Sarah Derouin
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 Angelesarea 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.
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