On the morning of January 9, 1857, a California rancher named John Barker led his horse to the shores of Tulare Lake, located halfway between San Francisco and Los Angeles. Before his horse could take a drink, though, Barker was struck by a jarring nausea.

“The ground seemed to be violently swaying from east to west,” reported Barker years later. “The water splashed up to my knees; the trees whipped about, and limbs fell on and all around me … The lake commenced to roar like the ocean in a storm, and, staggering and bewildered, I vaulted into the saddle and my terrified horse started, as eager as I was to get out of the vicinity.”

The powerful earthquake lasted nearly three minutes. When Barker came out of hiding, he found the fields around the lake littered with dead fish. The water had sloshed so violently that it stranded the fish three miles inland, where they became a feast for vultures.

It’s estimated that the Fort Tejon earthquake of 1857 was a magnitude 7.9 or 8.0—equal to, if not greater than, the legendary California earthquake of 1906 that nearly destroyed San Francisco. The Fort Tejon quake was the last true “big one” to occur along the southern stretch of the San Andreas Fault, which ruptured along the surface for more than 215 miles.

Eyewitness reports from the 1857 quake described wide fissures tearing open in the earth, rivers changing course, trees being swallowed up by liquefaction, and solid ground rolling like waves in the ocean. In Los Angeles, panic-stricken residents staggered into the streets and fell to their knees in prayer.

“If you live in California and have felt an earthquake, you think you know what strong shaking is, but very few people experience one of these great earthquakes,” says Susan Hough, a seismologist with the U.S. Geological Survey. “If the shaking got so bad that rivers started to flow backward, it wasn’t uncommon for people to think it was the end of the world.”

Even though the record-setting 1857 quake and its aftershocks were felt from San Francisco to San Diego, there were only two reported fatalities. That’s because California’s total population was only 330,000 people at the time. If the same quake struck the San Andreas today, the damage and loss of life would likely be staggering.

The historic Fort Tejon earthquake got its name because the worst damage was inflicted at Fort Tejon, a U.S. Army garrison located 75 miles northwest of Los Angeles. (The actual epicenter of the quake was 100 miles away, in Parkfield, California.) Fort Tejon was a remote Army outpost responsible for patrolling the Sebastian Indian Reservation.

Around 8:30 a.m. on January 9, the quartermaster at Fort Tejon described feeling “the most terrific shock imaginable, tearing the officer’s quarters to pieces …” In a letter to the Los Angeles Star dated two days later, he catalogued the widespread damage: “Immense trees have been snapped off close to the ground, and every building between Fort Tejon and Lake Elizabeth leveled with the ground. Many persons have been seriously injured, and one woman killed at ‘Reed’s Rancho.’”

That unlucky woman was the one direct fatality from the Fort Tejon earthquake, killed by the collapse of her adobe house. There were other reports of an elderly man who collapsed and died in a plaza in Los Angeles.

The San Andreas Fault marks the boundary between the earth's Pacific Plate and the North American Plate. The fault roughly bisects the state of California from north to south, running from Mendocino through the Central Valley, past Los Angeles and down to the Mexican border. Unlike most other faults around the Pacific Rim, which are buried deep underground in subduction zones, the San Andreas sits right on the surface—a visual reminder of its raw, geological power.

“It's pretty dramatic,” says Hough. “It runs nearly the whole length of California, it breaks the surface, it’s got a poetic name… In terms of faults that are well-known and that impact people, the San Andreas is kind of in a league of its own.”

The San Andreas is a “strike-slip” fault, meaning the two massive plates are sliding past each other. The average motion of the plates is only 2 inches a year, roughly the same rate that fingernails grow. But that motion isn’t constant. The plates will sometimes get stuck or locked for years, storing up “elastic strain” energy that is released in one jolting lurch. As shockwaves from that sudden movement propagate through the ground, they’re felt as earthquakes.

“Geologic processes seem slow by our human scale, but 2 inches a year is really quite fast,” says Hough. “It doesn't take long to build up really significant levels of stress.”

