The San Andreas Fault

The San Andreas Fault is the most famous fault line in the world. It is a long crack in Earth's crust running approximately 1,200 km through California, marking the boundary between two of Earth's biggest tectonic plates: the Pacific Plate (on the western side) and the North American Plate (on the eastern side). The two plates are sliding past each other horizontally at a speed of approximately 5 cm per year, making the San Andreas a strike-slip fault. The fault has caused many of California's biggest earthquakes, including the catastrophic 1906 San Francisco earthquake. Scientists believe the next "Big One" along this fault is geologically overdue.

  • LengthApprox. 1,200 kmFrom northern California to the Mexican border
  • TypeStrike-slip faultPlates sliding sideways past each other
  • Plate movementApprox. 5 cm/yearPacific Plate moving NW past North American Plate
  • Famous earthquake1906 San FranciscoMagnitude 7.9, approx. 3,000 deaths
  • Other big one1989 Loma PrietaMagnitude 6.9, stopped the World Series
  • Next "Big One"OverdueCould be magnitude 7 to 8+

What the fault is

The San Andreas Fault is the boundary between two of the world's largest tectonic plates. On the western side is the Pacific Plate (which underlies most of the Pacific Ocean). On the eastern side is the North American Plate (which carries North America and Greenland). The Pacific Plate is moving northwest relative to the North American Plate, at approximately 5 cm per year.

Because the two plates are moving past each other rather than colliding or pulling apart, the boundary is a strike-slip fault. The rocks on either side of the fault are not pushed up or down; they grind past each other horizontally.

The 1906 San Francisco earthquake

On 18 April 1906, at 5:12 am, the northern section of the San Andreas Fault ruptured along approximately 480 km. The resulting earthquake was about magnitude 7.9 and lasted about 45 seconds. In some places, the ground shifted sideways by up to 6 metres in a single moment. Fences, roads, pipes and railway lines that crossed the fault were suddenly offset by that distance.

The shaking damaged most of downtown San Francisco. Worse, broken gas pipes started fires that quickly spread out of control. With water mains also broken, firefighters could not put the fires out. They burned for three days, destroying much of the city. The official death toll was 700, but historians now believe the true figure was closer to 3,000. Around 250,000 people were made homeless out of a city of 400,000.

The 1906 earthquake is a landmark event in earthquake science. It gave geologists their first detailed look at a major fault rupture and led directly to many of the ideas that became the modern theory of plate tectonics.

The 1989 Loma Prieta earthquake

On 17 October 1989, just before game 3 of the baseball World Series in San Francisco, a magnitude 6.9 earthquake struck along a southern section of the San Andreas system. It caused 63 deaths, collapsed a section of the upper deck of the Bay Bridge, and led to the famous "World Series earthquake" being broadcast live on television. The earthquake also led to better earthquake-resistant building codes across the Bay Area.

Fact The San Andreas Fault is moving at approximately 5 cm per year. That means Los Angeles (on the Pacific Plate side) is slowly inching towards San Francisco (on the North American side). At current rates, in approximately 15 million years Los Angeles will be sitting next to where San Francisco is today. In approximately 50 million years, the western strip of California may have completely detached and become an island in the Pacific.

The "Big One"

The southern part of the San Andreas Fault (between San Bernardino and the Mexican border) has not had a major earthquake in over 320 years. Geological studies of past earthquakes show that this section has historically ruptured every 100 to 200 years. It is therefore "overdue" for a major earthquake, sometimes called the Big One.

The US Geological Survey estimates a 75% chance of a magnitude 7 or greater earthquake along the southern San Andreas in the next 30 years. Such an earthquake could kill thousands of people and cause hundreds of billions of dollars of damage, especially in Los Angeles. California has spent decades trying to prepare: strict building codes, earthquake drills (the annual "Great ShakeOut" practice), early-warning systems and improved infrastructure.

Did you know? Geologists can see the San Andreas Fault from the air. From a plane or satellite, the fault appears as a long, almost straight line cutting across California, with rivers and streams making sharp 90-degree bends where they cross it (because the moving rocks on one side gradually drag the river channel sideways). The fault is one of the few plate boundaries on Earth where you can stand on dry land and see the boundary itself.
Deeper dive: how Californians live with earthquakes

Living in a major earthquake zone has shaped California in many ways. Decades of effort have gone into making the state as safe as possible.

  • Building codes: California has some of the strictest seismic building codes in the world. New buildings must be designed to flex and absorb shaking rather than collapse. Older buildings have been steadily retrofitted with extra steel or rubber-isolated foundations.
  • Early warning: the ShakeAlert system, launched in 2019, uses sensors near major faults to detect the start of an earthquake. Phone apps can warn users tens of seconds before damaging waves arrive, enough time to take cover, stop a train, or shut a gas line.
  • Annual drills: the Great ShakeOut, held every October, sees millions of Californians practise "drop, cover and hold on" at home, work and school.
  • Public education: every Californian learns basic earthquake safety from childhood. Most homes keep emergency kits with water, food, first aid and a torch.
  • Special engineering: famous buildings like the new San Francisco Bay Bridge and many tall structures use base isolators (giant rubber bearings) that let the building stay still while the ground shakes underneath. Some sit on sliders that let the foundation move with the earthquake.

Despite all this, a magnitude 7.8 earthquake on the southern San Andreas would still be a disaster. Estimates suggest 1,800 deaths and over $200 billion in damages. The goal of all the preparation is to make sure things are not as bad as they could be without it. Compared with similar magnitude earthquakes elsewhere (Haiti 2010, Turkey 2023), the California measures have already saved countless lives.

For more, see fault lines and what is an earthquake.