago. The San Gabriel Fault then became a primary focus of movement between 10 and 5 million years ago. Scientists blame this tangled history on the Big Bend, on a mismatch between the plate motion and the trend of the fault, or on both.

  Today the slippage across California averages about 33 to 37 millimeters a year. The Pacific plate, west of the fault, travels northwest. The North American plate to the east pushes southwest, though relative to the Pacific it drifts southeast. That southwestward push creates compression on the fault's eastern side, and the Coast Ranges are the result. The Pacific plate's northwest push squeezes hardest where the fault jogs westward. That squeeze built the Transverse Ranges in Southern California and, to a lesser extent, the Santa Cruz Mountains.

  The accounting does not quite add up. Studies of the relative motion of the two plates show that the San Andreas and its branch faults carry only about 75 percent of it. The rest turns up east of the Sierra Nevada, in the Walker Lane, also called the Eastern California Shear Zone. Nobody has settled why, and research continues. One hypothesis gained interest after the Landers earthquake in 1992. It suggests that the plate boundary may be shifting eastward, away from the San Andreas and toward Walker Lane. Researchers also believe the modern San Andreas will eventually pass its motion to a fault within that shear zone.

  If the boundary stays where it is, projected motion carries the land west of the fault, including Los Angeles, past San Francisco. Over perhaps twenty million years it would continue northwestward toward the Aleutian Trench.

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• 5. Where It Ends

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  Near Eureka, California, the fault slips offshore and ends at the Mendocino triple junction, where three tectonic plates meet. The Cascadia subduction zone also meets the San Andreas at that point. Scientists have hypothesized that a major earthquake on Cascadia could set off a rupture along the San Andreas.

  A 2008 paper that studied past earthquakes along the Pacific coastal zone found support for the idea. Seismic events on the northern San Andreas matched in time with events on the southern part of the Cascadia zone. That zone stretches from Vancouver Island to Northern California. Scientists believe Cascadia quakes may have triggered most of the major quakes on the northern San Andreas over the past 3,000 years. In each of those time-correlated events, the rupture ran from north to south.

  The 1906 San Francisco earthquake appears to be the exception. Its plate movement ran mostly from south to north, and no major Cascadia quake came before it.

  At the opposite end, the fault ends in the Salton Sea. There the plate motion shifts from right-lateral to divergent, in the manner of the East Pacific Rise further south. This region is the Salton Trough, where the boundary is rifting and pulling apart. The rifting is creating a new mid-ocean ridge, an extension of the Gulf of California. Sediment deposited by the Colorado River keeps the trough from filling with seawater from the gulf.

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• 6. The Parkfield Clock

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  Parkfield, in central California, has recorded seismic events in 1857, 1881, 1901, 1922-1934 and 1966. From this pattern, seismologists found that the fault there produces a magnitude 6.0 earthquake about once every 22 years. Scientists used the rhythm to predict that the next one should arrive in 1993.

  It did not. The quake finally came on the 28th of September 2004, at 10:15 a.m. PDT. It measured magnitude 6.0, and people felt it across the state, including in the San Francisco Bay Area. Even with the delay, Parkfield's predictable activity has made it one of the most important places in the world for large earthquake research.

  In 2004, work began just north of Parkfield on the San Andreas Fault Observatory at Depth, known as SAFOD. The National Science Foundation funded the project, which involved drilling through the fault from 2004 to 2007. The hole was meant to reach nearly 3 kilometers into the Earth's crust. Researchers wanted core samples and direct geophysical and geochemical observations of how the fault behaves at depth. Plans called for an array of sensors to record earthquakes near the site.

  The wider San Andreas Fault System has drawn a flood of studies. Research from the last 23 years alone has produced about 3,400 publications.

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• 7. Quakes on Record

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  In 1812, an earthquake near San Juan Capistrano ruptured at least 25 kilometers, or 16 miles, of Southern California. Several Roman Catholic missions in the area suffered heavy damage. Early studies placed the quake on the Newport-Inglewood fault because the damage lay close to it. A 2002 study pointed to the San Andreas instead. More recent work favors a joint rupture involving both the San Andreas and the San Jacinto faults. Around 40 people died, and estimates of the magnitude range from 6.9 to 7.5.

