Article: Southern California—The Earthquake Laboratory

The earthquake hazard in Southern California is larger than anywhere else in the United States


- Lucy Jones, U.S. Geological Survey


Every day, people pack up their belongings and head to Southern California, their heads filled with visions of a sunny playground where hills and mountains overlook the golden shore. A few may hesitate because of California’s notorious history of devastating earthquakes. Ironically, it is exactly that long legacy of earthquakes that created the landscape that attracts people to Southern California. “Between the beaches and the snow on Mount Baldy, you can go skiing and surfing in the same day here in Southern California,” says geophysicist Ken Hudnut of the U.S. Geological Survey. “And the reason those mountains are so close to the ocean is because of the movement between the plates pushing up those mountains.” In other words, the mountains are there because of earthquakes.

All along the west coast of the United States, two enormous chunks of Earth, the Pacific and the North American tectonic plates, are slowly sliding past each other. Many places near this plate boundary are at high risk for earthquakes, including the San Francisco area, the Pacific Northwest, and Alaska, yet fully half the nation’s earthquake hazard is in Southern California.

Why is the potential for disaster so much greater in Southern California? Part of the answer is that the region is scarred with hundreds of active faults. But earthquake hazard is a measure not only of the expected number and magnitude of earthquakes but also of the potential damageto people, to property, to infrastructure. A large network of faults plus a large number of people equals a large earthquake hazard. 

Each year, more than 15,000 earthquakes rumble underneath Southern California. Although most of these earthquakes are too small to be felt, the region averages two felt earthquakes per week. Fifteen million people living on top of such an active geologic area makes Southern California a natural focal point for earthquake research. 

Shattered Earth

To have an earthquake you need a fault, and Southern California has an exceptional abundance of them. The main reason for this is the Big Bend, a large curve in the boundary between the Pacific and North American plates. Up and down the entire boundary, the plates are slowly grinding past one another. But in the Los Angeles area, the Big Bend prevents them from sliding past each other as easily as they do elsewhere. Lucy Jones, the scientist in charge of Southern California for the U.S. Geological Survey’s Earthquake Hazards Program, compares it to trying to move a piece of plate glass around a corner. The glass gets smashed and “you have all these little pieces that need to be moved around.” This shattered glass effect has left Southern California with hundreds of faults capable of producing damaging earthquakes.      

The movement of tectonic plates does more than just shake the ground; it literally reshapes the landscape. As the Pacific Plate pushes northward, trying to force its way past the Big Bend, it is compressing the Los Angeles Basin and uplifting the San Gabriel Mountains to the north of Los Angeles. The 1971 San Fernando earthquake raised the San Gabriel Mountains by about 2.5 meters (eight feet). 

The Big One

The Big Bend is actually a 300-kilometer (200-mile) stretch of the San Andreas Fault, which runs most of the length of California--from Point Arena, about 175 kilometers (110 miles) north of San Francisco, to the Salton Sea near the Mexican border. Flying over parts of California, the San Andreas Fault looks like a giant scar cutting through the countryside. Though hundreds of other faults crisscross Southern California, in the public imagination the San Andreas is synonymous with catastrophic earthquakes. This isn’t completely irrational.

“The San Andreas Fault concentrates about two-thirds of the motion between the North American and the Pacific Plates,” Ken Hudnut points out, “so the San Andreas Fault has been the source of the greatest historical earthquakes.” Both the 1857 Fort Tejon earthquake in the Los Angeles area and the 1906 San Francisco earthquake were larger than magnitude 8. “The really big earthquakes, what we call ‘The Big Ones,’ tend to be on the San Andreas Fault,” says Hudnut.

The reasons for this are twofold. The size of an earthquake is directly related to the length of a fault, and the San Andreas is the longest fault in California by far. The San Andreas is also the fastest-moving fault in Southern California, which means that it has earthquakes more frequently than other major faults. While many other faults go tens of thousands of years between damaging earthquakes, each section of the San Andreas Fault has a big earthquake on average every 100 to 250 years.          

