The strange way the Los Angeles Basin influences earthquakes

Southern California was recently hit by two magnitude 4.0 earthquakes, and the way they were felt in Los Angeles has a lot to do with the sediment-filled basin on which the city sits.

Just over an hour after sunset on August 6, 2024, a sparsely populated farm belt near Bakersfield in Southern California was awakened from its peaceful evening slumber. Earthquake of magnitude 5.2followed by Hundreds of smaller aftershocksTremors shook the region as a fault ruptured at the southern end of the Central Valley.

This latest 5.2 magnitude earthquake is the strongest in Southern California in three years. The epicenter was about 27 kilometers south of Bakersfield, California, and people reported tremors nearly 90 miles (145 km) away in parts of Los Angeles and as far away as San Diego. Then, a few days later, Another jolt shook the Los Angeles due to a crack in a small section of the dangerous Puente Hills fault systemThe epicenter of the 4.4 magnitude earthquake was just six kilometers northeast of the city center.

Although there may be various reasons for this – including human activities at the time of the earthquake – five miles deep (8 km)The sediment-filled basin on which LA is built plays a surprising role in the impacts felt above ground.

While the soil appears stable on the surface, deeply buried bedrock can resemble a broken window pane. These cracks or faults are where earthquakes occur. Errors are put under enormous stress by the slow and steady movement of the Earth’s tectonic plates.

In California, the North American Plate and the Pacific Plate rub against each other along the infamous San Andreas Fault, on average about 30-50 millimeters (1-2 inches) per year. The movement is anything but fluid. Fractured rocks are rough and wedge together, sometimes remaining stuck for thousands of years. Over time, the stress created by the slowly moving tectonic plates builds up – when the fracture reaches its stress limit, it “slips” and breaks. cause an earthquake.

A fracture starts in one place and spreads a direction along the faultthat extend for hundreds of kilometers. The longest fracture ever recorded was a 1,600 km long section of a fault that caused the Great Sumatra-Andaman Earthquake and resulting tsunami on Boxing Day 2004. “The farther it spreads, the longer (the earthquake) lasts and the more energy is released. So the longer the fault, the stronger 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 energy stored in the body Sticky Fault is suddenly releasedSeismic waves emanate from the fault, like the waves created when you throw a stone into a pond, and spread in all directions through the surrounding rock and soil.

The magnitude of an earthquake tells scientists about the length of the rupture and the duration of the quake, Jones says. But the intensity of an earthquake – the ground movements we feel in one place – depends on how close we are to the epicenter, the direction the rupture ruptured and the geological layers beneath our feet.

Los Angeles is south of a huge bend in the San Andreas Fault where the plate boundary is clearly changing direction. “When you see it from the air, it’s amazing,” Jones says. “It’s so bizarre – you can look down and see the fault valley and then it just spins.”

The region around the bend is full of faults. Over millions of years, the faults have churned up slabs of rock and pushed them into several mountain ranges and deep basins. Gravity, water and wind act like sandpaper, eroding the mountains and carrying the debris into the basins. Over time, the basins have been filled with sediment.

During an earthquake, seismic waves are modulated by geology, says John Vidale, a professor of seismology at the University of Southern California. “The main factor is how hard the ground is and how deep the structure is that has soft (material) near the surface,” he says. Seismic waves travel faster in denser materials like rock than in softer and less dense sediments.

As seismic waves travel through the basin, their behavior changes when they encounter the loose sediment. “(The wave) now has to travel at a much lower speed, but it still has to transport the same amount of energy per unit time,” Jones said. As the wave fights its way through the sediment, the amplitude, or wave height, gets larger.

Put another way, imagine the Los Angeles Basin as a giant bowl of jelly – the dense mounds of rock and the underlying rock form the bowl, while the sedimentary fill is represented by the gelatinous mixture. “If you shake the bottom (of the bowl) a little bit, the top rocks back and forth quite a bit,” says Vidale. And on top of this quivering jelly lies the megacity of Los Angeles.

This means that the amplitude of waves within a basin can be considerably larger than that of waves moving through rock. In a studyResearchers who took earthquake measurements in the Los Angeles area from 1992 Landers earthquake found that seismic waves in the Los Angeles Basin were three to four times stronger than in locations outside the basin.

In addition to amplification, seismic waves can also reverberate in a sediment-filled basin. Think of the shaking bowl of jelly and how the lid bounces off the sides of the bowl. Scientists at the Statewide California Earthquake Center simulated earthquakes in the Los Angeles area and found that the Basin can capture seismic wave energy in a similar way. This reverberation can mean that shaking often continue longer than the duration of the fault itself, which increases the danger to the city built above it.

Even within a basin, there can be differences in the way sediment interacts with seismic waves. “There are differences in the shaking… there are differences in the geology,” Vidale says. Sediment in the top 100 meters of the basin tends to be looser and less dense than the deeper, more compacted sediment below. Sediment changes can also happen rapidly at the surface. “Old riverbeds, for example, may be filled with some kind of wet, soft material,” Vidale says. “So if you happen to be in an old riverbed, you’re going to get hit much harder than someone who’s even a quarter mile away on firmer ground.”

Even people in the same house can have different experiences, especially if the earthquake is relatively weak. “I’m in Pasadena, on the sediments in the San Gabriel Valley,” Jones says. Although they were both in the house, she and her husband experienced the August 6 earthquake differently. “I felt it, my husband didn’t,” she says.

Although Los Angeles is at risk for earthquakes in many ways, it is not the only city to worry about. Throughout human history, people have tended to build their cities on flat ground near bodies of water.

It is simply that these places tend to spread over geological basins and sometimes near faults.

While there are some famous cities in the United States that were built on basins – Seattle, Portland and Salt Lake City – there are many other cities around the world that are exposed to increased seismic waves due to their location. After European settlers drained Lake Texcoco in the 16th century, Mexico City was built on the flat bottom of the old lake. In the years 1985 and 2017 suffered significant damage of earthquakes that shook the sediments of the basin.

Understanding earthquake risk is the first step in improving a city’s protection against strong shaking. Implementing strict building codes can be another way to protect people and infrastructure, but it often takes a major event to force stricter regulations to be implemented. For example, after the devastating earthquake in 1985, Mexico City enacted strict building regulationsand older buildings retrofitted.

“The very first earthquake codes (in California) after the Long Beach earthquake in 1933Jones adds. Back then, schools were built of fireproof, unreinforced brick. “Seventy schools were completely destroyed – luckily it was at six o’clock in the evening,” Jones says. The horror of collapsing schools led to regulations, but the initial regulations were flimsy. “They basically just said, ‘Don’t build unreinforced masonry in California.’ That was kind of the first basic regulation.”

Nowadays, the assessment of earthquake risks is much more differentiated.

In the USA, a team of seismologists, geoscientists and geophysicists has Earthquake hazard mapwhich shows the probability of a destructive earthquake in the next 100 years. In their latest version of the report, the team found that that almost 75 % of the United States could experience destructive earthquakes. To help decision makers and engineers, the team has included information on the impacts on building and structure designs.

While building codes can protect lives, scientists like Jones want the codes to go further. Designing buildings so they can be repaired more easily rather than being torn down would cost one percent more during the construction phase, Jones estimates. “We call this ‘functional restoration,'” she says.

“What we’re saying is that ‘just won’t kill you’ is not a sufficient benchmark. The reality is: If your building is badly damaged and has to be demolished after the earthquake, you’ve hurt your tenants, your neighbors and the local economy.”

Fortunately, Los Angeles’ buildings have survived the recent earthquakes that struck Southern California quite well. But at some pointthe city will not be so lucky.

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