The weird way the Los Angeles basin alters earthquakes

Southern California has been shaken by two recent earthquakes greater than magnitude 4.0. The way they were experienced in Los Angeles has a lot to do with the sediment-filled basin the city sits upon.

A little over an hour after sunset on 6 August 2024, a sparsely populated belt of farmland near Bakersfield, Southern California, was shaken from a restful evening. A magnitude 5.2 earthquake, followed by hundreds of smaller aftershocks, shuddered through the area as a fault near the southern end of the Central Valley ruptured.

It wasn't a terribly unusual event, by California's standards. The state is the second-most seismically active in the United States behind Alaska, with Southern California experiencing an earthquake on average every three minutes. While most are too small to be felt, around 15-20 events exceed magnitude 4.0 each year.

This latest magnitude 5.2 earthquake is the largest to hit Southern California in three years. The epicenter was about 17 miles (27km) south of Bakersfield, California, and people reported shaking nearly 90 miles (145km) away in portions of Los Angeles and as far away as San Diego. Then, a few days later, another jolt rattled the Los Angeles area due to a rupture on a small section of the dangerous Puente Hills fault system. The resulting magnitude 4.4 earthquake had its epicentre just four miles northeast of the city's downtown area.

Although there was minimal damage caused by both quakes, they have highlighted just how the geology under California's largest city can alter the effects of fault movements in the area. The relatively shallow depth of the 6 August earthquake appeared to create more intense or prolonged shaking in some parts of the city, while others felt almost nothing at all.

While there are various reasons for why this might be – including what people were doing at the time of the earthquake – the enormous five-mile-deep (8km), sediment-filled basin that LA is built upon plays a surprising role in the effects felt above ground.

While the ground feels steadfast at the surface, deeply buried bedrock can resemble a shattered window pane. These cracks, or faults, are where earthquakes occur. Faults are put under tremendous stress by the slow and steady movement of the Earth's tectonic plates.

In California, the North American plate and the Pacific Plate are grinding past each other along the infamous San Andreas fault, averaging about 30-50 millimeters (1-2 inches) every year. The movement is anything but fluid. Cracked rocks are rough and wedge against each other, sometimes staying stuck for thousands of years. Over time, stress created by the slow marching tectonic plates builds – when the fault reaches its stress limit, it "slips" and ruptures, causing an earthquake.  

A rupture begins at one location and travels in one direction along the fault, stretching up to hundreds of kilometers. The longest rupture ever recorded was a 994 mile (1,600km) portion of a fault that caused the Great Sumatra-Andaman earthquake and resulting tsunami on Boxing Day 2004. "The farther it goes, the longer [the earthquake] lasts, and the more energy that's released. So the longer the fault, the bigger the earthquake," explains seismologist Lucy Jones, a researcher at the California Institute of Technology and former seismologist with the US Geological Survey.

During an earthquake, the stored energy saved within the sticky fault is released suddenly. Seismic waves radiate out from the rupture like the ripples created by throwing a rock into a pond, spreading in all directions through the surrounding rock and earth. 

The magnitude of an earthquake tells scientists about the length of the ruptured fault as well as the duration of shaking, says Jones. But the intensity of an earthquake – the ground motions we feel at a location – is shaped by how close we are to the epicenter, which direction the fault ruptured, and the geological layers under our feet.

Los Angeles is located south of a giant a bend in the San Andreas fault where the plate boundary clearly changes direction. "If you see it from the air, it's amazing," says Jones. "It's so bizarre – you can look down and see the fault valley and then it just turns."

Around the turn, the region is chock full of faults. Over millions of years, the faults churned and pushed slabs of bedrock into multiple mountain ranges and deep basins. Gravity, water and wind act like sandpaper, wearing down the mountains, and carrying the debris into the basins. Over time, the basins have been filled with sediment.

The bowl-shaped basin of rock under Los Angeles is up to five miles (8km) deep, filled with a mixture of gravel, sand and clay. The contrast between the hard rock and softer sediment are big factors that cause some seismic weirdness for cities like Los Angeles.

During an earthquake, seismic waves are modulated by geology, says John Vidale, professor of seismology at University of Southern California. "The primary factor is just how hard is the ground and how deep is the structure that has soft [material] near the surface," he says. Seismic waves will move faster in denser material like rock, versus softer and less dense sediment.

As seismic waves travel through the basin, their behaviour changes when they encounter the loose sediment. "[The wave] is now having to travel at a much slower speed, but it still has to carry the same amount of energy per unit time," said Jones. As the wave slogs through the sediment, the amplitude, or wave height, gets bigger.

