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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.
The travelling earthquake
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.
Imagine the Los Angeles basin as a giant bowl of jelly – the mountains and underlying rock make up the bowl, while the sediment is represented by the gelatinous mixture
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.
Geology-induced complications
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.