Although a large earthquake strikes the section of the San Andreas Fault nearest Los Angeles on average every 130 years, the last one was the Fort Tejon quake in 1857, when the area was relatively unpopulated. In that quake, there was about six meters (20 feet) of slip along the fault. The section of the fault farther south, near Palm Springs, hasn’t had a major earthquake since 1680. The fact of the matter is that Southern California is overdue for a big earthquake on the San Andreas Fault. “The next big one could happen any moment,” Hudnut says. “You just never know.”

But the San Andreas Fault is only a part of Southern California’s earthquake hazard. Southern California’s shattered ground has more than 300 active faults, most of which are part of the San Andreas Fault system. A few of these, including the San Jacinto and the Elsinore, are long, fast-moving faults likely to produce major earthquakes. But the most devastating quakes in recent years, such as the 1971 San Fernando earthquake and the 1994 Northridge earthquake, were on relatively minor faults that were not expected to move. These smaller faults actually produce the majority of damaging earthquakes in Southern California.

Shaking Like a Bowl of Jello 

Although Southern California as a whole has half the nation’s earthquake hazard, that hazard is not evenly distributed throughout the region. Los Angeles County alone can proudly claim a quarter of the nation’s earthquake hazard. Part of the reason is the sheer number of people living there. When the Fort Tejon quake hit in 1857, the area was relatively unpopulated. Today, about ten million people make Los Angeles County their home. Similarly, in 1857 few large structures had been constructed in Los Angeles. Today, office buildings, freeways, bridges, and ports blanket the area. 

But there is a second, geologic, reason that the Los Angeles Basin has a particularly high earthquake hazard. The mountains surrounding Los Angeles are granite, a very hard rock, but the Los Angeles Basin is made up of very soft, loose sediment. When the seismic waves from an earthquake that began in the hard mountains hit the soft basin, they slow down. To carry the same amount of energy, the waves must then get bigger in amplitude. These amplified waves cause greater shaking. In the L.A. Basin, the shaking is amplified by a factor of five to ten, with the worst shaking striking the center of the basin, where the sediment is the thickest.

Many earthquake experts compare the Los Angeles Basin to Jello inside a hard granite bowl. “If you jiggle that bowl of Jello, it jiggles for a long time,” says Andrea Donnellan, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena. “The L.A. Basin just sloshes back and forth as a result of earthquakes, so they can be very damaging here.” 

Living in Earthquake Country 

Southern California’s high earthquake hazard makes it a very desirable place to live if you happen to be an earthquake researcher. “We’ve got an earthquake-producing machine below us,” says Tom Henyey, a professor of geological sciences at the University of Southern California. “If I’m going to study earthquakes, I’ve got to live in earthquake country.”

Yet other people are also drawn to Southern California because of its earthquakes, though unwittingly, for it is the region’s active geology that gives it a dramatic landscape and pleasant climate. It is earthquakes that have built up Southern California’s hills and mountains, which in turn catch the clouds that drop rain on the region. “If we gave up our mountains, we’d have an empty desert that wouldn’t be a whole lot of fun to live in,” says Lucy Jones. 

“You have to take both,” explains Nancy King, a geophysicist with the U.S. Geological Survey. “You take an active geologic area and you get a lot of beauty. If you don’t have active plate tectonics, erosion is the major geologic force operating, and gradually over millions of years you get flat topography.”

If they are wealthy enough, people in California often take full advantage of the spectacular scenery earthquakes have provided them by building their homes on top of hills, often on stilts. Ironically, this puts them at greater risk from earthquakes. “One rule of thumb in Southern California is that if you live on a hill, you’re probably living near a fault, because the hills are uplifted by faults,” Tom Henyey says. Henyey thinks this is a reasonable risk since most faults go thousands of years between major quakes.

 Likewise, when most people weigh the pros and cons of living in Southern California, they find living in earthquake country a risk worth taking. No matter where you live in the world, you are in the shadow of some natural hazard. Although people in other parts of the country may face little danger from earthquakes, they have to put up with hurricanes and tornadoes, winter storms and withering heat. In Southern California, earthquakes are the big danger. Since earthquakes are inevitable, the question becomes, how do you minimize the damage they cause? In recent years, scientists have begun using space-based technology to paint a more detailed picture of Southern California’s earthquake hazard, information that engineers and planners can use to focus their attention and resources on the areas that need it most.