Put another way, imagine the Los Angeles basin as a giant bowl of jelly – the dense rocky mountains and underlying rock make up the bowl, while the sediment fill is represented by the gelatinous mixture. "If you shake the bottom [of the bowl] a little bit, the top flops back and forth quite a bit," says Vidale. And atop this quivering mass of jelly is the megacity of Los Angeles.

This means the amplitude of the waves within a basin can be significantly bigger than those moving through rock. In one study, researchers using earthquake measurements in the Los Angeles region from the 1992 Landers earthquake found that seismic waves inside the Los Angeles basin were three to four times larger than sites outside the basin.

In addition to amplification, seismic waves can also reverberate within a sediment-filled basin. Think back to that shaking bowl of jelly and how the flopping top bounces off the sides of the bowl. Scientists from the Statewide California Earthquake Center simulated earthquakes in the Los Angeles region and found that the basin can trap seismic wave energy in a similar way. This reverberation can mean shaking can often go on for longer than the duration of the fault rupture itself, increasing the hazard for the city built on top.

As if that wasn't enough for Los Angeles, the close proximity of the San Bernadino and San Gabriel Basins to the Los Angeles Basins can create a funneling effect, directing seismic waves towards Los Angeles.

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Even within a basin, there can be differences in how the sediment interacts with seismic waves. "There's variability in the shaking… there's variations in the geology," says Vidale. Sediment in the upper 330ft (100m) of the basin tends to be looser and less dense than the deeper, compacted sediment below. Sediment changes can also happen quickly at the surface. "Old stream channels, for example, can be filled with a kind of wet, soft material," Vidale says. "So, if you happen to be in an old stream channel, you'll get hit a lot harder than somebody even a quarter mile away that's on firmer ground."

Even those in the same house can have different experiences, especially if the earthquake is on the smaller side. "I'm in Pasadena, on the sediment in the San Gabriel Valley," say Jones. Despite both being in the house, she and her husband had different experiences of the earthquake on 6 August. "I felt it, my husband didn't," she says.

While the city of Los Angeles ticks a lot of seismic hazard boxes, it is not the only urban center that needs to worry. Throughout human history, people have tended to build cities on flat ground near water bodies.

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

While the US has a few famous cities built on basins – Seattle, Portland, and Salt Lake City – there are many others around the world that experience amplified seismic waves due to where they are situated. After the European settlers drained Lake Texcoco in the 1500s, Mexico City was built on the flat, old lake bottom. In 1985 and 2017, the city experienced significant damage from earthquakes that shook the basin sediments.

The desert megacity of Tehran in Iran also sits atop a geologic basin filled with river sediments, and there is growing concern about the risk of a major earthquake in the area.

Understanding the earthquake risk is the first step in bolstering protection for a city against significant shaking. Enacting robust building codes can be another way to protect people and infrastructure, but it often takes a major event for stricter regulations to be implemented. After the devastating earthquake in 1985, for example, Mexico City enacted stringent building codes, and retrofitted older buildings. 

"The very first earthquake codes [in California] went in after the 1933 Long Beach earthquake," adds Jones. At that time, schools were built out of firesafe, unreinforced brick. "Seventy schools were completely destroyed – luckily, it was at six o'clock at night," says Jones. The horror of collapsing schools spurred regulation, but initial codes were meagre. "They basically just said, 'don't build unreinforced masonry in California'.  That was sort of the first basic code."

Today, assessing earthquake risks is a lot more nuanced.

In the US, a team of seismologists, geoscientists, and geophysicists have created a seismic hazard map, showing the chances of a damaging earthquake shaking in the next 100 years. In their latest version of the report, the team found that that nearly 75% of the US could experience damaging shaking. To help policymakers and engineers, the team included information on the implications for building and structural designs.

While building codes can protect lives, scientists like Jones want building codes to go further. Designing buildings so they can be more easily repaired rather than needing to be demolished would cost an extra 1% in the construction phase, Jones estimates. "We're calling it 'functional recovery'," she says.

"We are trying to say that 'just not killing you' is an insufficient standard. The reality is, if your building's badly damaged and now has to be torn down after the earthquake, you've hurt your tenants, you've hurt your neighbors, you've hurt the local economy."

Fortunately, the buildings of Los Angeles rode out the latest quakes to rattle Southern California pretty well. But at some point, the city won't be so lucky.

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