Los Angeles stands on a 'jelly bowl': earthquakes in the city will cause dire consequences
Southern California recently experienced two earthquakes measuring over 4,0 in magnitude. The way they felt in Los Angeles is largely due to the city's location above an underground basin that collects sediment, writes Air force.
Photo: Francesco Scatena | Dreamstime.com
Earthquakes are not unusual. California is the second most seismically active state after Alaska. On average, there is an earthquake every three minutes in Southern California. True, most of them are too small for a person to feel. But about 15-20 earthquakes annually exceed a magnitude of 4,0.
About an hour after sunset on August 6, 2024, a sparsely populated belt of farmland near Bakersfield, Southern California, experienced a magnitude 5,2 seismic thunderstorm, followed by hundreds of smaller tremors.
This latest quake is the largest to hit Southern California in three years. Its epicenter was about 27 miles south of Bakersfield. People reported shaking about 145 miles away, in parts of Los Angeles and as far away as San Diego. Then, a few days later, another quake shook the Los Angeles area. The epicenter of the magnitude 4,4 quake was just four miles northeast of the city’s downtown.
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While both tremors caused minimal damage, they once again demonstrated how the geology beneath California's largest city can influence ground movements in the area. The relatively shallow depth of the Aug. 6 quake appears to have created more intense or longer-lasting shaking in some parts of the city.
The huge sediment-filled basin on which Los Angeles is built, located at a depth of 8 km, greatly influences how tremors are felt on the surface of the earth.
"Traveling" earthquake
While the earth appears solid on the surface, the deep-lying rocks resemble broken glass. These cracks, or faults, are where earthquakes occur. The faults are subject to enormous stress due to the slow and steady movement of the Earth's tectonic plates.
In California, the North American and Pacific plates move along the infamous San Andreas Fault at an average rate of about 30-50 millimeters per year. The movement is not smooth. The cracked rocks push into each other like wedges. Over time, the stress created by the slowly moving tectonic plates builds up. When the fault reaches its limit of stress, it ruptures and causes an earthquake.
Think of the Los Angeles Basin as a giant bowl full of jelly: the mountains and rocks lining the valley floor form the bowl, while the soft soil sediments become sort of the jelly-like contents of the bowl.
The rupture starts in one place and extends along the fault for hundreds of kilometers. The longest rupture ever recorded was 1600 kilometers long, and caused the Great Sumatra-Andaman Earthquake of December 26, 2004. (It was one of the most powerful earthquakes in history, and caused a devastating tsunami that affected many countries in the Indian Ocean.)
"The farther the fault goes, the longer the seismic storm lasts and the more energy is released. So the longer the fault, the stronger the earthquake," said seismologist Lucy Jones, a research scientist at the California Institute of Technology and a former seismologist with the U.S. Geological Survey.
During an earthquake, the energy stored in the fault is suddenly released. Seismic waves spread out from the fault in all directions, like ripples from a stone dropped into a pond.
The size of an earthquake tells scientists how long the fault is, as well as how long the shaking lasts, Jones says. But the intensity of the tremors (the ground movement we feel in a particular location) is shaped by several factors: how close we are to the epicenter, which direction the fault ruptured, and the geologic layers beneath our feet.
Complications caused by geology
Los Angeles is located just south of a giant bend in the San Andreas Fault, where the plate boundary changes direction. Over millions of years, the faults pushed slabs of bedrock into numerous mountain ranges and deep depressions. Gravity, water, and wind acted like sandpaper, wearing down the mountains and carrying debris into the depressions. Over time, these depressions filled with sediment.
The bowl-shaped depression beneath Los Angeles is nearly 8 miles deep and filled with a mixture of gravel, sand, and clay. The contrast between the hard rock and the soft mixture that fills the bowl is an important factor in causing the interesting seismic effects in Los Angeles. John Vidale, a professor of seismology at the University of Southern California, says the main factor is how hard the ground is and how deep the soft-filled structure is near the surface. Seismic waves will travel faster in denser material like rock than in softer, less dense soils.
