The San Andreas Fault: Is the Big One Coming?


San Andreas fault San Andreas fault
The most significant of the seven faults in California's Bay Area is the San Andreas fault, a 750-mile-long (1,207-kilometer-long) transform fault that runs down most of the state. Michael R. Perry/Flickr (CC By-2.0)

After a 6.4 magnitude quake hit Southern California on the morning of July 4, 2019, near the city of Ridgecrest, about 100 miles (161 kilometers) to the northeast of Los Angeles, followed by a 7.1 quake in the same area the following evening, it was a disturbing reminder that California faces the possibility of a vastly more catastrophic major earthquake sometime in the future.

Where Is the San Andreas Fault?

According to the U.S. Geological Survey, the big quakes that hit in early July occurred on a complex array of faults that lie near Ridgecrest. Meanwhile, the monster that lies to the west of that area remained quiet — at least for now. The massive San Andreas Fault Zone, is the boundary between the Pacific and North American tectonic plates that runs more than 800 miles (1,300 kilometers) from Cape Mendocino, 200 miles (322 kilometers) north of San Francisco, and extends southward to the Salton Sea, a shallow saline lake about 120 miles (193 kilometers) to the east of San Diego.

The San Andreas is the stuff of nightmares because back on April 18, 1906, it caused the most catastrophic event in California history, the great San Francisco earthquake, which was so powerful that it caused a rupture in the land that stretched for 296 miles (477 kilometers). While its magnitude is uncertain, scientists have estimated that it may have been as large as 8.3. The quake and the massive fire that swept through San Francisco afterwards and killed more than 3,000 people, while leaving homeless another 225,000 — more than half the then-population of San Francisco. It destroyed 28,000 buildings and caused the equivalent of more than $11 billion in today's dollars in monetary losses.

But despite its enormous destructive potential, the northern part of San Andreas has been largely quiet since then, and the southern portion hasn't had a major earthquake since the 7.9 Fort Tejon earthquake on Jan. 9, 1857. That's led to widespread worry that California is overdue for a devastating quake somewhere along the San Andreas.

When Could the Big One Happen?

When and where is anyone's guess. The 2014 Working Group on California Earthquake Probabilities predicted that there was a 72 percent probability of a 6.7 or larger magnitude quake hitting the San Francisco Bay area by 2043, but only a 22 percent chance of a quake that big or bigger on the northern portion of the San Andreas. Along the southern portion of the San Andreas, closer to Los Angeles, researchers projected a 19 percent chance of a 6.7 or greater magnitude quake.

But earthquake prediction isn't an exact science, and those numbers shouldn't give too much comfort. "There are many damaging earthquakes possible on the San Andreas fault system," John Vidale, the Dean's Professor of Earth Sciences at the University of Southern California and former head of the Southern California Earthquake Center, explains via email. "While the 'big ones' are inevitable on the timescale of centuries, many other scenarios would do great damage with magnitudes as low as M6 to M7."

"In particular, both the Bay Area and Los Angeles are riven with fairly active faults directly underfoot most the cities, many of which are not even yet known and named," Vidale continues. "All the faults in the Bay Area and Los Angeles are within the San Andreas fault system. Napa in 2014, Northridge in 1994, Loma Prieta in 1989 and San Fernando in 1971 are just four examples of such events."

While people often think of San Andreas as a single huge fault, that's not actually the case. "The San Andreas fault system consists of many parallel fault strands with variable rates of motion," Vidale explains. "In southern California, the main strands are the San Andreas fault, the San Jacinto fault, and the Elsinore fault. In northern California, the most dangerous are the San Andreas, the Hayward-Calaveras fault system, and the Greenville and Green Valley faults farther east. (Here's a handy guide to the major Bay Area faults.)

If you saw the 2015 thriller "San Andreas," you're probably wondering: What about the chances of a truly apocalyptic event, in which a series of massive quakes would ripple up and down the entire fault? Fortunately, that's something that's likely to happen only in the feverish mind of a Hollywood screenwriter. "The earthquakes in northern California are thought to occur independently from the events in southern California," according to Vidale. "Each segment could host ruptures up to 248.6 miles [400 kilometers] long. There is a slight chance that a single earthquake could traverse longer stretches of fault, conceivably breaking both at once, but it would be a very rare occurrence. The creeping section of fault between Parkfield and San Juan Bautista is thought to separate the two regions."

But it's also important to remember that the San Andreas Fault Zone isn't the only system in California that's capable of generating a big quake. "The faults in eastern California, the ones along the Eastern California Shear Zone, are also capable of M8 earthquakes," Vidale says. "In fact, there was a near-M8 event in 1872 near Lone Pine, and the 1952 M7.3 Kern County earthquake was along it, as well as the 1972 M7.3 Landers and the1999 M7.1 Hector Mine quakes. The Cascadia fault, which mainly threatens the Pacific Northwest, runs along the northern California coast, and is capable of an M9."