During the 1857 earthquake, the average movement—or “displacement”—of the San Andreas Fault was 15 feet, but some locations experienced a shift of nearly 30 feet as the plates released decades or even centuries of pent-up energy.

California has always been “earthquake country.” California’s Indigenous tribes tell stories of ancient quakes and destructive tsunamis from the distant past. And from the moment the first Europeans arrived in California, they got a taste of a big earthquake.

On July 28, 1769, a Spanish expedition led by Gaspar de Portolá was camped by a river in modern-day Orange County when the ground began to shake violently. The God-fearing Spaniards measured the length of each aftershock by how many “Hail Marys” they could mutter before the trembling finally subsided.

Earthquakes weren’t only a New World phenomenon, of course. Places like Italy and Turkey had recorded their own historic quakes for centuries, but there was little understanding of what caused the earth to tremble. When the Fort Tejon earthquake struck California in 1857, geologists still didn’t understand plate tectonics and knew nothing about the existence of fault lines.

At the time, the best explanation for earthquakes was that they had something to do with volcanoes. Reporting on the 1857 earthquake, the Los Angeles Star wrote that earthquakes were produced “by gasses confined in the molten interior of the earth. Such gasses, prevented by local circumstances from escaping, may, it is thought, shake the solid ground over a large tract, and even cause it to rise to a certain extent above its former level.”

The quartermaster at Fort Tejon reported that the 1857 earthquake was preceded by a terrible rumbling from the nearby mountains and a “powerful volcanic eruption.” Other eyewitnesses described a mountaintop explosion that spewed “a mass of rock and earth” into the sky.

The role of faults in earthquakes didn’t become clear until a half-century later when another massive earthquake struck California.

The 1906 California earthquake was historic for many reasons. Estimated at a magnitude 7.7 to 7.9, the earthquake was triggered by a violent rupture in the northern San Andreas fault that stretched 270 miles. The 1906 earthquake claimed more than 3,000 lives in San Francisco, mostly due to the catastrophic fires that consumed 80 percent of the city and burned for three days.

But the 1906 quake is also important for scientific reasons. In the immediate wake of the destruction, geologists from University of California, Berkeley and Stanford fanned out across Northern California to map the earthquake’s line of displacement. What they found was a fault beginning northwest of San Francisco, in Mendocino and extending south to where damage from the 1857 quake was still visible at the surface. Following that line took them all the way to the Salton Sea, near the Mexico border.

“1906 is what put the San Andreas fault on the map,” says Hough.

Careful analysis of data from the 1906 earthquake led to the development of the “elastic rebound” theory, which argued that earthquakes were caused by the release of elastic stress along fault lines. It would still be decades before geologists developed a comprehensive theory of plate tectonics.

Because of the high death toll in San Francisco, the 1906 earthquake is much more famous than the Fort Tejon quake of 1857. That said, Hough thinks that the 1857 quake may have been a little bigger based on eyewitness descriptions of the shaking. Ultimately, it’s impossible to know, because seismograms were in their infancy in 1906 and nonexistent in 1857.

A more important question is whether Southern California is due for another “big one” to rupture the San Andreas fault in the same vicinity as the Fort Tejon quake. According to the math, the San Andreas should produce a magnitude 8.0 quake every 200 years, but Hough says that fault systems are complicated and there are too many variables to predict when exactly a large earthquake will occur.

One thing is for sure—if the 1857 earthquake struck today, it would impact areas now populated by tens of millions of Californians. Hough says the quake would “blast toward San Bernardino” along the San Andreas before pumping a lot of energy into the Los Angeles basin.

“In LA, you're going to get this sloshing—these sediments just shaking back and forth,” says Hough. “Can all of those high-rise buildings really withstand that? They’ve been built to code, but they’ve also never been tested by a magnitude 8.0 earthquake.”

Like the 1906 quake, Hough adds, the real damage could be done by fires, especially if the next “big one” strikes during Santa Ana conditions with hot, dry weather and high winds.