As seismic waves travel through the depression, their behavior changes when they encounter loose sediment.
"The seismic wave now has to travel at a much slower speed, but it still has to transfer the same amount of energy per unit of time," Jones said.
As the wave passes through the sediment, its height increases. For example, imagine the Los Angeles Basin as a giant bowl of jelly—the dense rocky mountains and underlying rocks form the bowl, while the sediment fill is a gelatinous mixture.
"If you shake the bottom of the bowl a little bit, the top of it will wiggle quite a bit," Vidale concluded, adding that Los Angeles is on top of this trembling mass of jelly.
That is, the amplitude of waves inside the basin can be significantly larger than those moving through the rocks. In one study, scientists using measurements of tremors in the Los Angeles area after the 1992 Landers earthquake found that seismic waves inside the Los Angeles basin were three to four times larger than those outside it.
Scientists at the California National Earthquake Center modeled tremors in the Los Angeles area and found that the basin can trap energy from seismic waves bouncing off the bowl's walls. That means the shaking can often last longer than deep-seated soil movements, increasing the danger to the city built on top.
Additionally, the close proximity of the San Bernardino and San Gabriel basins to the Los Angeles basin could create a funnel effect and direct seismic waves toward Los Angeles.
Even within a basin, there can be differences in how sediment interacts with seismic waves. Sediment in the top 100 meters of a basin is generally looser than the deeper, more compacted lower layers.
"The old river beds are filled with some kind of wet, soft material," Vidale said. "So if you're in an old river bed, you're going to get hit a lot harder than somebody a quarter mile away. Approx. Ed.) from you, but on firmer ground."
Even people living in the same house experience different sensations, especially if the earthquake was weak. Lucy Jones was in her home in Pasadena, a city located on the sedimentary rocks of the San Gabriel Valley, during one of the quakes.
“My husband and I were both at home at the time, but I felt the earthquake, but he didn’t,” she shared.
Swimming pools everywhere
Although the city of Los Angeles meets many of the seismic hazard boxes, it is not the only metropolitan area with such problems. Throughout human history, people have tended to build cities on flat land near bodies of water.
It just so happens that these places tend to form over geological basins and sometimes near faults.
While the U.S. has a few famous cities built on basins (Seattle, Portland, and Salt Lake City), there are many cities around the world that experience increased seismic activity due to their location. When European settlers drained Lake Texcoco in the 1500s, Mexico City was built on the flat old lake bed. The city suffered significant damage from earthquakes in 1985 and 2017, which aggravated the basin’s sediments.
Tehran, Iran, also sits on a geological basin filled with river sediments, and there is growing concern about the destruction a major earthquake could cause in the area.
Understanding why the earth shakes is the first step to making cities safer. Adopting strong building codes is another way to protect people and infrastructure in megacities. For example, after the devastating 1985 earthquake, Mexico City adopted strict building codes and retrofitted older buildings.
“The very first seismic codes in California were put in place after the Long Beach earthquake of 1933,” Jones said. “Back then, schools were built of fire-resistant, unreinforced brick. Seventy schools were completely destroyed — fortunately, it was XNUMX p.m. and they were virtually empty.”
In the United States, a group of seismologists and geophysicists have created a seismic hazard map showing the likelihood of a destructive earthquake over the next 100 years. In the latest version of the report, the group found that nearly 75% of the United States could experience destructive tremors.
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While building codes can protect lives, scientists like Jones want to see seismologists' findings carefully considered when designing buildings. Building codes should be designed to make it easier to repair a building if an earthquake occurs, rather than tear it down. That, Jones calculates, would cost 1% more during construction.
"The experts call it functional recovery," she explained. "What we're trying to say is that 'just not killing you' is not a good enough standard. The reality is that if your building is badly damaged by an earthquake, it's going to hurt the local economy."
Luckily, Los Angeles' buildings have weathered the recent tremors that rocked Southern California fairly well. But it's worth taking precautions for the future.
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