The Weird Way the Los Angeles Basin Affects Earthquakes

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Los Angeles sits atop a huge bowl of sediment that affects how seismic waves move beneath the city (Image credit: Getty Images)

Southern California has recently been hit by two earthquakes with magnitudes greater than 4.0 on the Richter scale. The way Los Angeles has experienced such earthquakes has a lot to do with the sedimentary basin in which the city sits.

A little more than an hour after sunset on Aug. 6, 2024, a belt of sparsely populated farmland near Bakersfield, Southern California, shook after a quiet evening. A 5.2-magnitude earthquake, followed by hundreds of smaller aftershocks, rattled the area as a fault ruptured near the southern end of the Central Valley.

The latest 5.2-magnitude quake was the largest to hit Southern California in three years. The epicenter was about 17 miles (27 kilometers) south of Bakersfield, California, and people reported shaking about 90 miles (145 kilometers) away in parts of Los Angeles and as far away as San Diego. Then a few days later, another tremor shook the Los Angeles area, caused by a rupture in a small section of the dangerous Puente Hills fault system. The resulting 4.4-magnitude quake was centered just four miles northeast of downtown.

While there are various reasons behind this — including what people were doing at the time of the quake — the massive, five-mile-deep (8 km) sediment-filled basin on which Los Angeles is built plays a surprising role in the effects felt by people above ground.

mobile earthquake

While the Earth appears solid on the surface, deeply buried rocks can resemble broken window glass. Earthquakes occur in these cracks, or fissures. Faults are under tremendous stress due to the slow, steady movement of the Earth's tectonic plates.

In California, the North American Plate and the Pacific Plate are moving along the famous San Andreas Fault, at a rate of 30 to 50 millimeters (1 to 2 inches) each year. The movement is not fluid at all. The cracked rocks are jagged and stuck together, sometimes for thousands of years. Over time, stresses from the slow-moving tectonic plates build up—and when the fault reaches its maximum stress, it “slips” and bursts, causing an earthquake.

Imagine the Los Angeles Basin as a giant bowl of jelly – the bowl is made up of mountains and bedrock, while the sediments are represented by the gelatinous mixture.

The fracture starts in one place and runs in one direction along the fault, extending for hundreds of kilometers. The longest fracture ever recorded was a 994-mile (1,600-kilometer) section of the fault that caused the Great Sumatra-Andaman Earthquake and resulting tsunami on Boxing Day 2004. “The farther out the fracture, the longer the fracture lasts. [the earthquake] “The longer the fault is, the more energy is released. So the longer the fault is, the bigger the earthquake,” explains seismologist Lucy Jones, a researcher at Caltech and former seismologist with the U.S. Geological Survey.

During an earthquake, the energy stored inside the viscous fault is suddenly released. Seismic waves radiate out from the fault like ripples from a stone thrown into a pond, and travel in all directions through the surrounding rock and ground.

The size of an earthquake tells scientists how long the fault has ruptured and how long the shaking will last, Jones says. But the intensity of an earthquake—the ground movements we feel in a given location—is shaped by how close we are to the epicenter, the direction the fault has ruptured, and the geological layers beneath our feet.

Complications caused by geology

Los Angeles is located south of a giant curve in the San Andreas Fault, where the plate boundary changes direction dramatically. “If you saw it from the air, it would be amazing,” Jones says. “It’s very strange—you can look down and see the rift valley and then suddenly it changes direction.”

At the bend, the area is filled with faults. Over millions of years, faults have shifted slabs of bedrock, pushing them into multiple mountain ranges and deep basins. Gravity, water, and wind act like sandpaper, eroding the mountains and carrying debris into the basins. Over time, the basins fill with sediment.

Seismic waves during an earthquake are modified by geology, says John Vidal, a professor of seismology at the University of Southern California. “The main factor is how solid the ground is and how deep the structure is that contains soft layers. [material] “Seismic waves move faster in denser materials like rocks than in softer, less dense sediments,” he says.

As seismic waves travel through the basin, their behavior changes when they encounter loose sediments.[The wave] “The waves are now moving much slower, but they still carry the same amount of energy per unit time,” Jones said. As the wave moves through the sediment, the wave’s amplitude, or height, increases.

