What’s Worse Than One, Big Earthquake? A String of Slightly Smaller Ones

Historical earthquake activity shows that California may not just be set for one big earthquake, but a cluster of smaller shakers

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A view looking down Pine Street in the wake of the 1906 Earthquake. Waldemar Lindgren/National Geographic Society/Corbis

In California, everyone knows that the Big One—a massive, inevitable earthquake—is coming. New research, however, is suggesting that the Bay Area in particular shouldn't just be bracing for a single big earthquake but a swarm of slightly smaller ones. “If that sounds like good news, it isn’t,” says Joel Shurkin for Inside Science.

A single massive earthquake can causing shaking that lasts for minutes, felling buildings and cracking highways. This is what California is preparing for. Every year since 2008 millions of Californians have taken part in the Great ShakeOut—the trains slow down, students duck and cover, and emergency managers run through a mock catastrophe. 

But the authors of the new paper think it's "somewhat likelier," says Shurkin, that a series of individually smaller yet still sizable earthquakes could strike over the span of a century.

This idea—that the Bay Area may be facing an earthquake cluster—is grounded in a new observational record of historical earthquake activity, says the Seismological Society of America, describing the new research. Researchers dug down into the major fault lines in the area and found that, over the past 400 years, accumulated stress has been released in two ways: as a huge earthquake, like the massive 1906 earthquake, or as a series of smaller earthquakes, as happened in the 1700s.

“F]rom 1690 to 1776,” says the San Jose Mercury News, “[a]t least six earthquakes, ranging from 6.3 to 7.7 magnitude, rattled the region's major faults during that period."

And a series of quakes like that one could be even worse than one big earthquake for the modern Bay Area. Rather than a single blow and a long stretch of recovery, the region could be battered by intermittent, quite destructive earthquakes.

Which of these two options will play out in the future is unclear, but neither would be good. The Wall Street Journal:

Scientists say the new information should shift the region's focus to preparing for the possibility of a potentially devastating quake on one fault followed by one on another fault within the same decade. A number of major faults in the Bay Area haven't seen large quakes in decades, so they haven't had a chance to catch up to the movements of the Earth's tectonic plates, the scientists behind the study said.

"So, as the region is recovering from one event, another event happens," said Mr. Schwartz, a geologist with the U.S. Geological Survey. "In a sense, it's a much more difficult hazard to deal with."

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