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. But a previously overlooked danger lurks below this frenzy: A fault capable of generating earthquakes magnitude 6.3 or greater.
The Wilmington fault, as it’s called, is an elusive type of fracture. 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. 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.
Now, a new analysis of the system, published in Bulletin of the Seismological Society of America, suggests that isn’t the case. 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.
“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.
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. And researchers worry that the Wilmington could link with other nearby faults to produce a temblor as strong as a magnitude 7.4.
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. Scientists have made major strides characterizing the geology of the region, but there’s still more to do.
“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. 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?”
When most people think about California earthquakes, they think of the San Andreas 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. 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. (Learn what’s happening as the Farallon plate dies under Oregon.)
This sent North America for a wild ride: Its west coast first stretched out then sections later compressed. 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.