In other words, think of the Los Angeles Basin as a giant bowl of jelly—the bowl is made up of dense rocky mountains and the rocks beneath them, and the gelatinous mixture is the sediment fill. “If you shake the bottom, the jelly will build up in the bowl, eroding it.” [of the bowl] “The top moves back and forth a lot,” Vidal says, “and above this quivering mass of jelly lies the metropolis of Los Angeles.

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The San Andreas Fault between the Pacific and North American tectonic plates can be clearly seen from the air in some places (Source: Getty Images)

This means that the amplitude of waves inside the basin can be much larger than waves moving through the rocks. In one study, researchers using seismic measurements in the Los Angeles area from the 1992 Landers earthquake found that seismic waves inside the Los Angeles basin were three to four times larger than at locations outside the basin.

In addition to amplification, seismic waves can also reverberate within a sediment-filled basin. Think of a vibrating jelly bean and how its quivering top bounces off the sides of the bowl. Scientists at the California Earthquake Center simulated earthquakes in the Los Angeles area and found that the basin is able to trap the energy of seismic waves in a similar way. This reverberation can mean that shaking can often last longer than the fault itself ruptures, increasing the risk to the city built above.

Even within the basin, there can be differences in how sediments react to seismic waves. “There’s variation in the vibrations … and there are differences in the geology,” Vidal says. The sediments at the top of the 330-foot-deep (100-meter-deep) basin tend to be looser and less dense than the deeper, compacted sediments below. And sediment changes can happen quickly at the surface. “For example, old sewage channels can be filled with some kind of soft, wet material,” Vidal says. “So if you’re in an old sewage channel, you’re going to get hit much harder than someone a quarter-mile away on harder ground.”

Even those who live in the same house may have different experiences, especially if the quake was smaller. “I’m in Pasadena, on the sediment in the San Gabriel Valley,” Jones says. Despite being home, she and her husband had different experiences of the Aug. 6 quake. “I felt it, my husband didn’t,” she says.

Sinks, sinks everywhere

While Los Angeles has many seismic hazards, it’s not the only urban center that should be a concern. Throughout human history, people have tended to build cities on flat land near bodies of water.

It so happens that these sites tend to form above geological basins and sometimes near faults.

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Scientists are trying to better understand how large earthquakes propagate across the Los Angeles Basin to assess where the risk of damage is greatest (Image credit: Getty Images)

While the United States has a few famous cities built on basins—Seattle, Portland, Salt Lake City—there are many others around the world that experience amplified seismic waves because of their location. After European settlers drained Lake Texcoco in the 16th century, Mexico City was built on the former flat lake bed. In 1985 and 2017, the city suffered significant damage from earthquakes that shook up the sediments of the basin.

Understanding earthquake risk is the first step in strengthening a city’s defense against strong tremors. Enacting strong building codes can be another way to protect people and infrastructure, but it often takes a major event to enforce stricter regulations. For example, after a devastating earthquake in 1985, Mexico City enacted strict building codes and retrofitted older buildings.

“First Earthquake Rules” [in California] “We came in after the Long Beach earthquake of 1933,” Jones adds. At the time, schools were built of fireproof, unreinforced brick. “Seventy schools were completely destroyed—luckily, it was at 6 p.m.,” Jones says. The horror of school collapses prompted regulations, but the initial rules were meager. “They basically said, ‘Don’t build unreinforced brick in California.’ That was the first law.”

Today, earthquake risk assessment has become more accurate.

In the United States, a team of seismologists, earth scientists, and geophysicists created a seismic hazard map that shows the likelihood of a damaging earthquake occurring in the next 100 years. In the latest report, the team found that nearly 75% of the United States is likely to experience damaging quakes. To help policymakers and engineers, the team included information on the implications for building and structural design.

While building codes may save lives, scientists like Jones want them to go further. Jones estimates that designing buildings so they can be repaired rather than torn down would cost an extra 1 percent of the construction cost. “We call this ‘functional recovery,’” she says.

“We’re trying to say that just not killing you is not a sufficient criterion. The reality is that if your building is badly damaged and you have to tear it down after an earthquake, you’ve hurt your tenants, your neighbors, and your local economy.”

Fortunately, Los Angeles' buildings have withstood the recent earthquakes that hit Southern California. But at some point, the city won't be so lucky.

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