  The 1857 Fort Tejon earthquake ruptured about 350 kilometers of central and southern California. Despite its name, its epicenter is thought to lie far to the north, just south of Parkfield. Two deaths were reported, and the moment magnitude was 7.9.

  The 1906 San Francisco earthquake ruptured about 430 kilometers of Northern California. At least 3,000 people died in the quake and the fires that followed. Its magnitude is estimated at 7.8.

  A smaller quake struck in 1957. It measured magnitude 5.7, and its epicenter lay on the fault in the ocean west of San Francisco and Daly City.

  The Loma Prieta earthquake ruptured about 40 kilometers near Santa Cruz, although the break never reached the surface. It happened on the 17th of October 1989, at approximately 5:04 pm PDT. Its moment magnitude was about 6.9. It caused 63 deaths and moderate damage in certain vulnerable locations around the Bay Area.

  None of these quakes has settled the question that matters most, which is what the southern section will do next.

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• 8. The Next Big One

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  In 2006, Yuri Fialko, an associate professor at the Scripps Institution of Oceanography, published a study in the journal Nature. It found that the fault had reached a stress level sufficient for an earthquake above magnitude 7.0. It also found that the risk may be growing faster than scientists had believed. The danger is concentrated on the southern section, because the central segment broke in 1857 and the northern in 1906. The southern section has not seen a similar rupture for at least 300 years.

  According to the study, a major quake there would badly damage the Palm Springs-Indio area. It would also hit cities in San Bernardino, Riverside and Imperial counties, as well as Mexicali in Baja California. The shaking would be strongly felt across much of Southern California. That includes Los Angeles, Ventura, Orange and San Diego counties, along with Ensenada, Tijuana, San Luis Rio Colorado in Sonora and Yuma, Arizona. Older buildings would be especially prone to damage or collapse. So would buildings on unconsolidated gravel, or in coastal areas where high water tables invite soil liquefaction.

  Fialko said that the data suggest "the fault is ready for the next big earthquake." He added that "exactly when the triggering will happen and when the earthquake will occur we cannot tell." In his words, "It could be tomorrow or it could be 10 years or more from now."

  No substantial quake has since struck the Los Angeles area. Two major United States Geological Survey reports have made variable predictions. Predicting a major earthquake precisely enough to justify extra precautions has remained out of reach. The agency's most recent forecast, UCERF3, was released in November 2013. It estimated that a quake of magnitude 6.7 or greater, equal to the 1994 Northridge earthquake, occurs about once every 6.7 years statewide. It also gave a 7 percent probability of a magnitude 8.0 or greater quake somewhere on the San Andreas in the next 30 years. A 2008 USGS study imagined a magnitude 7.8 quake on the southern San Andreas. It projected about 1,800 deaths and 213 billion dollars in damage.

  A 2023 study added a new clue about the southern section. It linked the water level of Lake Cahuilla, now the Salton Sea, to seismic activity. Major quakes on that section coincided with high water in the lake. The weight of the water can more than double the stress on the fault, likely enough to trigger an earthquake. The lake has since dried up, which may explain the unusually long wait since the region's last major quake.

  The HayWired Scenario turns to the Bay Area. It imagines a magnitude 7.0 earthquake on the San Andreas there and estimates the effects on infrastructure and the rebuilding that follows. Experts in the physical and social sciences and in engineering, from the public and private sectors, built it together. Its first volume appeared in 2017, and a second, titled Engineering Implications, followed in 2018. The aim is preparedness for Bay Area residents and a nudge for policy makers to build protections and resilience.

  The study estimates that hazards such as liquefaction, landslides and fire ignition would disrupt utilities, transportation and emergency services. It warns that low-income people, racially and culturally diverse communities and people with literacy hardships would face the greatest risk of displacement. Business interruptions in the first 6 months could cost the state an estimated 44 billion dollars in gross state product, about 4 percent of the California economy. In heavily affected places such as Alameda County, recovering the lost jobs could take up to 10 years.

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