One such thrust fault was the focus of this latest work, but it’s still hidden below ground. The rift valley is a classic example of a divergent plate boundary. The Virunga chain is part of the East African Rift Valley system, which marks the boundary between two plates: the Nubian plate to the west and the Somalian plate to the east. Lava spews out of a fissure in the Virunga mountains in the Democratic Republic of the Congo. It’s as if “you put the whole thing it in a vise and started turning the handle,” says Goldfinger. That means the faults in the basin where Los Angeles sits today first slid in one direction and then reversed, forming thrust faults in which blocks of land are shoved on top of others. This sent North America for a wild ride: Its west coast first stretched out then sections later compressed. ( Learn what's happening as the Farallon plate dies under Oregon.) Ever since, the North American plate has met face to face with the Pacific plate, the two grinding past each other at a boundary called a strike-slip fault. The mighty feature formed some 30 million years ago as the North American plate swallowed almost all of what’s known as the Farallon plate. When most people think about California earthquakes, they think of the San Andreas fault. But the discovery that the Wilmington fault is likely still active, he says, “sort of leaves this big hanging question mark: What about all the others?” Tectonic smashup
“Just by itself, if you take the implications of a relatively slow-moving fault like this, it won’t change the hazard very much,” he says. Scientists have made major strides characterizing the geology of the region, but there’s still more to do. The study also emphasizes just how many faults crisscross Southern California, adds Chris Goldfinger, an earthquake geologist at Oregon State University, who was not part of the research team. Learn about the geophysics behind earthquakes, how they are measured, and where the most powerful earthquake ever witnessed occurred. And researchers worry that the Wilmington could link with other nearby faults to produce a temblor as strong as a magnitude 7.4.Įarthquakes can leave behind incredible devastation, while also creating some of the planet's most magnificent formations.
While the fault is slow moving and likely ruptures only once every 3,200 to 4,700 years, it underlies two of the United States’ busiest ports. “I hope bringing attention to it can potentially increase safety in the region,” says study author Franklin Wolfe, a doctoral candidate who is part of Harvard’s structural geology and Earth resources group. Using a cluster of clues incorporated into a three-dimensional model, the study authors posit that the fault has been active much more recently than once thought-and likely still poses a risk to people on the surface.
Now, a new analysis of the system, published in Bulletin of the Seismological Society of America, suggests that isn’t the case. So while scientists have long known the fault is present-stretching 12.4 miles under southern Los Angeles into San Pedro Bay-it was presumed to have sat quiet for millions of years.
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Unlike many faults, which crack Earth’s surface like an egg, the Wilmington fault is “blind,” which means it’s concealed beneath the surface, making it especially difficult to study. The Wilmington fault, as it’s called, is an elusive type of fracture. But a previously overlooked danger lurks below this frenzy: A fault capable of generating earthquakes magnitude 6.3 or greater. The ports of Los Angeles and Long Beach bustle with activity-their colorful array of shipping containers are stacked and unstacked in a never-ending, multibillion-dollar game of Tetris.
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